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==CHAPTER  VIII - THE  ELECTRICAL  THEORY  OF  MATTER==
==CHAPTER  VIII - THE  ELECTRICAL  THEORY  OF  MATTER==
 
[[Aston 1922/Chapter8]]
===84.  The  Whole  Number  rule===
By  far  the  most  important
result  of  the  measurements  detailed  in  the  foregoing  chapters  is
that,  with  the  exception  of  hydrogen,  the  weights  of  the  atoms
of  all  the  elements  measured,  and  therefore  almost  certainly
of  all  elements,  are  whole  numbers  to  the  accuracy  of  experi-
ment, in  most  cases  about  one  part  in  a  thousand.  Of  course,
the  error  expressed  in  fractions  of  a  unit  increases  with  the
weight  measured,  but  with  the  lighter  elements  the  divergence
from  the  whole  number  rule  is  extremely  small.
 
This  enables  the  most  sweeping  simplifications  to  be  made  in
our  ideas  of  mass,  and  removes  the  only  serious  objection  to  a=
 
unitary  theory  of  matter.
 
===85.  The  Unitary  Theory  of  the  constitution  of  matter===
From  the  very  earhest  times  it  has  been  a  favourite  hj'pothesis
that  all  matter  is  really  composed  of  one  primordial  substance,
Air,  Fire,  Earth  and  Water  have  all  been  suggested  in  the  past.
The  first  definite  theory  of  the  constitution  of  the  atoms  of
the  elements  out  of  atoms  of  a  primordial  element  (Protyle,
Urstoff,  etc.)  was  made  by  Prout  in  1815.  ProuVs  Hypothesis
was  that  the  atoms  of  the  elements  were  different  aggregations
of  atoms  of  hydrogen .  On  this  view  it  is  obvious  that  the  atomic
weights  should  all  be  expressed  by  whole  numbers  when  the
atomic  weight  of  hydrogen  itself  is  taken  as  unity.  Owing  to
the  roughness  of  the  methods  available  and  the  considerable
inaccuracies  of  the  atomic  weight  determinations  made  at  that
time  there  was  fittle  to  disprove  the  hypothesis,  and  its  marked=
 
simplicity  gained  it  many  adherents.  But  as  time  went  on
chemical  methods  grew  more  precise  and  it  became  more  and
more  impossible  to  reconcile  experimental  results  with  integral
 
90
 
 
THE  ELECTRICAL  THEORY  OF  MATTER      91
 
combining  weights  until  the  evidence  against  it  was  strong
enough  to  cause  J.  S.  Stas  (1860-1865)  to  state:  "  I  have
arrived  at  the  absolute  conviction,  the  complete  certainty,  so
far  as  it  is  possible  for  a  human  being  to  attain  to  certainty =
in
such  matters,  that  the  law  of  Prout  is  nothing  but  an  illusion, =
a
mere  speculation  definitely  contradicted  by  experience."
 
Nevertheless,  though  abandoned  temporarily  by  the  chemist
as  impracticable,  the  idea  of  primordial  atoms  appealed  strongly
to  the  mind  of  the  philosopher  and  the  physicist.  Herbert
Spencer,  in  his  hypothesis  of  the  constitution  of  matter,  says :=
 
"All  material  substances  are  divisible  into  so-called  elementary
substances  composed  of  molecular  particles  of  the  same  nature
as  themselves  ;  but  these  molecular  particles  are  complicated
structures  consisting  of  congregations  of  truly  elementary
atoms,  identical  in  nature  and  differing  only  in  position,
arrangement,  motion,  etc,  and  the  molecules  or  chemical  atoms
are  produced  from  the  true  or  physical  atoms  by  processes  of
evolution  under  conditions  which  chemistry  has  not  been  able
to  reproduce."
 
The  discovery  of  the  electron,  the  proof  that  it  was  the  same
whatever  the  atom  from  which  it  was  detached  and,  most
important  of  all,  the  demonstration  by  Sir  J.  J.  Thomson  and
others  that  electricity  could  simulate  the  known  properties
of  matter,  gave  us  the  key  to  the  riddle  of  what  these  primordial
atoms  really  are.  The  only  serious  obstacle,  the  fractional
atomic  weights,  has  now  been  removed  so  that  there  is  nothing
to  prevent  us  accepting  the  simple  and  fundamental  conclusion :
The  atoms  of  the  elements  are  aggregations  of  atoms  of
positive  and  negative  electricity.
 
===86.  The  Atom  of  Negative  Electricity,  or  Electron===
The  fundamental  unit  of  negative  electricity  makes  its  appear-
ance in  physical  phenomena  in  many  guises,  such  as  the  cathode
ray  of  electrical  discharge,  the  beta  ray  of  radioactive  change,
the  thermion  of  the  wireless  valve.  A  very  complete  account  of
it  has  recently  been  published  by  Millikan^  so  that  it  is  not
proposed  to  describe  its  history  and  properties  at  any  length
 
^  The  Electron,  by  R.  A.  Millikan,  University  of  Chicago  Press,  =
1917.
 
 
92  ISOTOPES
 
here.  It  will  be  sufficient  to  note  a  few  of  its  more  important
constants.
 
Its  charge  e  is  given  by  Millikan  as  4-774  X  10  "  lo  E.S.U.
The  most  reliable  measurements  of  e/m  for  the  electron,  at  low
velocity,  give  the  value  5-30  x  10"  E.S.U.  Hence  its  mass  is
almost  exactly  9*00  X  10~  ^s  grg,^  i845  times  less  than  the  mass
of  the  hydrogen  atom,  or  0-00054  on  the  ordinary  scale  of  atomic=
 
weights  (Oxygen  =3D16).
 
===87. The  atom  of  Positive  Electricity,  or  Proton===
Our physical  knowledge  of  this  body  is  not  nearly  so  complete  as
that  of  its  counterpart  the  electron.  It  is  very  significant  that
in  no  analysis  of  positive  rays  so  far  performed  have  we  been
able  to  discover  a  particle  of  mass  less  than  that  of  the  hydrogen
atom.  This  direct  result,  supported  as  it  is  by  many  less  direct
lines  of  evidence,  leads  logically  to  the  conclusion  that  the
hydrogen  positive  ray,  i.e.  the  positively  charged  part  remain-
ing when  an  electron  is  detached  from  a  neutral  hydrogen  atom,
is  the  atom  of  positive  electricity  itself.  The  name  "proton"
was  suggested  for  it  by  Sir  Ernest  Rutherford  at  the  Cardiff
meeting  of  the  British  Association  in  1920.  The  charge  on  a
proton  is,  of  course,  equal  and  of  opposite  sign  to  that  on  the
electron.  Its  mass  in  the  free  state  has  been  measured  directly
and  is  practically  identical  with  that  of  the  neutral  atom  of
hydrogen  1-66  x  10"  ^^  grs.,  or  1-007  on  the  oxygen  scale.
 
===88.  The  Nucleus  Atom===
Certain  experimental  results,
notably  the  scattering  of  alpha  rays,  led  Sir  [[wikipedia:Ernest  Rutherford| Ernest  Rutherford]]
in  1911*  to  formulate  an  atom  model  which  has  resulted  in  the
most  remarkable  advances  in  both  physics  and  chemistry,  and
is  now  almost  universally  accepted  as  correct  in  fundamental
principle.  This  is  that  an  atom  of  matter  consists  of  a  central
massive  nucleus  carrying  a  positive  charge  which  is  surrounded,
at  distances  relatively  great  compared  with  its  diameter,  by
"planetary"  electrons.  The  central  nucleus  contains  aU  the
positive  electricity  in  the  atom,  and  therefore  practically  all  its
mass.  The  weight  of  the  atom  and  its  radioactive  properties
are  associated  with  the  nucleus;  its  chemical  properties  and
 
^  From  Greek  Trpwrov  first  the  primary  substance.
 
=C2=BB P.  67.  '  V.  p.  105.
 
*  Rutherford,  Phil.  Mag.  41,  669,    1911.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER        93
 
spectrum,  on  the  other  hand,  are  properties  of  its  planetary
electrons.  It  is  clear  that  in  a  neutral  atom  the  positive  charg=
e
on  the  nucleus  must  be  equal  to  the  sum  of  the  negative  charges=
 
on  the  planetary  electrons.
 
===89.  Moseley's  Atomic  Numbers===
The  scattering  experiments mentioned  above  indicated  that  the  net  positive  charge
on  the  nucleus  (expressed  in  terms  of  the  natural  unit  e)  was
roughly  equal  to  half  the  atomic  weight.  Now  if  we  arrange  the
elements  in  order  of  atomic  weight,  starting  with  hydrogen,  each
element  will  have  a  position  the  number  of  which  will  be  about
half  its  atomic  weight.  It  was  suggested  by  Van  den  Broek
that  this  atomic  number  might  be  equal  to  the  charge  on  the
nucleus.  Two  years  after  the  formulation  of  the  nucleus  atom
theory  Moseley  undertook  an  investigation  of  the  changes  which
took  place  in  the  wave-length  of  the  X-rays  given  off  when
various  elements  were  used  in  turn  as  anticathodes.  The  result
of  this  piece  of  work,  now  classical,^  was  the  estabhshment  of  the
most  important  generaUsation  in  the  history  of  chemistry  since
Mendeleef 's  Periodic  Law.  Discussing  a  quantity  Q  related  to
wave-length  Moseley  writes:
 
"It  is  at  once  evident  that  Q  increases  by  a  constant  amount
as  we  pass  from  one  element  to  the  next,  using  the  chemical
order  of  the  elements  in  the  periodic  system.  Except  in  the
case  of  Nickel  and  Cobalt ,2  this  is  also  the  order  of  the  atomic
weights.  While,  however,  Q  increases  uniformly  the  atomic
weights  vary  in  an  apparently  arbitrary  manner,  so  that  an
exception  in  their  order  does  not  come  as  a  surprise.  We  have
here  a  proof  that  there  is  in  the  atom  a  fundamental  quantity,
which  increases  by  regular  steps  as  we  pass  from  one  element  to
the  next.  This  quantity  can  only  be  the  charge  on  the  central
positive  nucleus,  of  the  existence  of  which  we  already  have
definite  proof.  Rutherford  has  shown,  from  the  magnitude  of
the  scattering  of  a  particles  by  matter,  that  the  nucleus  carries
a
positive  charge  approximately  equal  to  that  of  A/2  electrons
when  A  is  the  atomic  weight.  Barkla,  from  the  scattering  of
X-rays  by  matter,  has  shown  that  the  number  of  electrons  in  an=
 
 
1  Moseley,  Phil.  Mag.,  26,  1031,    1913.
=C2=BBCf.  Barkla,  Phil.  Mag.,  14,  408,    1907.
 
 
94  ISOTOPES
 
atom  is  roughly  A/2,  which  for  an  electrically  neutral  atom  is
the  same  thing.  Now  the  atomic  weights  increase  on  the
average  by  about  2  units  at  a  time,  and  strongly  suggest  the
view  that  N  increases  from  atom  to  atom  always  by  a  single
electronic  unit.  We  are  therefore  led  by  experiment  to  the
view  that  N  is  the  same  as  the  number  of  the  place  occupied  by=
 
the  element  in  the  periodic  system.  This  atomic  number  is
then  for  H  1,  for  He  2,  for  Li  3 for  Ca  20 for
Zn  30,  etc.
This  theory  was  originated  by  Broek  ^  and  since  used  by  Bohr.
We  can  confidently  predict  that  in  the  few  cases  in  which  the
order  of  the  atomic  weights  A  clashes  with  the  chemical  order  of
the  periodic  system  the  chemical  properties  are  governed  by  N,
while  A  itself  is  probably  some  compHcated  function  of  N."
 
Subsequent  work  has  supported  in  an  unquestionable  manner
the  ideas  so  expressed  by  Moseley.  That  the  number  of  the
element  in  the  order  of  the  periodic  table  is  actually  the  same as
the  positive  charge  on  the  nuclei  of  its  atoms,  expressed  of
course  in  terms  of  the  natural  unit  of  electric  charge  e,  has  been
proved  by  direct  experiment  for  some  of  the  heavier  elements.
The  recent  work  of  Chadwick^  leaves  little  room  for  doubt  on
that  point.  At  the  other  end  of  the  scale  all  the  known  pro-
perties of  hydrogen  point  to  the  conclusion  that  its  atomic
number  is  1 ;  its  exceptional  atomic  weight,  as  will  be  seen  later,
is  itself  strong  corroborative  evidence  of  this.
 
A  complete  table  of  the  elements  with  their  Atomic  Numbers,
Atomic  Weights  and  isotopes  (where  these  are  known)  is  given
on  page  142.
 
From  Moseley's  law  of  atomic  numbers  the  explanation  of  the
empirical  rule  of  radioactive  transformation  given  on  page  11
follows  at  once.  An  alpha  particle  carries  two  positive  charges,
a  beta  particle  one  negative  one.  If  therefore  the  atom  of
a  radioactive  substance  emits  one  alpha  particle  from  its
nucleus  it  naturally  descends  two  units  in  atomic  number,  that
is  moves  back  two  places  in  the  periodic  table.  If  on  the  other
hand  it  emits  one  beta  particle  it  clearly  moves  forward  one
place,  for  by  the  operation  the  nucleus  has  acquired  one
additional  charge.
 
1  Van  den  Brock,  Phys.  Zeit.  14,  33,    1913.
'Chadwick,  Phil.  Mag.,l40,  734,    1920.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER        95
 
===90.  The  Bohr  Atom===
In  this  atom  model  the  electrons
outside  the  nucleus  are  supposed  to  be  in  a  state  of  continual
revolution  about  it,  like  planets  round  the  sun.  This  rotation
is  considered  to  take  place  in  orbits  defined  in  a  very  special
manner  by  means  of  a  "  quantum  relation."  This  postulates
that  when,  and  only  when,  an  electron  changes  its  orbit,
radiation  is  given  out  and  the  energy  acquired  by  the  change  of
orbit  is  entirely  given  off  as  radiation  of  frequency  v  where  the
change  of  energy  equals  hv  where  h  is  Planck's  quantum  or
element  of  action  (6-55  x  10"  ^^  C.G.S.).^  This  theory  lends  itself
to  exact  mathematical  analysis  but  unfortunately  it  can  only  be
worked  out  adequately  for  the  two  simplest  cases,  the  neutral
hydrogen  atom  and  the  singly  charged  heUum  atom.  Here,
however, its  success  is  most  remarkable;  for  not  only  is  itpossible
to  calculate  by  its  means  the  wave  length  of  the  chief  series  lines
of  the  hydrogen  spectrum,  to  an  accuracy  almost  unprecedented
in  physics,  but,  by  applying  the  relativity  correction  for  change
of  mass  with  velocity  to  the  rotating  electron,  the  fine  structure
of  the  fines  and  the  effects  of  electric  and  magnetic  fields  have
been  predicted  with  the  most  astonishing  exactness  by  Sommerf eld,  Epstein  and  others.  Bohr  has  recently  expressed  the  hope
of  extending  his  theory  to  heavier  atoms  by  means  of  a  new
device  which  he  terms  the  principle  of  "correspondence."^
 
===91.  The  Lewis  Langmuir  Atom===
This  form  of  atom
model  was  primarily  designed  to  afford  some  theoretical  basis
for  the  numerous  general  quahtative  properties  of  elements  and
their  compounds.  In  it  the  electrons  outside  the  nucleus  are
supposed  to  be  at  rest  at,  or  vibrating  about,  definite  points.
The  first  two  electrons  wiU  form  a  pair,  the  next  eight  will  tend
to  set  in  positions  corresponding  to  the  eight  corners  of  a  cube,
or  some  other  soUd  figure,  and  so  on.  In  this  way  we  shall  get a
series  of  shells  or  sheaths  one  outside  the  other.  Langmuir  has
recently  *  reduced  his  postulates  to  the  following  three:
 
(1)  The  electrons  in  atoms  tend  to  surround  the  nucleus  in
 
iBohr,  Phil  Mag.  36,  1,  476,  857,    1913.
 
2  Sommerfeld,  Atombau  and  SpektralUnien,  Brunschweig,  1921.
 
3  Bohr,  Nature,  107,  104,    1921.
 
*  Langmuir,  Brit.  Assoc.  Edinburgh  meeting,  1921.
 
 
96  ISOTOPES
 
successive  layers  containing  2,  8,  8,  18,  18,  32  electrons  re-
spectively.
 
(2)  The  atoms  may  be  coupled  together  by  one  or  more
"  duplets  "  held  in  common  by  the  complete  sheaths  of  the
atoms.
 
(3)  The  residual  charge  on  the  atom  and  on  each  group  of
atoms  tends  to  a  minimum.
 
This  atom  model  is  not  amenable  to  mathematical  treatment,
but  it  has  been  exceedingly  successful  in  accounting  for  the
general  chemical  qualitative  properties  of  many  of  the  elements
and  in  predicting  those  of  their  compounds.
 
===92.  Diagrammatical  representation  of  atoms  of Isotopes  and  Isobares===
The  accompanying  diagrams  (Fig.
16)  are  intended  to  indicate  the  sort  of  arrangements  which
may  take  place  in  atoms.  The  smaU  dark  circle  is  the  nucleus,
the  number  of  protons  and  electrons  comprising  it  being  indi-
cated by  the  numerals.  The  electrons  outside  the  nucleus  are
indicated  by  small  Light  circles.
 
(1)  is  an  atom  of  atomic  weight  6.  Its  nucleus  contains  6
protons  and  3  electrons,  hence  its  atomic  number  is  3.  It  is  in=
 
fact  the  atom  of  the  lighter  isotope  of  lithium  of  atomic  weight=
 
6,  Li^.  To  be  electrically  neutral  it  must  have  3  electrons  out-=
 
side the  nucleus.  Now  the  principles  underlying  Langmuir's first
postulate  are  derived  from  the  Periodic  Table  and  are  certainly
correct.  Langmuir  explains  this  by  saying  that  the  first  two  of=
 
these  electrons  will  form  an  innermost  ring  or  shell  of  two.
This  sheU  being  now  complete,  any  more  electrons  will  go  out-
side and  start  the  next  shell  of  eight,  so  we  indicate  this  by=
 
putting  the  third  electron  in  a  circle  of  greater  diameter.
 
Now  suppose  we  add  one  electron  and  one  proton  to  this  atom.
If  both  enter  the  nucleus  we  shaU  get  the  configuration  repre-
sented by  (2)  The  nuclear  charge  is  unaltered,  so  that  the
arrangement  of  the  exterior  electrons  will  be  precisely  the  same-=
 
It  follows  that  all  properties  depending  on  these  electrons  such=
 
as  atomic  volume,  spectrum,  chemical  properties,  etc.,  will  be
quite  unaltered.  But  the  weight  of  this  atom  is  now  7,  so  it
is  an  isotope  of  lithium  ; '  it  is  actually  the  atom  of  the  he=
avier
constituent  Li^    (1)  and  (2)  are  Isotopes.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER        97
 
But  now  suppose  we  add  a  proton  and  an  electron  to  (1)  so
that  the  proton  only  enters  the  nucleus  and  the  electron  remains=
 
outside  as  shown  at  (3)  We  shall  now  have  a  charge  4  on  the
nucleus  and  two  electrons  in  the  outer  ring.    The  chemical
 
 
0)  Atom  of  Li^
 
CZ)  Atom  of  Li^
 
(3)  Atom  of  hypothetical  isotope  of  Beryllium
 
(4--)  Atom  of  F
 
(5)  Atom  of  Ne^=C2=B0
 
re)  Atom  of  Ne^^
 
(7)  Atom  of  Na        O  =3D  Electron
 
Fig.  16.  Diagrammatic  Representation  of  Nucleus  Atoms.  The =
planetary
electrons  are  shown  as  lying  on  plane  circles,  the  first  containi=
ng  2,  the
second  8  and  so  on.  The  dark  circle  is  the  nucleus  and  the  +  =
and 
charges  within  it  are  indicated  by  figures.  (1)  and  (2)  are  Isot=
opes.  (2)
and  (3)  are  Isobares  and  (5)  and  (6)  are  Isotopes.
 
 
properties  of  such  an  atom,  if  it  could  exist,  would  be  completely
different  from  those  of  Uthium,  but  would  be  identical  with  those
of  beryUium,  of  which  it  would  be  an  isotope.  But  its  mass  is
clearly  identically  the  same  as  that  of  (2)  so  that  (2)  and  (3)
ARE  Isobares.
 
H
 
 
98  ISOTOPES
 
In  the  same  way  (4)  will  be  recognised  as  the  atom  of  fluorine=
 
(5)  and  (6)  as  the  atoms  of  the  two  isotopes  of  neon  and  (7)  =
as
the  atom  of  sodium.
 
===93.  The  relation  between  Isotopes  and  Elements  in  the same  Group===
As  far  as  can  be  seen  the  chemical  properties  by
which  the  elements  are  divided  into  groups  depend  practically
entirely  on  the  outermost  shell  of  electrons,  which  are  therefore
called  valency  electrons.  Now  consider  all  that  part  called  by
Langmuir  the  kernel  of  the  atom  lying  within  the  shell  of  these
valency  electrons.  The  movements  or  configuration  of  the
outermost  electrons  will  depend  in  the  first  degree  on  the  charge
on  the  kernel,  which  may  be  looked  upon  as  a  virtual  nucleus.
The  kernels  of  atoms  (1)  and  (7)  both  have  the  same  net  charge
1,  so  that  the  elements  they  represent  should  have  many  chemi-
cal similarities.  These  they  certainly  have  as  both  are  alkaU
metals.  In  general  the  atoms  of  elements  belonging  to  the
same  group  chemically  have  the  same  number  of  electrons  in
the  outer  shell  and  the  same  net  charge  on  the  kernel.  On  this
view  it  will  be  seen  that  the  similarity  of  isotopes  may  be
regarded  as  the  extreme  limiting  case  of  the  similarities  long
observed  between  elements  of  the  same  chemical  group.
 
===94.  Abnormal  compounds  formed  by  charged  atoms===
 
The  tendency  of  elements  to  form  compounds  with  each  other,
that  is  the  property  with  which  the  idea  of  valency  is  associated,
is  ascribed  to  the  tendency  of  the  atom  to  complete  its  outer
shell.  This  it  can  do  either  by  parting  with  the  electrons  in
this  sheU  and  so  promoting  the  next  inner  completed  shell  to
outer  position,  or  by  sharing  the  electrons  in  the  atom  of
another  element  so  that  they  fill  the  gaps  in  its  own  outer  shell.
We  have  already  alluded  to  the  success  which  has  attended
this  idea  in  explaining  valency  and  the  properties  of  chemical
compounds.  For  the  present  argument  it  will  be  enough  if
it  is  understood  that  lithium  and  sodium  (1)  (2)  (7)  will  very
readily  part  with  their  solitary  valency  electron  and  become
positively  charged,  i.e.,  will  be  strongly  electropositive  elements
with  valency  +  1  ;  whereas  a  fluorine  (4)  wiU  have  an  equally
powerful  tendency  to  take  up  an  electron  and  become  nega-
tively  charged    and    so    will  be    a    strongly    electronegative
 
 
THE  ELECTRICAL  THEORY  OF  MATTER        99
 
element  with  valency    1,  Both  of  these  tendencies  will  b=
e
satisfied  if  (4)  and  (7)  combine  forming  the  compound  molecule
NaF,  for  the  outer  electron  of  (7)  will  enter  the  outer  shell  o=
f
(4)  thus  forming  two  complete  shells  of  eight  (Langmuir's
octets).  We  trace  the  tendency  of  the  atom  of  fluorine,  or  any=
 
other  halogen,  to  form  compounds  with  the  atom  of  an  electro-
positive element,  or  with  the  atom  of  hydrogen,  to  the  fact
that  it  has  one  too  few  electrons  in  its  outer  shell.
 
Now  the  only  way  we  can  give  a  positive  charge  to  an  atom
of  neon  (5)  or  (6)  is  by  knocking  one  or  more  electrons  out  of=
 
its  outer  shell.  Suppose  we  remove  one  from  (5)  as  indicated
by  the  dotted  line.  (5)  now  will  have  a  similar  outer  shell  to=
 
(4)  and  a  valency    1,  so  we  may  expect  that  atoms  of  =
the
inert  gases  carrying  a  single  positive  charge  will  behave
chemically  in  a  similar  manner  to  neutral  halogen  atoms  and
will  therefore  be  capable  so  long  as  they  are  charged  of  forming=
 
hydrides.  This  very  important  idea  was  first  suggested  by
Sir  J.  J.  Thomson  in  connection  with  the  charged  atoms  of
chlorine  ^  and  certainly  supplies  a  very  satisfactory  explana-
tion of  the  very  abnormal  hydrides  of  inert  gases  and  com-
pounds such  as  OH3  discovered  in  positive  rays.  The  line
at  41  (Spectrum  VI,  Plate  III)  is  probably  to  be  put  down
to  a  charged  hydride  of  argon  of  this  type.  Exceedingly
faint  lines  at  5  in  the  case  of  hehum,  and  21  in  the  case  of=
 
neon,  are  probably  to  be  ascribed  to  similar  abnormal  compound
(HeH)  and  (NeH)  respectively.  In  the  case  of  atoms  carrying
more  than  one  charge  it  can  be  generally  stated  that  each
positive  charge  given  to  an  atom  wiU  increase  its  negative
valency  by  one.
 
===95.  The  failure  of  the  additive  law  in  respect  to  mass===
 
We  have  seen  that,  for  velocities  smaU  compared  with  that  of
light,  the  masses  of  the  proton  and  the  electron  may  be
regarded  as  universal  constants.  If  the  additive  law  were
strictly  true  as  regards  the  summation  of  their  masses  it  is
clear  that  any  mass  whatever,  whether  it  were  that  of  an  atom
or  a  molecule,  a  planet  or  a  star,  or  even  the  universe  itself=
 
could  be  expressed  in  the  form  NM  where  N  is  a  pure  integer
 
1  J.  J.  Thomson,  Proc.  Roy.  Soc.  99A,  90,    1921,
 
 
100  ISOTOPES
 
and  M  the  mass  of  the  neutral  system  1  proton  +  1  electron
( =3D  the  atom  of  hydrogen).  The  simplicity  of  this  idea,  which=
 
is  Prout's  theory  in  the  language  of  modern  physics,  is  extremely
attractive  ;  but  we  know  it  to  be  false,  for  although  the
discovery  of  isotopes  has  removed  the  difficulty  of  the  grosser
fractions  associated  with  such  elements  as  neon  and  chlorine,
we  are  still  left  with  the  more  minute  but  none  the  less  real
one  associated  with  hydrogen  itself.  To  explain  this  the
additive  law  must  be  qualified  by  some  such  reasoning  as  is
contained  in  the  following  paragraph.
 
===96.  The  explanation  of  the  fractional  mass  of  the hydrogen    atom    by the    hypothesis    of    ''packing" ===
 
According  to  generally  accepted  views  the  proton  and  the
electron  possess  mass,  or  what  on  the  relativity  theory  is
regarded  as  the  same  thing,  weight,  by  virtue  of  the  energy
in  the    electromagnetical  field    which    surrounds  them. It
can  easily  be  shown  on  classical  lines  that  if  we  give  it  a
spherical  form  a  charge  e  spread  uniformly  over  the  surface
of  the  sphere  will  have  a  mass  m  when  its  radius  a  is  such  th=
at
 
2    e^
m  =3D    ;  hence  to  give  the  electron  its  proper=
  mass  its  charge
 
must  be  compressed  to  a  sphere  of  diameter  about  3-8  x  lO"  ^^=
cm.  By  the  same  argument  the  proton  will  be  nearly  two
thousand  times  smaller  and  have  a  diameter  2-06  x  10"  ^^  cm.
The  extreme  range  of  the  diameter  of  atoms  themselves  is
1    5  X  10~^  cm.,  so  that  it  will  at  once  be  realised =
that  the
structure  of  an  atom  is  an  exceedingly  open  one,  even  more
so  than  that  of  our  solar  system.
 
Now  it  can  be  shown  that  if  we  bring  two  charges  of  opposite
sign  sufficiently  close  together  their  fields  will  aflfect  each
other  in  such  a  way  that  the  mass  of  the  system  will  be  reduced.
This  effect  is  quite  inappreciable  for  distances  comparable
with  the  diameter  of  an  atom,  but  begins  to  make  itself  felt
when  the  distance  apart  is  of  the  order  of  the  size  of  the  ele=
ctron
itself  as  given  above.  The  nucleus  of  the  atom  of  an  ordinary
element  (not  hydrogen)  contains  both  protons  and  electrons
and  is  very  small  compared  with  the  atom  itself.  Its  dimen-
BJons  can  be  roughly  determined  by  actual  experiment  in  the
 
 
THE  ELECTRICAL  THEORY  OF  MATTER      101
 
case  of  the  heavy  elements  and  are  found  to  be  so  small  that
even  to  get  in  the  electrons  alone  these  would  have  to  be
packed  very  closely  together.  Such  a  nucleus  wiU  contain
more  protons  than  electrons,  roughly  twice  as  many,  so  that
it  may  be  regarded  as  practically  certain  that :  In  the  =
nuclei
of  normal  atoms  the  packing  of  the  electrons  and  protons  is  so=
 
close  that  the  additive  law  of  mass  will  not  hold  and  the  mass =
of
the  nucleus  will  be  less  than  the  sum  of  the  masses  of  its  con=
-
stituent charges.
 
The  nucleus  of  a  hydrogen  atom  consists  of  one  single  free
proton,  its  planetary  electron  is  too  far  away  to  cause  any
effect  so  that  it  is  clear  that  we  shaU  find  the  mass  associate=
d
with  the  atom  of  hydrogen  greater  than  one -fourth  the  mass
of  a  hehum  atom  or  one-sixteenth  the  mass  of  an  oxygen
atom.  The  mass  lost  when  four  free  protons  and  two  free
electrons  are  packed  close  together  to  form  a  helium  nucleus
(No.  9,  p.  106)  is  roughly  0-7  per  cent,  of  the  whole  and  it  =
can
be  calculated  that,  if  we  take  the  value  of  the  diameter  of
the  electron  given  above,  the  protons  must  approach  nearer
than  half  of  this  to  give  so  large  a  reduction.  This  means  that=
 
the  charges  must  be  so  closely  packed  that  the  electrons  are
actually  deformed.
 
The  whole  number  rule  may  now  be  simply  translated  into
a  statement  that  the  mean  packing  effect  in  all  atoms  is
approximately  constant,  and  the  unit  of  mass  1  when  0=3D16
wiU  be  (mass  of  a  packed  proton)  -f  |  (mass  of  free  electron)=
 
+  I  (mass  of  packed  electron).  The  whole  number  rule  is  not,
and  never  was  supposed  to  be,  mathematically  exact,  for  this
would  imply  an  identical  packing  effect  in  the  case  of  all
atoms,  an  exceedingly  improbable  supposition.  It  is  almost
certain  that  atoms  of  some  elements,  such  as  nitrogen,  weigh
slightly  more  than  a  whole  number  (looser  packing)  while
those  of  others  such  as  caesium  or  iodine  may  weigh
slightly  less  (closer  packing).  The  Umit  of  accuracy  so  far
attained  in  mass-spectrum  measurement  is  not  sufficient  to
detect  a  change  of  the  order  expected,  except  in  the  case  of
hydrogen,  where  the  variation  in  mass  is  exceptionally  high.
 
===97. The  structure  of  the  nucleus===
The  manner  in  which
 
 
102  ISOTOPES
 
the  units  of  electricity  are  arranged  in  the  nucleus  of  an  atom
has  received  a  good  deal  of  attention  from  theorists  but  ideas
on  this  subject  are  almost  entirely  of  a  conjectural  character.
Thus  Harkins  ^  has  proposed  a  constitutional  formula  for  the
nuclei  of  all  the  elements.  In  this,  besides  electrons  and
protons,  he  uses  as  building  units  a  particles  (4  protons  +  2
electrons)  of  mass  4,  and  hypothetical  units  of  mass  3  with  a
single  positive  charge  (3  protons  +  2  electrons).  The  matter
has  been  more  recently  discussed  and  nucleus  models  sug-
gested by  Rutherford, 2  E.  Gehrcke  ^  and  others.
 
The  fact  that  the  helium  nucleus  is  almost  exactly  an
integer  on  the  oxygen  scale  that  is  to  say  helium  has  a=
pproxi-
mately normal  packing  gives  a  distinct  balance  of  probabili=
ty
that  helium  nuclei  actually  exist  as  such  in  the  nuclei  of  norma=
l
elements.  In  support  of  this  idea  it  has  been  stated  that  the
presence  of  heUum  nuclei  inside  the  nuclei  of  radioactive
atoms  is  definitely  proved  by  the  ejection  of  a  particles  by
the  latter.  In  the  writer's  opinion  this  is  much  the  same  as
saying  that  a  pistol  contains  smoke,  for  it  is  quite  possible
that  the  a  particle,  Hke  the  smoke  of  the  pistol,  is  only  forme=
d
at  the  moment  of  its  ejection.  Brosslera  *  defends  this  view
and  points  out  that  if  the  alteration  from  looser  to  closer
packing  of  the  charges  forming  the  particle  is  at  all  large
energy  will  be  liberated  amply  sufficient  for  the  purpose  of
detaching  it  and  giving  it  the  energy  of  an  a  ray.  The
reason  to  expect  that  this  energy  will  be  set  free  will  be
described  in  the  next  section.  Brosslera's  suggestion  that
in  the  nuclei  of  radioactive  atoms  there  are  loosely  bound
protons  and  electrons  and  that  these,  given  something  of  the
nature  of  a  certain  exact  and  instantaneous  correlation,
might  combine  to  form  an  a  particle  is  in  good  accordance
with  the  most  reasonable  theory  of  radioactive  disintegration,
which  was  first  put  forward  by  Lindemann.^
 
There  are  therefore  two  different  ideas  which  we  may  regard
 
1  Harkins,  Phys.  Rev.,  15,  73,    1920.
 
2  Rutherford,  Proc.  Roy.  Soc,  97A,  374,    1920.
 
3  Gehrcke,  Phys.  ZeiL,  22,  151,  1921.
 
*  Brosslera,  Rev.  Chwi.,  1,  42,  74,    1921.
=C2=BB  Lindemann,  Phil.  Mag.,  30,  560,    1915.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER      103
 
as  working  hypotheses.  According  to  the  first  the  nuclei  of
atoms  consist  of  helium  nuclei,  or  a  particles,  held  together
in  some  way  so  that  their  packing  effect  wpon  each  other  is
small ;  and,  in  the  case  of  atoms  not  having  a  mass  of  the
type  4w,  additional  protons  and  electrons.  According  to
the  second  we  only  have  to  suppose  that  the  mean  packing
of  all  the  charges  in  the  nucleus  is  such  as  will  account  for  =
the
whole  number  rule  with  sufficient  exactness,  but  that  the
actual  arrangement  of  the  protons  and  electrons  need  not
necessarily  be  at  aU  similar  to  that  in  a  helium  nucleus.
 
The  experimental  evidence  is,  so  far,  definitely  in  favour
of  the  first  of  these  views.  In  their  remarkable  work  on  the
disintegration  of  light  atoms  by  the  collision  of  swift  a  rays
Rutherford  and  Chadwick  i  show  that  as  the  result  of  such
collisions  swift  hydrogen  rays,  i.e.  free  protons,  are  liberated
from  the  atoms  of  boron,  nitrogen,  fluorine,  sodium,  aluminium
and  phosphorus.  They  point  out  that  the  masses  of  the
atoms  of  aU  these  elements  are  of  the  types  4w  +  2  and
4w  -|-  3.  The  effect  is  not  obtained  from  atoms  of  the  type
471  so  that  this  result  suggests  that  in  these  the  protons  are=
 
already  all  bound  together  to  form  helium  nuclei.
 
===98.  Cosmical  effects  due  to  change  of  mass===
It  has
long  been  known  that  the  chemical  atomic  weight  of  hydrogen
was  greater  than  one  quarter  of  that  of  helium,  but  so  long  as
fractional  weights  were  general  there  was  no  particular  need
to  explain  this  fact,  nor  could  any  definite  conclusions  be
drawn  from  it.  The  results  obtained  by  means  of  the  mass-
spectrograph  2  remove  all  doubt  on  this  point,  and  no  matter
whether  the  explanation  is  to  be  ascribed  to  pacldng  or  not,
we  may  consider  it  absolutely  certain  that  if  hydrogen  is
transformed  into  helium  a  certain  quantity  of  mass  must  be
annihilated  in  the  process.  The  cosmical  importance  of  this
conclusion  is  profound  and  the  possibilities  it  opens  for  the
future  very  remarkable,  greater  in  fact  than  any  suggested
before  by  science  in  the  whole  history  of  the  human  race.
We  know  from  Einstein's  Theory  of  Relativity  that  mass
 
1  Rutherford  and  Chadwick,  Phil.  Mag.,  42,  809,    1921.
^  V.  p.  70.
 
 
104  ISOTOPES
 
and  energy  are  interchangeable  i  and  that  in  C.G.S.  units  a
mass  m  at  rest  may  be  expressed  as  a  quantity  of  energy
mc"^,  where  c  is  the  velocity  of  light.  Even  in  the  case  of  th=
e
smallest  mass  this  energy  is  enormous.  The  loss  of  mass  when
a  single  helium  nucleus  is  formed  from  free  protons  and
electrons  amounts  in  energy  to  that  acquired  by  a  charge  e
falling  through  a  potential  of  nearly  thirty  million  volts.  A
swift  a  ray  has  an  energy  of  three  to  four  million  volts  so  th=
at
the  change  of  packing  suggested  by  Brosslera  need  not  be
nearly  so  great  to  provide  the  energy  needed.  If  instead  of
considering  single  atoms  we  deal  with  quantities  of  matter
in  ordinary  experience  the  figures  for  the  energy  become
prodigious.
 
Take  the  case  of  one  gramme  atom  of  hydrogen,  that  is  to
say  the  quantity  of  hydrogen  in  9  c.c.  of  water.  If  this  is
entirely  transformed  into  helium  the  energy  liberated  will  be
 
0077  X  9  X  1020  =3D  6-93  X  lO^^  ergs.
 
Expressed  in  terms  of  heat  this  is  1-66  x  10^^  calories  or  in=
 
terms  of  work  200,000  kilowatt  hours.  We  have  here  at  last
a  source  of  energy  sufficient  to  account  for  the  heat  of  the
Sun.2  In  this  connection  Eddington  remarks  that  if  only
10  per  cent,  of  the  total  hydrogen  on  the  Sun  were  trans-
formed into  helium  enough  energy  would  be  liberated  to  main-
tain its  present  radiation  for  a  thousand  million  years.
 
Should  the  research  worker  of  the  future  discover  some
means  of  releasing  this  energy  in  a  form  which  could  be
employed,  the  human  race  will  have  at  its  command  powers
beyond  the  dreams  of  scientific  fiction ;  but  the  remote
possibility  must  always  be  considered  that  the  energy  once
liberated  will  be  completely  uncontrollable  and  by  its  intense
violence  detonate  all  neighbouring  substances.  In  this  event
the  whole  of  the  hydrogen  on  the  earth  might  be  transformed
at  once  and  the  success  of  the  experiment  published  at  large
to  the  universe  as  a  new  star.
 
^Eddington,  Time,  Space  and  Gravitation,  p.  146,  Cambridge,  1920.=
 
Eddington,  Brit.  Assoc,  address,  1920  ;    Perrin,  Scientia,=
  Nov.,  1921.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER      105
 
===99.  The  stable  systems  of  protons  and  electrons known  to  occur===.  Starting  with  our  standard  bricks,  the
protons  and  electrons,  we  may  make,  theoretically  at  least,
an  infinity  of  systems  by  the  combination  of  any  number  of
each.  It  is  interesting  to  consider  the  systems  actually  occur-
ring in  practice,  that  is  to  say  those  which  are  sufficiently  sta=
ble
to  give  definite  evidence  of  their  existence.  The  follow-
ing table  gives,  in  order  of  mass,  the  first  twenty-four  known.
Where  the  circles  representing  the  charges  touch  each  other,
to  form  nuclei,  the  packing  is  extremely  close,  where  they  do
not  touch  they  are  to  be  taken  as  distant  thousands  of  times
further  from  each  other.  The  masses  of  the  first  twelve  are
deduced  as  follows  :  The  most  accurate  value  for  the  che=
mical
atomic  weight  of  hydrogen  is  1-0077  (0  =3D  16),  and  as  it  is  v=
ery
improbable  that  it  consists  of  isotopes  we  take  this  as  the
mass  of  a  neutral  hydrogen  atom.  The  mass  of  the  electron
is  0*00054  and  as  the  packing  effect  is  nil  we  arrive  at  the
figure  1-0072  for  the  mass  of  the  proton,  and  this  agrees  within=
 
the  experimental  error  with  that  directly  determined  by  the
mass-spectrograph.  The  most  probable  value  of  the  mass  of
a  neutral  helium  atom  is  4-00(0)  we  wiU  assume  the  last  figure=
 
for  the  sake  of  simplicity.  The  masses  of  (13)  to  (24)  are  less=
 
accurately  known.
 
The  stability,  where  known,  is  expressed  in  volts  and
represents  the  potential  through  which  a  charge  e  must  faU
in  order  to  acquire  sufficient  energy  to  disrupt  the  particular
configuration  concerned.  This  is  the  ionisation  potential  in
the  case  of  atoms.
 
 
106
 
 
ISOTOPES
 
 
^:s
 
 
i-e    =C2=A3
 
 
Nuclear
institution.
 
6
 
r
 
a
 
i^
 
O
 
0
 
-1
 
0-00054
 
1  +
 
+1
 
1-0072
 
1  +
 
0
 
1-0077
 
1  +
 
-1
 
1-0082
 
1  +
 
+1
 
2-0149
 
1  +
 
0
 
2-0154
 
1  +
 
+1
 
3-0226
 
1  +
 
0
 
3-0231
 
4+2-
 
+2
 
3-999
 
4+2-
 
+  1
 
3-999
 
4+2-
 
0
 
4-000
 
Description.
 
 
o
 
 
o
 
 
o        o
 
 
    o   
 
 
o        o
 
 
    o        o
 
 
  o      o      o
 
 
10
 
 
11
 
 
o
 
 
O  CO  o
 
 
14
 
 
4-3
 
 
small
 
 
small
 
 
>3x  106
 
 
55
 
 
25
 
 
Electron
 
 
Proton      or
positively
charged  H
atom
 
Neutral      H
atom
 
Negatively-
charged  H
atom
 
Positively
charged  H
molecule
 
Neutral    Hg
molecule
 
Positively
charged
H3
 
Neutral  H,
 
 
Doubly
charged
heUum
atom      or
alpha  ray
 
Singly
charged
helium
atom
 
Neutral
helium
atom.
 
 
THE  ELECTRICAL  THEORY  OF  MATTER      107
 
 
^i
 
a  3
 
sl
 
>>
 
o
 
=C2=A71
^1
 
II
 
S
 
Description.
 
12
 
m
 
lo.
 
2
 
4+2-
 
+  1
 
5-007
 
Positively
charged
HeH
 
13
 
=C2=B0l
 
%
 
3
 
6  +  3-
 
+  1
 
6-0
 
Positively
charged
Li8  atom
 
14
 
J
 
io  o
 
3
 
6  +  3-
 
0
 
6-0
 
4-9*
 
Neutral  Li''
atom
 
15
 
6\
 
i
 
^o
 
3
 
7+4-
 
+  1
 
7-0
 
Positively
charged
Li'  atom
 
16
 
M
 
^  0    o
 
3
 
7+4-
 
0
 
7-0
 
4-9*
 
Neutral  Li'
atom
 
17
 
O    O  J
 
3
 
6+3-
 
0
 
6-0(07)
 
Neutral
 
Li^H
 
{
 
l^k      0
 
molecule
 
18
 
O  O  j
 
^^ 
 
3
 
7+4-
 
0
 
7-0(07)
 
Neutral
 
{
 
Li'H
 
#04
 
IC#
 
molecule
 
19
 
=C2=B08
 
<^o  o
 
4
 
9  +  5-
 
+1
 
9-0
 
Positively
charged
Be  atom
 
20
 
4
 
9+5-
 
0
 
9-0
 
3-3*
 
Neutral    Be
atom
 
21
 
5
 
10  +  5-
 
+2
 
10-00
 
Doubly
charged
B^o  atom
 
22
 
oo|
 
^      o
)#0=C2=A9      o
 
5
 
10  +  5-
 
0
 
10-00
 
Positively
charged
B^  atom
 
23
 
o  ot
 
1
 
5
 
10  +  5-    0
 
10-00
 
Neutral    B^"
 
-
 
atom
 
24
 
=C2=B0^
 
3=C2=B0  =C2=B0
 
5
 
11+6-
 
+2
 
11-00
 
Doubly
charged
B"  atom
 
*  Calculated  from  frequency  of  radiation.


==CHAPTER  IX -  ISOTOPES  AND  ATOMIC  NUMBERS==
==CHAPTER  IX -  ISOTOPES  AND  ATOMIC  NUMBERS==

Revision as of 17:58, 3 July 2025

ISOTOPES[1]

F. W. ASTON, M.A., D.Sc, A.I.C., F.R.S.

Fellow of Trinity College, Cambridge

LONDON

EDWARD. ARNOLD & CO.

1922

[All rights reserved]

Printed in Great Britain

PREFACE

I have undertaken the preparation of this book on isotopes in response to many requests made to me by teachers of physics and chemistry and others working in these subjects that I should publish the results obtained by means of the mass spectrograph in a form more convenient to the public than that in which they first appeared. This is one of the reasons why the space allotted to the inactive isotopes may appear, in the light of the general title of the book, somewhat disproportion- ately large. Another is that the subject of radioactive isotopes really requires a book to itself, and I am in the hope that the inadequacy of my account may stimulate the production of such a volume by hands more competent than mine to deal with this very special and remarkable field of modern science. The logical order of exposition of a scientific subject is to start with the simple and from that build up the more complex. Unfortunately the sequence of events in experimental research is the exact opposite of this so that a compromise must be effected, unless one is content to sacrifice historical treatment altogether. The latter seems very undesirable in a new subject. I have endeavoured in Chapters I, II and IV, and elsewhere when possible, to adhere strictly to the historical order of events even at the cost of some reiteration.

I wish to take this opportunity of expressing my indebted- ness to Mr. C. G. Darwin for his timely criticism and unfailing assistance throughout the work, and also to Mr. R. H. Fowler for help with the proofs. My thanks are also due to Professor Soddy for his diagram of the radioactive isotopes, to Mr. A. J. Dempster for kindly sending me the illustrations of his work, to the proprietors of the Philosophical Magazine and to the Council of the Chemical Society for permission to use the plates and figures of my original papers, and to Messrs. Macmillan & Co., for the diagram of the radioactive trans- formations.

F. W. Aston
Cambridge,
January, 1922.

CONTENTS

Aston 1922/Contents

CHAPTER I - INTRODUCTION

Aston 1922 Chapter 1

CHAPTER II - THE RADIOACTIVE ISOTOPES

Aston 1922/Chapter 2

CHAPTER III - POSITIVE RAYS

Aston 1922/Chapter 3

CHAPTER IV - NEON

Aston 1922/Chapter 4

CHAPTER V - THE MASS-SPECTROGRAPH

Aston 1922/Chapter 5

CHAPTER VI - ANALYSIS OF THE ELEMENTS

Aston 1922/Chapter 6

CHAPTER VII - ANALYSIS OF THE ELEMENTS (Continued)

Aston 1922/Chapter 7

CHAPTER VIII - THE ELECTRICAL THEORY OF MATTER

Aston 1922/Chapter8

CHAPTER IX - ISOTOPES AND ATOMIC NUMBERS

100. The relation between chemical atomic weight and atomic number. Inasmuch as it is now recognised to be in general merely a statistical mean value the importance=

of the chemical atomic weight has been greatly reduced by the discovery of isotopes. Its position as the natural numerical=

constant associated with an element has been taken by the atomic number, though from the point of view of chemical analysis the chemical atomic weight is just as important as it ever was.

The possibility of anomalies in the order of the elements in the periodic table when their chemical atomic weights are considered, is now obvious enough. The true weights of the atoms as directly determined, are so intermingled in the order of the natural numbers and the proportions present in complex elements so varied that such anomalies are bound to occur, indeed it is rather surprising there are not more.

The following table (Fig, 17) shows the masses of the isotopes of three groups of elements now completely investigated. The approximate proportions present are indicated by the heights of the columns  ; plain for the alliali metals, black for the in= ert gases, and hatched for the halogens. The anomalous order of argon and potassium is at once seen to be due to the fact=

that whereas the heavier constituent of argon is present in much the greater proportion, in potassium the reverse is the case. Had the proportions of heavier and Ughter isotopes been similar in each case the atomic weight of potassium would have been greater instead of less than that of argon.

108


ISOTOPES AND ATOMIC NUMBERS


109


s


19


20


21


22 23


Fluorine (9) Neon (10) Sodium (11)


35 36 37 38 39 40 41

(Chlorine 17) Argon (18) Potassium (19)


=C2=A5


1


I-D


78


I

127


79 80 81 82 83 84 85 86 = 

     87

Bromine (35) Krypton (36) Rubidium (37).


I n


W


136


128 129 130 131 132 133 134 135 Iodine (53) Xenon (54) Caesium (55)

Fig. 17. Isotopes of the Halogens, the inert gases and t= he alkali metals.

101. Statistical relations exhibited by elements and their isotopes. -Although our knowledge of true atomic weights is far from complete, for out of eighty-seven existing elements only twenty-seven have been analysed, of which thirteen are simple, interesting relations have already become clear which are stated in the form of rules as follows  : =E2=80= =94

In the nucleus of an atom there is never less than one electron= 

 to

every two protons. There is no known exception to this law. It is the expression of the fact that if an element has an ato= mic number N the atomic weight of its lightest isotope cannot be less than 2N. Worded as above, the exception in the case of hydrogen is avoided. True atomic weights corresponding exactly to 2N are known in the majority of the Ughter elements=

]jp to A^^ Among the heavier elements the difference between


110 ISOTOPES

the weight of the lightest isotope and the value 2N tends to increase with the atomic weight ; in the cases of mercury it amounts to 37 units. The corresponding divergence of the mean atomic weights from the value 2N has of course been noticed from the beginning of the idea of atomic number.

The number of isotopes of an element and their range of atomic weight appear to have definite limits. Since the atomic number only depends on the net positive charge in the nucleus there is no arithmetical reason why an element should not have any number of possible isotopes. An examination of the tables of results given on p. 89 and at the end of the book show that so far the largest number determined with certainty is 6 in the case of krypton. It is possible that xenon has even more, but the majority of complex elements have only two each. The maximum differ- ence between the lightest and heaviest isotope of the same element so far determined is 8 units in the cases of krypton and xenon. The greatest proportional difference, calculated on the lighter weight, is recorded in the case of lithium, where=

it amounts to one-sixth. It is about one-tenth in the case of boron, neon, argon and krypton.

The number of electrons in the nucleus tends to be even. This rule expresses the fact that in the majority of cases even atomic number is associated with even atomic weight and odd with odd. If we consider the three groups of elements, the halogens, the inert gases and the alkah metals, this tendency is very strongly marked. Of the halogens od= d atomic numbers all 6 ( + 1  ?) atomic weights are odd. = Of the inert gases even atomic numbers 13 (+ 2  ?) = are even and 3 odd. Of the alkali metals odd atomic numbers 7= 

 are

odd and 1 even. In the few known cases of elements of the other groups the preponderance, though not so large, is still very marked and nitrogen is the only element yet discovered to consist entirely of atoms whose nuclei contain an odd number=

of electrons.

A further interesting result is the absence of isobares. So far none have been definitely identified, but it is quite obvious=

that in the cases of elements such as calcium and selenium they=

must exist, for the supply of integers in the region of their


ISOTOPES AND ATOMIC NUMBERS HI

atomic weights have been exhausted by the needs of other elements.

A table of the first 40 natural numbers and the true atomic weights corresponding to them is given in Fig. 18. The gaps


H

He

Li

Li

Be

B

B

c

N

O

F

Ne

1

2 3 4^ 5 6 7 8 9 10 11 = 

 12

13 14

15 16 17

18

19 20

Ne

Na

Mg

m

Mg

Al?

Si

Si Si?

P

s

CI

A

CI

K

A

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

39

40

Fig. 18. The first 40 natural numbers, showing those occ= upied by atomic weights of known elements.

are particularly interesting and seem to show no semblance of regularity. It is very clear that many more experimental results will have to be obtained before any satisfactory theory for the occurrence of these, or of the other laws, is to be fo= rmu- lated.

102. The preponderance of elements of even atomic number. In discussing the nuclear structure of elements the question of their relative abundance in nature is one of great interest. This may be estimated by direct chemical analysis of the Earth's crust, and such extra-terrestrial sources=

as are available in the form of meteorites. The spectroscope will teU us what elements are present in the stars, but unfortu-=

nately it does not give much direct information as to their relative quantities.

On this question we can classify to use biological terms= 

either  by  individuals  or  by  species.  We  may  examine  the=

percentage composition, which wiU give a measure of the total number of individual atoms of each element present, or we may inquire into the number of different nuclear species which occur and classify them without respect to their individual abundance.

A very valuable discussion from the first point of view has been published by Harkins,^ who considers the percentage composition of meteorites and of parts of the Earth's crust. He demonstrates in a most convincing manner that there are

^Harkins, Jour. Amer. Chem. Soc, 39, 856, 1917.


112 ISOTOPES

immensely more atoms of elements of even atomic number. This interesting preponderance can, with a reasonable amount of probability, now be extended to even atomic weight, by the statistics given in the preceding paragraphs, but it will not be certain until the constitution of certain abundant elements such as iron has been actually determined.

The second point of view can be examined by means of the atomic weights of the radioactive isotopes and also by the true=

atomic weights given by the mass-spectra. In both cases nuclear systems of even atomic number are found to predomi- nate. The mass-spectra of 13 elements of even, and 14 ele- ments of odd atomic number indicate 32 isotopes of even atomic number and 20 of odd. The average element of even atomic number has therefore 2-5 isotopes to 1-4 for each element=

of odd atomic number.

The table on p. 15 shows that among the radioactive isotopes the preponderance is greater 32 as against 10 but= 

 it  is

possible that the former figure may include some atomic systems absolutely identical though of different origin.

103. The constancy of chemical atomic weights.

One of the first difficulties in the way of accepting the idea = of the complex constitution of an element such as chlorine was the constancy of its atomic weight. This had been determined by many different observers using different methods and the results were always the same within a very small experimental error. This difficulty may be met, in the first place, by noting=

that the vast majority, if not all, of the really accurate value= s were obtained from chlorine which must have been originally derived from the sea. The sea has been mixed for so long that it would be absurd to expect to find chlorines of different=

chemical atomic weights in it. Had ordinary galena been the only source of lead used in the atomic weight determina- tions given on page 16 no difference would have been found. It was only by examining the lead from extraordinary radio- active sources that the results were obtained which gave such definite and valuable support to the theory of isotopes.

The atomic weight of chlorine from sources other than the sea is now receiving the attention of chemists, though it is


ISOTOPES AND ATOMIC NUMBERS 113

naturally very difficult to be at all sure that any known source=

of chlorine is not of marine origin. Mile. Irene Curie ^ has examined the atomic weight of chlorine from three minerals whose marine origin seems unlikely. The values obtained from a sample of sodalite (sodium aluminium chlorosilicate) from Canada, and from a sample of calcium chlorophosphate from Norway agree with the value for chlorine from sea-water. The value 35-60, for chlorine from a sample of sodium chloride from a desert region in Central Africa was slightly high.

The comparison of the atomic weights of terrestrial and meteoric nickel made by Baxter and Parsons ^ is interesting in this connection. As a mean of nine determinations with the terrestrial material the figure 58-70 was found, whilst three experiments with meteoric nickel gave 58-68. The standard value found by Richards and Cushman was 58-68 (Ag =3D 107-88). The difference found between terrestrial and meteoric nickel is considered to be within the limits of experi-=

mental error, but further comparisons are to be made.

The writer regards these negative results as having a cause probably much more fundamental than the mere mechanical mixing of the different constituent isotopes during the history of the body containing them, namely a constancy of proportion during the evolution of the elements themselves. This will be considered later. The case of the radioactive leads is entirely exceptional. These substances have been produced continuously during the history of the earth's crust and are being so produced to-day. Although ordinary lead may con- sist of isotopes which is practically certain and = these isotopes may be identical in every respect with those produced in the last stage of radioactive disintegration, yet there is no reason=

whatever to assume that ordinary lead is itself the accumulated result of these processes. It takes its place among the other ordinary elements and would doubtless have done so had thorium and uranium never existed.

104. The agreement between the chemical atomic weight and the mean atomic weight deduced from the mass spectrum. The mean atomic weight of the isotopes

II. Curie, Compt. Retid. 172, 1025, 1921.

  • Baxter and Parsons, Jour. Amer. Chem. Soc, 43, 607, 1921.

I


114


ISOTOPES


of a complex element can be calculated if the relative intensitie= s of their lines in the mass-spectrum is known. This has been directly measured by Dempster. ^ The charged particles of isotopes of the same element are practically certain to afEect the photographic plate to the same extent as each other, hence we can obtain a rough estimate of their relative pro- portion by comparing the intensities of the lines. If this is done it is found that the great majority of the elements so far tested give mean results in good agreement with the accepted chemical values. The following table gives the data concerning four in which the difference is noteworthy  : =


Element.

Atomic Weight.

Mean from Mass-spectrum.

Difference.

Per cent. Difference.

Boron .... Krypton . Xenon Caesium

10-90 82-92 130-2 132-81

10-75^0-07 83-5 =C2=B10-3 131-3 =C2=B10-3 133 =C2=B10-3

0-15 0-6 1-1 0-2

1-37

0-72 0-85 0-05

The case of boron is the most difficult to account for. The masses of its isotopes 10 and 11 certainly do not differ from integers by more than one or two parts in a thousand. The ratio of the intensities of their second order lines 5 and 5-5=

(and there were no other substances present which could possibly give such lines) is equally certainly not as high as 9:1. It was for this reason that a third isotope 12 was suspected, but as no evidence of this has been found it seems most probable that the chemical atomic weight is still slightly too high.

The atomic weights of krypton and xenon are not of course chemical in the ordinary sense, as they are deduced direct from density determinations. Any trace of the impurity most likely to be present, argon in the first case, krypton in the second, would tend to make the densities too low, and this appears the most hkely explanation.

In the case of caesium the chemical result may be correct, for the probable error in the determination of mass is at least=

as large as the discrepancy. On the other hand caesium

1 V. p. 81.


ISOTOPES AND ATOMIC NUMBERS 115

appears to be a simple element, in which case its chemical atomic weight must represent the true weight of its atoms. Any error in this figure would probably be of the sign suggested= , for it is the heaviest member of its chemical group. If, how- ever, as is possible, the true mass of its atom differs from an=

integer by as much as 0-2 it is a fact of the greatest interes= t.

105. The meaning of the word " element." The

exact idea conveyed by the word " element " in chemistry and physics has given rise to endless difficulties in the past.=

In this connection Crookes in 1886 sums up the matter as follows  : " Of the attempts hitherto made to define or = explain an element, none satisfy the demands of the human intellect. The textbooks tell us that an element is ' a body which has not been decomposed '  ; that it is ' a something to which we=

can add, but from which we can take away nothing,' or ' a body which increases in weight with every chemical change.' Such definitions are doubly unsatisfactory  : they are provi- sional, and may cease to-morrow to be applicable to any given case. They take their stand, not on any attribute of things to be defined, but on the limitations of human power  ; they are confessions of intellectual impotence."

There was good reason for this dissatisfaction. The dis- covery ten years later of the electron, and the subsequent electrical theory of matter robbed the word of any pretence to its original meaning  ; for although Ramsay attempted to introduce into chemistry electricity itself as an element, it soon became obvious that this extension was unsuitable. The discovery of isotopes brings us face to face with two possible alternatives. The first is to call each isotope, as it is dis-=

covered, a new element. The second is to fix the word pre- cisely, now and for the future, as meaning a substance with definite chemical and spectroscopic properties which may or may not be a mixture of isotopes in other words to asso= ciate it exclusively with the conception of atomic number. On this view there would be, corresponding to Moseley's numbers, 92 possible elements, of which 87 are known.

If we adopt the first of these alternatives a new word will be necessary to express such substances as chlorine or mag-


116 ISOTOPES

nesium, hitherto called elements, and also the word element would mean something entirely different from what it has meant in all the chemical and physical Uterature of the past century. It would moreover be still subject to alterations in the future.

In the opinion of the writer the second alternative the=

association of element with atomic number is much the more preferable. The difficulties arising from it are practi- cally confined to the radioactive substances which can differ from one another even when their atomic numbers and atomic weights are identical. This is not very serious, for the radio-=

active elements are in a class by themselves and the special nomenclature already applied to them could be retained or re- vised as convenient without affecting that of general chemistry.

106. Disintegration theory of the evolution of the elements. A theory has been put forward by some writers=

that all the elements occurring in nature are the result of radioactive disintegrations of the ordinary type, but continued far beyond the ordinary limit observed at present. For instance, if we continue the a ray changes of the thorium series=

far enough we shall ultimately reach helium. The emission of an a particle is the only change known to occur which alters=

the atomic weight and it always does so by 4 units at a time.=

Hence from thorium we shall get a series of elements or iso- topes of atomic weights from 232 to 4 of the general type 4w. Uranium in the same way will yield a similar series of the type=

4:71 + 2. In order to obtain isotopes of odd atomic weight it is necessary to postulate parent elements of the type 4n + 1=

and 4:% -f 3.

Using hypotheses based on this general idea Van den Broek, ^ Harkins,^ Kohlweiler,^ Kirchoff ^ and others have built up the most elaborate systems of isotopes,

1 Van den Broek, Phys. Zeit., 17, 260, 579, 1 916  ; 23, = 164, 1921.

"Harkins and Wilson, Jour. Am. Chem. Soc, 37, 1367, 1915  ; Har-=

kins and Hall, ibid., 38, 169, 1916  ; Harkins, Phys. Rev., 15, = 73, 1920; Nature, 105, 230, 1920; Jour. Amer. Chem. Soc, 42, 1956, 1920  ; PhU. Mag., 42, 305, 1921.

  • Kohlweiler, Zeit. fur physikal. Chem. , 94, 51 3, 1 920  ; Phys.=
 Zeit. ,  21,

311, 543; 22, 243, 1921. * Kirchoff, idid., 21, 711, 1920.=


ISOTOPES AND ATOMIC NUMBERS 117

The writer regards this view as unhkely and misleading. In the first place it does not appear to succeed in its objects= . As an explanation of how the elements may have been evolved it starts with at least fom: elements as complicated as any known to exist, which does not advance the inquiry very much. On the other hand it may be used to predict the atomic weights of the isotopes composing known elements, and a great many predictions of this kind have been made. Here, though the measure of its success has varied to some extent with the particular modification of the theory employed, it has never been worthy of serious consideration. In cases where two or three isotopes of a given element were pre- dicted they proved as often wrong as right, and when the number of isotopes of integral atomic weights was so large that some agreements were inevitable the argument obviously loses all its force.

Another objection is that radioactive transformations do not continue, as far as we can see, beyond the stage (lead) indicated in the diagrams on p. 15. The lighter elements are definitely not radioactive. The radioactivity of potassium and rubidium is exceedingly small and its nature doubtful ; in any case it is best ascribed to minute vestiges of radioactiv= e isotopes, not to feeble radioactivity of the main constituents. It seems therefore more reasonable, for the present, to regard the property of radioactivity as absent entirely from the inactive elements than to suppose it present but too weak to be detected. It must not be gathered from these remarks that it is considered impossible to imagine physical conditions violent enough to disrupt the nuclei of light atoms, but rather=

that the mechanism causing such disruption need not be similar in any way to that causing normal radioactivity.

107. Grookes' theory of the evolution of the elements. 

 A  more  attractive  theory  than  the  one  given  abo=

ve is that the complex atoms of matter have been evolved by the aggre- gation of simpler atoms. This idea has received a good deal of=

attention in the past. Crookes^ remarks on it as follows  : =E2= =80=94 " Let us picture the very beginnings of time, before geological=


^ Crookes, Brit. Assoc, address, 1886.


118 ISOTOPES

ases, before the earth was thrown off from the central nucleus of molten fluid, before even the sun himself had consolidated from the original pivtyle. Let us still imagine that at this primal stage aU was in an ultra-gaseous state, at a temperature=

inconceivably hotter than anything now existing in the visible universe  ; so high indeed that the chemical atoms could not yet have been formed, being still far above their dissociation point. In so far as protyle is capable of radiating or reflectmg=

light, this vast sea of incandescent mist, to an astronomer in a=

distant star, might have appeared as a nebula, showing in the spectroscope a few isolated hues, forecasts of hydrogen, carbon and nitrogen spectra.

" But in due course of time some process akin to cooling, probably internal, reduces the temperature of the cosmic protyle to a pomt at which the first step in granulation takes=

place  ; matter as we know it comes into existence, and atoms are formed."

This vivid picture may be brought up to date by the sub- stitution of free protons and electrons for the hypothetical protyle. We can imagme regions containing matter where the temperature is so high that not only is the dissociation of=

atoms from atoms and nuclei from planetary electrons com- plete but also protons and electrons are in a state of agitation=

so violent that even the most stable nuclei cannot be formed. We should have here matter of the simplest form we can imagine, or rather of no form at all, simply a more or less neutral electric gas. Such a condition is by no means impos- sible in our miiverse and may actually occur during one of those=

excessively violent catastrophes occurring in far distant space and observed by us as new stars.

By some such cooling process as that suggested by Crookes we easily imagine the free charges combining to form the nuclei of elements. Whether those of heavier elements are formed direct by the charges getting into particular geometrical relations with each other, or whether hehum nuclei are formed first and then subsequently coalesce depends on which theory of nuclear structure is adopted. In any case vast quantities of=

energy will have to be radiated off and this radiation may be of such extremely high frequency that it is capable of dis-


ISOTOPES AND ATOMIC NUMBERS 119

rupting nuclei themselves, so that there might be at this stage=

rapid and continuous transformations from heavier to lighter nuclei and vice versa.

For the present we are interested in the number of each type of atom which survives. It is obvious that if the con- ditions of cooling are practically identical throughout the whole mass there is no reason why the composition of the matter produced should vary. If 3 atoms of CP^ are formed to every 1 of CP^ at any one point the same ratio must hold at every point so that a complex element of constant atomic weight will be formed. But it is much more likely that different parts=

of this primordial mass will undergo their transformations under different rates of cooling, etc., so it is worth while inquiring if variation in the mean atomic weight of a complex element is to be expected.

The quantity of one particular atomic nucleus formed will probably depend (a) on the probability of a certain configura- tion of charges happening as a chance event ; (b) the stability o= f the particular nucleus formed as the result of that event. Again to take the case of chlorine each isotope may be regarded=

as completely stable and the relative quantities formed will simply depend on condition (a). Now it is not unreasonable to suppose that this is not seriously affected by different rates=

of cooUng, and in this case the isotopes will be evolved in constant proportion. As we know of no natural process by which the proportion of isotopes can be altered appreciably the complex elements will have to-day the same chemical atomic weight as when they were first formed.

The above argument is of course purely a speculative one, and the conclusion drawn from it would fall to the ground at once if noteworthy differences of atomic weight in a single complex element were found supposing that element was not=

the product of a radioactive change at different points o= n the earth's surface. It may be worth noting that condition (a) suggests that, in general, the lighter atoms will outnumber the heavier ones. In aU matter available in nature this preponderance is actually enormous.

If the matter forming the earth ever went through a prim- ordial stage such as that suggested above it certainly did so


120 ISOTOPES

more than 10^ years ago. It follows that of the radioactive elements then formed only two, thorium and uranium, wiU now be found on the earth, for the other radioactive elements existing to-day are of such short period that they must have been formed since. Hence we may divide the original elements very simply and definitely into two groups  : (1) All the inactive elements, whose nuclei are sufficiently simple to be stable ; (2) Thorium and Uranium, whose nuclei are so complex that they are only partially stable.

Other less stable elements vfiay have been formed then but there can be no proof of this for they would, in any case, hav= e disappeared long ago, and it is clear that the other radioactive=

elements now found can all be regarded as formed from the two parent elements in comparatively recent times.


CHAPTER X - THE SPECTRA OF ISOTOPES

108. The Spectra of isotopes. As has already been stated^ the first experimental work on the spectra of isotopes was that of Russell and Rossi in 1912 who failed to distinguish=

any difference between the spectrum of thorium and that of a mixture of thorium and ionium containing a considerable percentage of the latter. The same negative result was obtained by Exner and Haschek.^ During the fractional diffusion of neon^ no spectroscopic difference was detected between the heaviest and the lightest fraction, though as the separation was small this negative evidence was not very strong. In 1914 Soddy and Hyman showed that the spectrum of lead derived from thorium was identical with that of ordinary=

lead.* Furthermore in the same year the experiments of Richards and Lembert,^ Honigschmidt and HoroAvitz,*^ and Merton proved the same result. Merton concluded from his 1914 experiments that the difference in wave-length for the A 4058 line must be less than 0-003 A. Before going on to consider the more recent results it will be as well to discuss = the magnitude of the difference to be expected from theory.

109. The magnitude of the Gravitational effect. In

the Bohr theory of spectra the planetary electrons of the atom rotate round the central positively charged nucleus in various

1 F. p. 9.

2 Exiier and Haschek, Sitz. Akad. Wiss. Wien, iia, 121, 175, = 1912.

3 V. p. 39.

  • Soddy and Hyman, Jour. Chem. Soc, 105, 1402, 1914.

^ Richards and Lembert, Jour. Amer. Chem. Soc, 36, 1329, 1914.=

^ Honigschmidt and Horowitz, Sitz. Akad. Wiss. Wien, iia, = 123, 1914.

' Merton, Proc. Roy. Soc, 91A, 198, 1914.

121


122 ISOTOPES

stable orbits. The frequencies of the spectral lines emitted by the element are associated in an absolutely definite manner with the rotational frequencies of these orbits which are calculated by what is known as a " quantum " relation. Without going further into the theory it will be seen at once that if we alter the force acting between the central nucleus and its planetary electrons these orbits will change and with them the frequency of the light emitted. It is therefore of interest to examine the magnitude of the change, to be expected=

from this theory, when we alter the mass of the nucleus without=

changing its charge, and so pass from one isotope to another.

The difference in the system which will first occur to one is that although the electrical force remains the same the gravi- tational force must be altered. The order of magnitude of the change expected in the total force will clearly be given by=

considering the ratio between the electrical and gravitational forces acting, to take the simplest case, between the protou and the electron in a neutral hydrogen atom.

Assuming the law of force to be the same in both cases, this ratio is simply e^/GMm  ; where e is the electronic charge 4-77 X 10~i", G the universal gravitational constant 6-6 x 10"^,=

M the mass of the proton 1-66 x lO"^*^ and m the mass of the=

electron 9-0 x 10~ 2^. Putting in these numerical values we obtain the prodigious ratio 2-3 x 10 ^9. In other words the effect of doubling the mass of the nucleus without altering its=

charge would give the same percentage increase in the total pull on the planetary electron, as would be produced in the pull between the earth and the moon by a quantity of meteoric dust weighing less than one million millionth of a gramme falling upon the surface of the former body. The gravitational effect may therefore be dismissed as entirely negligible.

110. Deviation of the Bohr orbits due to change in the position of the centre of gravity of the rotating system. Although we may neglect the gravitational effect there is another, of quite a different order, which arises in th= e following manner. The mass of the electron compared with that of the nucleus is small but not absolutely negligible, hence=

it will not rotate about the nucleus as though that were a


THE SPECTRA OF ISOTOPES 123

fixed point, but both will rotate about their common centre of gravity. The position of this centre of gravity will be shifted by any alteration in the mass of the nucleus. If E, M=

and e, m are the respective charge and mass of the nucleus and=

the rotating electron, the equation of motion is

rM , Ee

M + m r^

where r is the distance between the two charges and w the angular velocity. Bohr ^ introduced this effect of the mass of the nucleus in order to account for the results obtained by Fowler. 2 The Bohr expression for the frequency then becomes

where e, E and m, M are the charges and masses of the electron=

and nucleus respectively. If we suppose that the atomic weight of lead from radium to be one unit less than that of ordinary lead, this theory predicts a difference in wave-length, for the principle line, of 000005 A between the two, a quantity=

beyond the reach of the most delicate methods of spectrum analysis used up to the present.

111. Later experiments of Aronberg and Merton.

In 1917 Aronberg,^ applying the extremely high dispersion derived from the spectrum of the sixth order of a Michelson 10-inch grating to the line A 4058 emitted from a specimen of radio-lead of atomic weight 206-318, observed a difiference of 0-0044 A between this and ordinary lead, of atomic weight 207-20. This remarkable result has been since confirmed by Merton of Oxford* who gives the difference of wave-length between radio-lead from pitchblende and ordinary lead as 0-0050^2 0-0007, Merton made use of a totally different optical system, namely a Fabry and Perot etalon, so that the agreement is very striking.

It is to be noticed that the effect observed was not a mere

1 Bohr, Nature, 92, 231, 1913.

2 Fowler, Nature, 92, 95, 1913.

3 Aronberg, Proc. Nat. Acad. Sci., Z, 710, 1917, and Ast= rophys, Jour., 47, 96, 1918.

4 Merton, Proc. Boy. Soc, 96A, 388, 920.


124


ISOTOPES


broadening of the line but a definite shift, and that, though of the same sign, it is about one hundred times greater than that predicted by the Bohr theory, Merton also found a shift of 0-0022 =C2=B10-0008 A between the wave-length of thorite-lead and ordinary lead, differing in atomic weight by about 0-6. The heavier atom shows the higher frequency in all cases. This remarkable discrepancy between the shift predicted by theory and that actually observed has been discussed by Harkins and Aronberg.^

At a recent discussion on isotopes at the Royal Society ^ Merton commented upon the line 6708 A emitted by the element lithium, which consists of two components 0-151 A apart. If lithium is accepted as a mixture of isotopes 6 and 7,= ^ he calculated that each of these components should be accom- panied by a satellite, some sixteen times as faint, displaced by=

0-087 A. So far he had not been able to observe such satellites= . Previous experiments of Merton and Lindemann* on the expected doubling in the case of neon had given no conclusive results on account of the physical width of the lines. It was hoped that this difficulty could be overcome by the use of liquid hydrogen temperatures.

StiU more recently Merton^ has repeated his experiments on lead, using a very pure sample of uranium lead from Australian Carnotite. His final results are indicated in the following table  :


A

(Carnotite lead)"! . ^(ordinary lead) J

r Wave niimber (ordinary lead) ' . Wave-number (Carnotite lead).

4058 3740 3684 3640 3573

0-011 =C2=B10-0008 0-0074=C2=B10-0011 0-0048=C2=B10-0007 0-0070=C2=B10-0003 0-0048=C2=B10-0005

0-065=C2=B10-005 0-053=C2=B10-008 0-035=C2=B10-005 0-C52=C2=B10-002 0-037=C2=B10-004

1 Harkiiis and Aronberg, Jour. Am. Chem. Soc, 42, 1328, 

Merton,  Proc.  Roy.  Soc.=C2=BB  99A,  87,     1921.

=C2=BB V. p. 86.

  • Lindemann, ibid.
 Merton,  Roy.  Soc.  Proc,  lOOA,  84,     1921.


1920.


THE SPECTRA OF ISOTOPES 125

It will be noticed that the shift for the line A 4058 is rathe= r more than twice that obtained before. Merton suggests that the most probable explanation of this difference is evidently that the Carnotite lead used is a purer sample of uranium lead=

than that obtained from the pitchblende residues. It is also apparent that the differences are not the same for different lines, an interesting and somewhat surprising result.

112. "Isotope" effect on the Infra-red spectrum of molecules. The extreme smaUness of the isotope " shift "=

described above in the case of line spectra emitted by atoms is=

due to the fact that one of the particles concerned in the vibration is the electron itself, whose mass is minute compared with that of the nucleus. Very much larger effects should be expected for any vibration in which two atoms or nuclei are concerned, instead of one atom and an electron. Such a vibration would be in the infra-red region of the spectrum.

This effect was first observed by Imes^ when mapping the fine structure of the infra-red absorption bands of the halogen acids. In the case of the HCl " Harmonic " band at 1-76^, mapped with a 20,000 line grating, the maxima were noticed to be attended by satellites. Imes remarks  : " The apparent tendency of some of the maxima to resolve into doublets in the=

case of the HCl harmonic may be due to errors of observation, but it seems significant that the small secondary maxima are all on the long-wave side of the principal maxima they accom- pany. It is, of course, possible that still higher dispersion applied to the problem may show even the present curves to be composite."

Loomis^ pointed out that these satellites could be attributed to the recently discovered isotopes of chlorine. In a later paper ^ he has shown that, if mi is the mass of the hydrogen nucleus, and ma the mass of the charged halogen atom, the

difference should be expressed by the quanity ^ = ~ the

square root of which occurs in the denominator of the expression=


^ Imes, Astrophysical Journal, 50, 251, 1919.

2 Loomis, Nature, Oct. 7, 179, 1920.

^ Loomis, Astrophysical Journal, 52, 248, 1920.


126 ISOTOPES

for frequency. " Consequently the net difference between the spectra of isotopes will be that the wave-lengths of lines in the spectrum of the heavier isotope will be longer than the=

corresponding lines for the lighter isotope in the ratio 1 + 1/1330  : 1 for chlorine and 1 -f 1/6478  : 1 for bromine.=

Since the average atomic weight of chlorine is 35-46 the amounts=

of CP^ and CP' present in ordinary chlorine must be as 1-54  : 0-46 or as 3-35  : 1 and, if the lines were absolutely = sharp and perfectly resolved, the absorption spectrum of ordinary HCl should consist of pairs of lines separated by 1/1330 of their frequency and the one of shorter wave-length should have about 3-35 the intensity of the other. The average atomic weight of bromine is 79-92, hence the two isotopes are present in nearly equal proportions and the absorption spectrum of HBr should consist of lines of nearly equal intensity separated by 1/6478 of their frequency."

The latter will be too close to be observed with the dispersion=

employed. In the case of the HCl band at IIQ ju the difference=

of wave number on this view should be 4-3. The mean differ- ence of wave number given by Loomis' measurements of 13 lines on Imes' original curves for this band is 4-5 ^ 0-4 corre= - sponding to 14 A in wave-length.

The spectroscopic confirmation of the isotopes of chlorine has also been discussed by Kratzer,! who considers that the oscillation-rotation bands of hydrogen chloride due to Imes^ are in complete accordance with the theory.

1 H. Ivratzer, Zeit. Physik., 3, 60, 1920.

  • Loc. cit.


CHAPTER XI - THE SEPARATION OF ISOTOPES

113. The Separation of Isotopes

The importance, from purely practical and technical points of view, of the theory of isotopes would have been insignificant had its application been confined to the radioactive elements and their products, which are only present in infinitesimal quantities on the Earth. But now that the isotopic nature of many elements in everyday use has been demonstrated, the possi- bility of their separation, to any reasonable extent, raises questions of the most profound importance to applied science. In physics all constants involving, e.g., the density of mercury=

or the atomic weight of silver may have to be redefined, while=

in chemistry the most wholesale reconstruction may be necessary for that part of the science the numerical founda- tions of which have hitherto rested securely upon the constancy of atomic weights.

It is therefore of great interest to consider in turn the various methods of separation proposed and examine how far they have been successful in practice.

114. Separation by Diffusion

he subject of the separation of a mixture of two gases by the method of Atmolysis or has been thoroughly investigated by the late Lord Rayleigh. The diffusion is supposed to take place through porous material. The conditions under which maximum separation is to be obtained are that " mixing " is perfect, so that there can be no accumulation of the less diffusible gas at the surface of the porous material, and that the apertures in the material through which the gases must

iRayleigh, Phil. Mag., 42, 493, 1896. 127


128 ISOTOPES

pass are very small compared with the mean free path of the molecules. If these conditions are satisfied he obtains as an expression for the effect of a single operation  :


X + y _ ^ . _^ Y


r '^


X + Y X + Y "-'^ X + Y "-'-

where (X Y) {x, y) are the initial and final volumes of the gases, /I, V, the velocities of diffusion, and r the enrichment=

of the residue as regards the second constituent.

The velocity of diffusion of a gas is proportional to the square root of the mass of its molecules, so that if a mixture=

of two isotopes is allowed to diffuse a change in composition must be brought about. Now no known isotopes differ from each other much in mass, so the difference between their rates of diffusion will also be small, hence the above equation=

may be written in the approximate form

^- =3D rTc where h =3D ^ a small quantity and,

and, finally, the enrichment by diffusion of the residue as regards the heavier constituent may be expressed with sufficient accm'acy by the expression


mi-m /Initial volume


Final volume


where Wi, mg are the molecular masses of the lighter and heavier isotope respectively. In the most favourable case known at present, that of the isotopes of neon, the number over the root is 21 so that the change in composition obtain- able in a single operation will in practice be very small.

If we take the density of the original mixture as unity, the increase in density of the residual gas to be expected from the=

operation of diffusion will be approximately

(r 1) X ^ X 2 ^


X Wg + Wi

Now neon consists of monatomic molecules differing between each other in mass by 10 per cent, and the heavier is present=

to the extent of 10 per cent. In the diffusion experiments described on p. 39 the effective ratio of the initial volume to=


THE SEPARATION OF ISOTOPES 129

the final volume was estimated as certainly greater than 500 and probably less than 10,000, so that r lies between 1-3 and 1-5. Hence the increase of density of the heavier residue should have been between -003 and -005. It was actually 004.

115. The separation of the isotopes of chlorine by the diffusion of HCl

In the case of other isotopic gaseous mixtures the numerical obstacles in the way of practical separation wiU be correspondingly greater. Thus in the case of HCl the 36th root is involved, and in that of HBr the 80th root. The only way by which measurable increase in density may be hoped for wiU clearly be by increasing the effective ratio of the initial to final volumes to an heroic degree. This can be done by experiments on a huge scale or by a vast number of mechanical repetitions.

Harkins started to attack the HCl problem in 1916 using the first of these two alternatives. In 1920 he mentions a quantity of 19,000 litres of HCl as having been dealt with in these experiments. 2 In the following year^ he published numerical results indicating that a change in atomic weight of 0-055 of a unit had been achieved.

At the recent discussion on isotopes * Sir J. J. Thomson pointed out that a change in the molecular weight of HCl should be caused by allowing a stream of the gas to flow over=

the surface of a material which absorbed it. The higher diffusion coefficient of the lighter isotope would result in it being absorbed more rapidly than the heavier one, so that the residue of unabsorbed gas should give a higher molecular weight. This " free diffusion " without the interposition of porous material has been recently tried in the Cavendish Laboratory by E. B. Ludlam, but no measurable difference has so far been detected.

116. Separation by Thermal Diffusion

It has been

^ Harkins, Jour. Amer. Cheni. Soc, Feb., 1916.

2 Harkins, Science, Mar. 19, 1920 ; Nature, Apl. 22, 1920 ; see=

also Phys. Rev., 15, 74, 1920  ; Science, 51, 289, 1920 ; Jour. = Amer, Chem. Soc, 42, 1328, 1920.

3 Harkins, Science, Oct. 14, 1921  ; Nature, Oct. 3, 1921.=

  • J. J. Thomson, Proc. Roy. Soc, 99A, 98, 1921.

K

shown on theoretical grounds independently by Enskog ^ and Chapman ^ that if a mixture of two gases of different molecular weights is allowed to diffuse freely, in a vessel of which the ends are maintained at two different temperatures T,T', until equilibrium conditions are reached, there will be a slight excess of the heavier gas at the cold end, and of the=

lighter gas at the hot end. The separation attained depends on the law of force between the molecules and is a maximum if they behave as elastic spheres. The effect was experi- mentally verified for a mixture of CO2 and Ha by Chapman and Dootson,^ and recently Ibbs * has demonstrated that the separation can be carried out continuously and that the time for equilibrium to be established is quite short.

Chapman has suggested ^ that thermal diffusion might be used to separate isotopes. He shows that the separating power depends on a constant ^x. And when the difference between the molecular masses mi, ma is smaU the value of this is approximately given by

, _ 17 ma mi AiAj

^^ ~~ 3 ma + mi 9-15 8-25 AiAa where ^1,^2 denote the proportions by volume of each gas in the mixture  ; thus Ai -f Aa =3D=3D1. The actual separation = 

is

given by

Ai A'l =3D (Ai A'a) =3DA;t log T'/T.=


He gives the following numerical example  : " Suppose that it is=

desired to separate a mixture of equal parts of Ne^" and Ne^^,=

then, writing mi =3D 20, ma =3D 22, Ai =3D A3 =3D ^, we find = 

that

Ic,^ =3D 0-0095. Suppose that the mixture is placed in a vessel=

consisting of two bulbs joined by a tube, and one bulb is maintained at 80=C2=B0 absolute by liquid air, while the other is=

heated to 800=C2=B0 absolute (or 527=C2=B0 C). When the steady st= ate has been attained the difference of relative concentration between the two bulbs is given by the equation

1 Enskog, Phys. Zeit., 12, 538, 1911  ; Ann. d. Phys., 38, 75= 0, 1912.

2 Chapman, Phil. Trans., 217A, 115, 1916; Phil. Mag., 34, 146, 1917.

3 Chapman and Dootson, Phil. Mag., 34, 248, 1917.

  • Ibbs, Proc. Boy. Soc, 99A, 385, 1921.

^Chapman, Phil Mag., 38, 182, 1919.


THE SEPARATION OF ISOTOPES 131

Ai A'l =3D (A 2 A' 2) =3D 0-0095 lo= g, 800/80

=3D 0-022

or 2-2 per cent. Thus the cold bulb would contain 48-9 per cent. Ne^" to 51-1 per cent. Ne^^, and vice versa in the hot bulb. By drawing o=C2=A3f the contents of each bulb separately, and by repeating the process with each portion of the gas, the=

difference of relative concentrations can be much increased. But as the proportions of the two gases become more unequal, the separation effected at each operation slowly decreases. For instance, when the proportions are as 3  : 1, the variation=

at each operation falls to 1-8 per cent.  ; while if they are a= s 10  : 1 the value is 1-2 per cent. This assumes that the mole-=

cules behave like elastic spheres  : if they behave like point centres of force varying as the inverse nth. power of the distan= ce, the separation is rather less; e.g., ii n=3D9, it is just over=

half the above quantities."

Chapman points out that for equal values of log p/p and log T/T pressure diffusion (centrifuging) is about three times as powerful as thermal diffusion but suggests that it may be more convenient to maintain large differences of temperature than of pressure.

117. Separation by Gravitation or "Pressure Diffusion"

When a heterogeneous fluid is subjected to a gravitational field its heavier particles tend to concentrate in the direction of the field, and if there is no mixing to co= unter- act this a certain amount of separation must take place. If therefore we have a mixture of isotopes in a gaseous or liquid=

state partial separation should be possible by gravity or centrifuging.

The simplest case to consider is that of the isotopes of neon in the atmosphere and, before the matter had been settled by the mass-spectrograph, analysis of the neon in the air at very great heights was suggested as a possible means of proving its isotopic constitution. 1 The reasoning is as follows: =E2=80= =94

If M be the atomic weight, g the gravitational constant, p the pressure, and p the density, then if no mixing takes place dp =3D gpdh, h being the height. In the isother= mal

1 Lindemann and Aston, Phil. Mag., 37, 530, 1919.


132 ISOTOPES

layer convection is small. If it is small compared with diffusion the gases will separate to a certain extent. Since T is constant

RTp , dp Mp ,,

whence p =3D pffi Rt ,

Po being the density at the height Jiq at 'which mixing by convection ceases, about 10 kilometres, and A^ the height above this level. If two isotopes are present in the ratio 1 to Ko, so that the density of one is po and of the other Kopo=

at height Jiq, then their relative density at height h^ + /SJi = is given by

Putting T =3D 220 as is approximately true in England,

XT

A^ being measured in kilometres. If Mi Ma =3D 2, th= erefore

It might be possible to design a balloon which would rise to 100,000 feet and there fill itself with air. In this case the relative quantity of the heavier constituent would be reduced from 10 per cent, to about 8-15, so that the atomic weight of=

neon from this height should be 20-163 instead of 20-2. If one could get air from 200,000 feet, e.g. by means of a long-=

range gun firing vertically upwards, the atomic weight of the neon should be 20-12.

A more practicable method is to make use of the enormous gravitational fields produced by a high speed centrifuge.

In this case the same equation holds as above except that g varies from the centre to the edge. In a gas therefore <ip__Mv2 dr _ _Mo)'^ ~^ ~ Rf "y ~ RT '

whence p =3D poe 2rt,

Vq being the peripheral velocity. Here again, if Kq is the


THE SEPARATION OF ISOTOPES 133

ratio of the quantities present at the centre, the ratio at the=

edge will be

A peripheral velocity of 10^ cm,/s. or perhaps even 1-3 x 10^ cm./s. might probably be attained in a specially designed

rr

centrifuge, so that:^^ might be made as great as e"=C2=B0'2^^'^'~^=

  • ^ or

even e ~0'^'^^'^>~^2),

If Ml M2 is taken as 2 a single operation would there= fore give fractions with a change of K of 0-65. In the case of neon=

the apparent atomic weight of gas from the edge would be about 0-65 per cent, greater than that of gas from the centre,=

i.e. a separation as great as the best yet achieved in practice=

by any method could be achieved in one operation. By centrifuging several times or by operating at a lower tempera- ture the enrichment might be increased exponentially.

Centrifuging a liquid, e.g. liquid lead, would not appear so favourable, though it is difficult to form an accurate idea of the quantities without a knowledge of the equation of state. If compression is neglected and the one lead treated as a solution in the other, a similar formula to that given above holds. On assumptions similar to these Poole ^ has calculated that a centrifuge working with a peripheral velocity of about 10^ cm. /sec should separate the isotopes of mercury to an extent corresponding to a change of density of 0-000015.

The only experiments on the separation of isotopes by the use of a centrifuge, so far described, are those of Joly and Poole 2 who attempted to separate the hypothetical isotopic constituents of ordinary lead by this means. No positive results were obtained and the check experiments made with definite alloys of lighter metals with lead were by no means encouraging.

118. Separation by Chemical Action or Ordinary Fractional Distillation. The possibility of separating iso-=

topes by means of the difference between their chemical affinities or vapour pressures has been investigated very fully

1 Poole, Phil. Mag., 41, 818, 1921.

2 Joly and Poole, Phil. Mag., 39, 372, 1920.


134 ISOTOPES

from the theoretical standpoint by Lindemann. The thermo- dynamical considerations involved are the same in both cases. The reader is referred to the original papers ^ for the details=

of the reasoning by which the following conclusion is reached  : =

" Isotopes must in principle be separable both by fractiona- tion and by chemical means. The amount of separation to be expected depends upon the way the chemical constant is calculated and upon whether ' NuUpunktsenergie ' is assumed. At temperatures large compared with ^v,^ which are the only practicable temperatures as far as lead is concerned, the difference of the vapour pressure and the constant of the

Bv law of mass action may be expanded in powers of ^. The

Bv most important term of the type log "^ is cancelled by the

chemical constant if this is calculated by what seems the only

Bv reasonable way. The next term in is cancelled by the=


' NuUpunktsenergie ' if this exists. All that remains ar= e

Bv terms containing the higher powers of ^. In practice there- fore fractionation does not appear to hold out prospects of success unless one of the above assumptions is wrong. If the first is wrong a difference of as much as 3 per cent, should occur at 1200 and a difference of electromotive force of one miUivolt might be expected. Negative results would seem to indicate that both assumptions are right."

As regards experimental evidence it has already been pointed out that the most careful chemical analysis, assisted by radio- active methods of extraordinary delicacy, was unable to achieve the shghtest separation of the radioactive isotopes. The laborious efforts to separate the isotopes of neon by a differ-=

ence of vapour pressure over charcoal cooled in hquid air also gave a completely negative result.

119. Separation by evaporation at very low pressure

If a liquid consisting of isotopes of different mass is allowed

1 Lindemann, Phil. Mag., 37, 523, 1919  ; 38, 173, 1919.

  • (iv is the " characteristic " and T the " Absolute " tempera=

ture.


THE SEPARATION OF ISOTOPES 135

to evaporate it can be shown that the number of Hght atoms escaping from the sm'face in a given time will be greater than=

the number of heavier atoms in inverse proportion to the square roots of their weights. If the pressure above the surface is kept so low that none of these atoms return the concentration of the heavier atoms in the residue will steadily increase. This method has been used for the separation of isotopes by Bronsted and Hevesy, who appUed it first to the element mercury.

The mercury was allowed to evaporate at temperatures from 40=C2=B0 to 60=C2=B0 C. in the highest vacuum attainable. The eva= porat- ing and condensing surfaces were only 1 to 2 cms. apart, the latter was cooled in liquid air so that all atoms escaping reached it without coUision and there condensed in the sohd form.

It will be seen that the Uquid surface acts exactly Uke the porous diaphragm in the diffusion of gases. ^ The diffusion rate of mercury can be obtained approximately from the diffusion rate of lead in mercury ^ and is such that the mean=

displacement of the mercury molecule in Uquid mercury is about 5 X 10"^ cm. sec."^. It follows that if not more than 5 X 10"^ c.cm. per cm.^ surface evaporate during one second no disturbing accumulation of the heavier isotope in the surface layer takes place.

The separation was measured by density determination. Mercury is particularly well suited for this and a notable feature of this work was the amazing deUcacy with which it could be performed. With a 5 c.cm. pyknometer an accuracy of one part in two millions is claimed. The first figures pubhshed ^ were  :

Condensed mercury. . . . 0-999981

Residual mercury .... 1-000031

The densities being referred to ordinary mercury as unity.

The later work was on a larger scale.* 2700 c.cm. of mercm-y were employed and fractionated systematically to about

1 V. p. 127.

  • Groh and Hevesy, Ann. der Phys., 63, 92, 1920.

^ Bronsted and Hevesy, Nature, Sept. 30, 1920.

  • Bronsted and Hevesy, Phil. Mag., 43, 31, 1922.


136 ISOTOPES

1/100,000 of its original volume in each direction. The final=

figures were  :

Lightest fraction vol. 0-2 c.c. . . 0-99974

Heaviest fraction vol. 0-3 c.c. . . 1-00023

Mercury behaves as though it was a mixture of equal parts of two isotopes with atomic weights 202-0, 199-2 in equal parts or of isotopes 201-3, 199-8 when the former is four times=

as strong as the latter, and so on.

120. Separation of the isotopes of chlorine by free evaporation

The same two investigators were able to announce the first separation of the isotopes of chlorine ^ by applying the above method to a solution of HCl in water. This was allowed to evaporate at a temperature of 50=C2= =B0 C. and condense on a surface cooled in hquid air. Starting with 1 litre 8-6 mol. solution of HCl 100 c.c. each of the lightest=

and heaviest fraction were obtained.

The degree of separation achieved was tested by two difiEerent methods. In the first the density of a saturated solution of NaCl made from the distillate and the residue respectively was determined with the following results  :

Density (salt from distillate) =3D 1-20222 Density (salt from residue) =3D 1-20235

These figures correspond to a change in atomic weight of 0-024 of a unit.

In the second method exactly equal weights of the isotopic NaCls were taken and each precipitated with accurately the same volume of AgNOg solution, in shght excess. After pre- cipitation and dilution to 2,000 c.c. the approximate concen- tration of the filtrate was determined by titration, also the ratio of Ag concentration of the two solutions was measured in a concentration cell. Calculation showed that the difference in atomic weight of the two samples was 0-021 in good agree- ment with the density result.

121. Separation by Positive Rays

The only method which seems to offer any hope of separating isotopes completely, and so obtaining pure specimens of the constituents of a com-

1 Bronsted and Hevesy, Nature, July 14, 1921.


THE SEPARATION OF ISOTOPES 137

plex element, is by analysing a beam of positive rays and trapping the particles so sorted out in different vessels. It is=

therefore worth while inquiring into the quantities obtainable by this means.

Taking the case of neon and using the parabola method of analysis with long parabolic slits as collecting vessels we find=

that the maximum separation of the parabolas corresponding to masses 20 and 22 (obtained when electric deflexion d is haK the magnetic) is approximately

^ 1 M,-M, _ d_ V2 Ml 28"

Taking a reasonable value of 0 as -3 the maximum angular width of the beam for complete separation =3D 0-01. If the canal-ray tube is made in the form of a slit at 45=C2=B0 to ax= es, i.e. parallel to the curves, the maximum angular length of the beam might be say 5 times as great, which would collect the positive rays contained in a solid angle of -0005 sq. radian= .

The concentration of the discharge at the axis of the positive ray bulb is considerable, and may be roughly estimated to correspond to a uniform distribution of the entire current over a |- sq. radian. One may probably assume that half the current is carried by the positive rays, and that at least half=

the positive rays consist of the gases desired. If neon is analysed by this method therefore the total current carried by the positive rays of mass 20 is

0005 x4:Xixlxi=3D -0005 i.

If i is as large as 5 miUiamperes this =3D 1-5 x 10* E.S.U.=

1-5 X 10*


or


2-7 X 1019 X 4-77 X 10-1"


=3D 1-2 X 10"^ c.c./sec.


i.e. one might obtain about one-tenth of a cubic millimetre of Ne2o and 1/100 cubic miUimetre of Ne^^ per 100 seconds run. It is obvious that even if the difficulties of trapping the rays=

were overcome, the quantities produced, under the most favourable estimates, are hopelessly small.

122. Separation by photochemical methods

A remarkably beautiful method of separating the isotopes of=


138 ISOTOPES

chlorine has been suggested by Merton and Hartley which depends upon the following photochemical considerations. Light falling on a mixture of chlorine and hydrogen causes these gases to combine to form hydrochloric acid. This must be due to the activation of the atoms of hydrogen or those of=

chlorine. Supposing it to be the latter it is conceivable that the radiation frequency necessary to activate the atoms of Cl^^ will not be quite the same as that necessary to activate those of CP'^. CaUing these frequencies 5^35 and V37 respectively=

it would seem possible, by excluding one of these frequencies entirely from the activating beam, to cause only one type of chlorine to combine and so to produce pure HCI^^ or HCI^'. Now ordinary chlorine contains about three times as much CP^ as CP^ and these isotopes must absorb their own activat- ing radiation selectively. In this gas therefore light of frequency V35 will be absorbed much more rapidly than that of frequency V37, so that if we aUow the activating beam to pass through the right amount of chlorine gas V35 might be completely absorbed but sufficient V37 radiation transmitted to cause reaction. On certain theories of photo-chemistry light containing ^37 but no V35 would cause only atoms of CP^ to combine so that a pure preparation of HCP^ would result. Pure CP'^ made from this product could now be used as a filter for the preparation of pure HCP^, and this in its turn would yield pure CP^ which could then be used as a more efficient filter for the formation of more HCP^

Had this very elegant scheme been possible in practice it would have resulted in a separation of a very different order to those previously described and the preparation of un- limited quantities of pure isotopes of at least one complex element. There is however little hope of this, for so far the results of experiments on this method have been entirely negative.

123. Other methods of separation and general conclusions

The following methods have also been suggested. By the electron impact in a discharge tube, in the case of the inert gases, the Ughter atoms being more strongly urged towards


THE SEPARATION OF ISOTOPES 139

the anode  ;^ by the migration velocity of ions in gelatine  ; ^=

by the action of light on metallic chlorides,^

A survey of the separations actually achieved so far shows that from the practical point of view they are very small. In cases where the method can deal with fair quantities of the substance the order of separation is small, while in the case of complete separation (positive rays) the quantities produced are quite insignificant. We can form some idea by considering the quantity

Q =3D (difference in atomic weight achieved) X (average quantity of two fractions produced in grammes). As regards the first of these factors the highest figure so far was 0-13 obtained by the writer in the original diffusion experiments on neon, but as the quantities produced were only a few milli- grams Q is negligibly small. The highest values of Q have been obtained by Bronsted and Hevesy by their evaporation method.* It is 0-5 in the case of Hydrochloric Acid, 0-34 in that of Mercury.

When we consider. the enormous labour and difficulty of obtaining this result it appears that unless new methods are discovered the constants of chemical combination are not likely to be seriously upset for some considerable time to come.=


1 Skaupy, Zeitsch. Phys., 3, 289, 460, 1920.

2 Lindemann, Proc. Roy. Soc, 99A, 104, 1921.

3 Renz, Zeit. Anorg. Chem., 116, 62, 1921.

  • V. p. 134.


APPENDIX I

Table of atomic weights and isotopes of the elements. 

 The  elements  are  given  in  order  of  their  atomic=
 numbers.  The

different periods are indicated by gaps after the inert gases. A curious relation, pointed out by Rydberg, is that the atomic numbers of all the inert gases are given by taking the series 2 (P + 2^ + 22 + 3^ + 3^ + 4^ + = ) and stoppmg the summation at any term. This gives the numbers used by Langmuir (p. 95).

The atomic weights given are the International ones except in the cases marked with an asterisk, where the figures are taken f= rom some of the recent determinations given below.

The isotopes where known are given in order of their atomic masses. The proportion of an isotope in a complex element is indicated by the index letters a, 6, c ... in descending order.=

In the case of isotopes of the radioactive elements 81-92 the ro= man numeral gives the number of them believed to exist. The nomen- clature of some of the rare earths 69-72 is not yet standardised.=

The names here are those used by Moseley. Some of these elements= , though detected by their X-ray spectra, have never been isolated.=

The elements corresponding to atomic numbers 43, 61, 75, 85, 87=

(all odd) have not yet been discovered.

Recent atomic weight determinations. The following is a list of some of the elements whose atomic weights have been re-=

determined quite recently, together with references to the papers in which they were published. Where more than one value is given different methods were used  :

Fluorine 19-001. Moles and Batuecas, Jour. Chim. Phys., 18, 35= 3,

1920. Aluminium 26*963. Richards and Krepelka, Journ. Am. Chem. Soc,=


42, 2221, 1920. Silicon 28-111. Baxter, Weatherelland Holmes, ibid., 42, 1194, = 

1920.

Scandium 45-10. Honigschmid, Zeit. Electrochem., 25, 93, 1919.=

Tin 118-703. Baxter and Starkweather, Journ. Am. Chem. Soc, 42,=


905, 1920.

118-699. Brauner and Krepelka, ibid., 42, 917, 1920.

141


142


APPENDIX I


Tellurium 127-73, 127-79. Bruylants and Michielsen, Bull= . Acad.

Bdg., 119, 1919. Samarium 150 "43. Owens, Balke and Kremers, Journ. Am. Chem= .

Soc, 42, 515, 1920. Thtdium 169-44, 169-66. James and Stewart, ibid., 42, 2022, = 1920. Bismuth 209-02. Honigschmid, Zeit. Electrochem., 26, 403, 1920= .

208-9967. Classen and Wey, Ber., 53, 2267, 1920. Antimony 121-773. Willard and McAlpine, Jouryi. Am. Chem. Soc, = 43,

797, 1921. Lanthanum 138-912. Baxter, Tani and Chapin, Journ. Am. Chem.=


Soc, 43, 1085, 1921. Germanium 72-418. Miller, Journ. Am. Chem. Soc, 43, 1085, 19= 21. Zinc 65-38. Baxter and Hodges, i&id., 43, 1242, 1921. Cadmium 112-411. Baxter and Wilson, ibid., 43, 1230, 1921.


-Q

" m

o^

Element.

2

a

if

Masses of isotopes.

=C2=A3 -2 *^ Hydrogen . .

H

1

1-008

1

1-008

f^^'o Helium . . .

He

2

4-00

1

4

&> 1"

00 Lithivim .

Li

3

6-94

2

-

" Beryllium

Be

4

91

1

9

r^ Boron

B

5

10-9

2

10=C2=BB 11"

3 Carbon .

C

6

12-00

1

12

S Nitrogen .

N

7

14-008

1

14

^ Oxygen . . .

0

8

16-00

1

16

0 Fluorine .

F

9

19-00

1

19

^ Neon ....

Ne

10

20-20

2

20" 22* 23

oQ Sodium .

Na

11

2300

1

^ Magnesium .

Mg

12

24-32*

3

24-=3D 25* 26^

Aluminium .

Al

13

26-96*

_o Silicon

Si

14

28-3

2

28" 29* (30)

3 Phosphorus .

P

15

31-04

1

31

^ Sulphur . . .

s

16

3206

1

32

'S Chlorine . . .

CI

17

35-46

2

35" 37* (39)

^ Argon . . .

A

18

39-9

2

36* 40" 39" 41*

Potassium

K

19

39-10

2

Calcium .

Ca

20

40-07

(2)

40 (44)

Scandium

Sc

21

45-1*

Titanium .

Ti

22

48-1

Vanadium

V

23

510

0

2 Chromium .

Cr

24

52-0

H Manganese .

Mn

25

54-93

' Iron ....

Fe

26

55-84

n

^ Cobalt . . .

Co

27

58-97

J Nickel

Ni

28

58-68

2

58" 60*

P

n Copper .

Cu

29

63-57

J

=3D Zinc ....

Zn

30

65-37

(4)

(64=C2=B0 66* 68 7O<0

  • Galliimi . . .

Ga

31

70-10

Germanivmi .

Ge

32

72-5

Arsenic .

As

33

74-96

1

75

Seleniima .

Se

34

79-2

Bromine .

Br

35

79-92

2

79" 81*

Krypton .

Kr

36

82-92

6

78/ 80 82'^ 83-^ 84=C2=BB

86*

APPENDIX I


143


"S .

^

o *^

O^i

o ^^

Element

o

X!

E >,

00

Masses of Isotopes.

Rubidium

Rb

37

85-45

2

85" 87*

Strontium

Sr

38

87-63

Yttrium .

Y

39

89-33

Zirconium

Zr

40

90-6

Niobium .

Nb

41

93-1

00 Molybdenum

Mo

42

96-0

  • H _ ~


43


'-' Ruthenium .

Ru

44

101-7

'o Rhodium.

Rh

45

102-9

=C2=A7 Palladium

Pd

46

106-7

An Silver ....

Ag

47

107-88

X Cadmium

Cd

48

112-40

"O Indiimi .

In

49

114-8

Tin ... .

Sn

50

118-7

Antimony

Sb

51

120-2

Tellurium

Te

52

127-5

Iodine

I

53

126-92

1

127

L Xenon

X

54

130-2

(7)5

(128) 129" (130) 13P 132=C2=BB 134 136"

Caesium .

Cs

55

132-81

1

133

Barium .

Ba

56

137-37

Lanthanum .

La

57

139-0

Cerium

Ce

58

140-25

Praseodymium .

Pr

59

140-6

Neodymiimi .

Nd

60

144-3



61


Samarium

Sm

62

150-4

Europium

Eu

63

152-0

Gadolinium .

Gd

64

157-3

Terbium .

Tb

65

159-2

Dysprosium .

Ds

66

162-5

c

5 Holmium

Ho

67

163-5

J, Erbium .

Er

68

167-7

=C2=B0 Thulium . . .

Tu

69

168-5

1 Ytterbiiun . .

Yb

70

173-5

'C Lutecuim

Lu

71

175

Pm (Keltium) . .

(Kt)

72

ji Tantalum

Ta

73

181-5

<=C2=BB Tungsten.

W

74

1840



75


Osmium .

Os

76

190-9

Iridium .

Ir

77

193-1

Platinimi .

Pt

78

195-2

1

Gold ....

Au

79

197-2

Mercury .

Hg

80

200-6

(6)

(197-200) 202 204

Thallium . . .

Tl

81

204-0

IV

Lead ....

Pb

82

207-2

XI

Bismuth .

Bi

83

209-0*

V

Poloniuna

Po

84 85

z

VII

L Emanation

Em

86

222-0

III

i

87

.2 Radium . =C2=AE Actinium.

Ra

88

226-0

IV

Ac

89


II

^ Thorium . . .

Th

90

23215

VI

^ Uranium X .

UX

91

II

t_ Uranium

Ur

92

238-2

II

APPENDIX II

The Periodic Table of the Elements. The atomic numbers ar= e given in bold type, the atomic weights in italics and the isotopes, where = known, in ordinary numerals. The roman ntmierals indicate the chemical groups and the most important associated valencies are given below them. Elem= ents are placed to the left or to the right of the columns according= to their chemical properties, those in the same vertical line as each other have s= trong chemical similarities. The Rare Earth group is surrounded by a thick line.= Elements 59-72 have no properties pronounced enough to give them definite = places in the table. The properties of the missing elements can be p= redicted with

PERIODIC TABLE OF


IH

1-008


Valency

0

I

+ 1

II

+ 2

III

+ 3

IV

+ 4

2 He

4-00 4

3 Li

6-94 6, 7

4 Be

9-1

9

5B 10-9 10, 11

60

12-00 12

10 Ne

20-2 20, 22

11 Na

23-00 23

12 Mg

24-32

24, 25, 26

13 AI

26-96

14 Si 28-3 28,29

18 A

39-9 36, 40

19 K

39-1 39, 41

29 Cu

63-57

20 Ca

40-07

30 Zn

65-37

21 Sc 45-1

31 G

70-1

22 Ti 48-1

32 Ge

72-5

36 Kr

82-92

78, 80, 82, 83, 84, 86

37 Rb

85-45

85, 87

47 Ag 107-88

38 Sr

87-83

48 Cd 112-40

39 Y

89-33

49 In

114-8

40 Zr

90-6

50 Sn

118-7

54 Xe

130-2

129, 131, 132, 134, 136

55 Cs

132-81

133

56 Ba

137-37

57 La 58 Ce 139-0 140-25

59 Pr eONd 61 62 Sm 63 Eu = 

    64  Gd           65  Tb

140-6 144-3 150-4 152-0 = 

     157-3           159-2

66 Ds 67 Ho 68 Ev 69 Tu 70 Yb 7= 1 Lu 72 (Kt) 162-5 163-5 1677 168-5 173-5 = 

175

79 Au

197-2

80 Hg

200-6 197-204

81 Tl

204-0

82 Pb

207-2

86 Em

222-0

87-

88 Ra

226-0

89 Ac

90 Th

232-15

144

considerable certainty from the positions of their atomic numbers. From the point of view of the construction of the atom the inert gas= es should mark the end of the periods as they are shown to do ua the hst of = atomic weights in Appendix I, on the other hand it is more usual in chemistry = to start with valency 0. From principles of general convenience of arrangement t= he latter plan is adopted in this table, which is intended to give = the maximum amount of chemical information. Hydrogen, which belongs equally wel= l to group I or group VII, is best omitted from the. table altoget= her.

THE ELEMENTS


V

VI

VII

VIII

3

2

-

-1

7N

80

9F

14-01

16-00

1900

14

16

19

15 P

16 S

17 CI

31-04

32-06

35-46

31

32

35, 37

23 V

24 Cr

25 Mn

26 Fe

27 Co

28 Ni

Sl-O

33 As

74-96 75

52-0

34 Se

79-2

54-93

35 Br

79-92 79, 81

55-85

58-97

58-68 58.60

41 Nb

42 Mo

43

44 Ru

45 Rh

46 Pd

93-5

51 Sb 120-2

96-0

52 Te 127-5

531

126-92 127

101-7

102-9

106-7

73 Ta

74 W

7&-

76 0a

77 Ir

78 Pt

181-5

83 Bi

209-0

184-0

84 Po

85

190-9

1931

195-2

91 UX

ii

92 U

238-2

145

Recent results obtained by Dempster. Thanks to a private=

communication the writer is able to include some further results=

obtained by Dempster and a diagram of his apparatus for obtaining=


Fig. 19. Diagram of Anode in Dempster's latest apparatus.=


positive rays from metals. A full account is to appear in the Physical Review. Fig. 19 shows the new arrangement of vaporising furnace A and ionising filament C. The analysing apparatus has already been described on p, 31 and the results wi= th


.4F


5-9


f

'

1

k

Lithium.

\

1

\

1

\

)

J

[

<=3D/

v..

^^

/

K

9

30


ZO


10


60


6-1


6-9

Atomic Weight.


7-0


7-1


Fig. 20. Curve for Lithium. 146

APPENDIX III

147


magnesium on p. 81. Fig. 20 shows one of the curves obtained with lithium. It will be seen that the relative intensities of t= he isotopes is entirely different from that found by the writer (p. = 

86)

and also disagrees very definitely with the chemical atomic weight= . Dempster describes these relative intensities as varying very considerably. This is a most remarkable phenomenon and further information upon it is very desirable. There seems just a possibi= lity that the 6 line is enhanced by doubly charged carbon but it is = 

not

easy to see where such particles could be produced.

l/oltS 943 928 913-5 899-5 886 873 860 847-5=


J

\

Zinc.

1

t

\

1

\

1

\

f

\

r

\

\

1

1

\

\i

1

1

\

/

\

I

/

1

=C2=AE

l/

\

1

i^

\

^^

62 63 64 65 66 67 Atomic Weight.

Fig. 21. Curve for Zinc.


68 69


70


Fig. 21 gives a remarkable curve obtained from zinc. This indicates three strong isotopes and a faint fourth. The absolute=

scale of atomic weight is not known with certainty, and the valu= es 63, 65, 67, 69 are given by Dempster as those in best agreement=

with the atomic weight 65-37. Considering that the error in th= e


148 APPENDIX III

mean atomic weight of lithium, when calculated on these lines, is about 5 per cent, it would appear possible that these might = be a unit too high or too low. The probability of this is strengthene= d very much by the rule given on p. 110 connecting even atomic number with even atomic weight.

Results with calcium show only one line. This makes it extremely=

probable that this is a simple element of atomic weight 40 and=

therefore an isobare of argon. ^

Note. In a still later communication Dempster states that = 

he

has been successful in using an anode of calcium to which a sma= U quantity of zinc had been added. By this means he is able to compare the masses of the zinc isotopes with the strong calcium=

maximum, assumed as 40. This gives the atomic weights as 64, 66, 68 and 70. The intensities are quite different to those in = the curve given above for zinc. 64 is now the strongest, 66 and 68=

fainter, while 70 is very faint indeed. No explanation is yet advanced for these remarkable irregularities in relative intensity.=


He has also observed a small maximum at 44 invariably accom- panying the strong calcium maximum 40. This he considers to be probably due to an isotope of that element present in smaU quant= ity as suggested by the atomic weight 40 07.

The above values are included provisionally in the tables on pages 89 and 142.

" V. p. 88.


INDEX

Abnormal hydrides, 98

Abundance of the elements, 111

Accuracy of mass-spectrograph, 60

Actinivim chain, 14, 15

Additive law of mass, 99

Alkali metals, mass-spectra of, 83

Alpha ray changes, 13

Analysis of the elements, 63

Andrade and Rutherford, 11

Anode, composite, 80, 86 

      hot,  80,  83,  84

Anticathode, silica, 48

Antimony, 78

Argon, 66

Aronbeeg, 123

,, and Harkins, 124

Atmolysis, separation by, 127

Atomic number, 13, 93 

      theory,  2

,, volume of isotopes, 18 

      weights,  tables  of,  89,  141
      weights  of  radio -elements,  13,

141

Atoms, structure of, 90

Balke, Owens and Kremers, 142 Barkla, 93

Batuecas and Moles, 141 Baxter and Hodges, 142 and Parsons, 113 and Starkweather, 141 and Wilson, 142 Tani and Chapin, 142 Weatherell and Holmes, 73, 142 Beryllium, 88 Beta ray change, 13 Bohr, 94, 95, 121, 122, 123

,, atom, 95 BOLTWOOD, 1, 7 Boron, 72 

     anomalous  atomic  weight  of,

114 

     trifluoride,  73

Bracketing, method of, 59, 69 Brauner and Krepelka, 141 Broek, Van den, 93, 94, 116 Bromine, 76


Bronsted and Hevesy, 135, 136, 139

Brosslera, 102, 104

Bruylants and Michielson, 142

Caesium, 87

,, anomalous atomic weight of, 114 Calcium, 88, 148 Calibration curve, 55 Camera of mass-spectrograph, 51 

      positive  ray,  26

Canalstrahlen, 22 Carbon, 63

Carnotite, lead from, 124 Cathode rays, 22, 24 Chadwick, 94 

 and  Rutherford,  103

Chapin, Baxter and Tani, 142 Chapman, 130 

        and  DooTSON,  130

Chemical action, separation by, 133 

       law  of  radioactive  change,

11 Chlorine, 65, 113 

       separation  of   the  isotopes

of, 136 Classen, 31

and Wey, 142 Claude, 35 Cleveite, lead from, 17 Coincidence, method of, 57 Composite anode, 80, 86 Constancy of chemical atomic weights,

22 Cosmical effect of change of mass, 103 Crookes, 3, 4, 24, 115, 117 ,, dark space, 24, 35 

       theory  of  the  evolution  of

elements, 117 Curie, Mlle. I., 113 

    M.,  18

Dalton's hypothesis, 2 Darwin, 15

Davies and Horton, 68 Deflection of positive rays, 27 Dempster, 31, 80, 81, 86, 114, 146


149


150


INDEX


Dempster's method of analysis, 31,146 Density balance, 35

,, of isotopic leads, 17, 18 Diffusion of neon, 39

separation by, 127 velocity, determination of, 20 Disintegration theory of the evolu- tion of elements, 116 Distillation of neon, 37 Distribution of lines on mass-

spectrum, 64 DooTSON and Chapman, 130 Du Bois magnet, 61

Eddington, 104

Einstein's theory of relativity, 103 Electrical theory of matter, 90 Electric discharge in gases, 23

,, field of mass-spectrograph, 50 Electricity as an element, 115 Electrochemical properties of isotopes,

10 Electron, the, 91

Element, meaning of the word, 115 Enskog, 130 Epstein, 95 ExNER and Haschek, 121

Fa JANS, 11

First order lines, 61

Fleck, 12

Fluorine, 72, 97

Focussing positive rays, 44

FOWLEB, 123 

      and  Aston,  45

Fractional distillation, separation by,

133 Fbanck and Knipping, 68

Gehrcke, 102

,, and Reichenheim, 80, 83, 88 Geigek and Nuttall, 10, 13 Goldstein, 22 Gravitation effect on spectra, 121 

       separation  by,  131

Groh and Hevesy, 20, 135

Hahn, 8 

       and  Meitner,  8

Halation effect, 60 Half-tone plates, 25 Hall and Harkins, 116 Harkins, 102, 111, 116, 129 

        and  Aronberg,  124

        and  Hall,  116

,, and Wilson, 116 Haschek and Exner, 121 Helium, 67, 69, 106


Hevesy, 10, 12, 19 

      and  Bronsted,  136,  136,

139 

      and  Groh,  20,  135
      and  Paneth,  11
      and  Zechmeisteb,  20

Hodges and Baxter, 142 Holmes, Baxteb and Weathebell,

73, 141 Honigschmid, 17, 18, 141, 142 

 and     Horovitz,     18,

121 Horovitz and Honigschmid, 18, 121 HoBTON and Davies, 68 Hot anode, 80, 83, 84 Hydrochloric acid, diffusion of, 129 Hydrogen, 67, 69, 106 Hyman and Soddy, 17, 121

Ibbs, 130

Imes, 125, 126

Indicators, radioactive, 19

Infra-red spectrum of isotopes, 125

Intensity of positive rays, 44

Iodine, 78

Ionic dissociation theory, proof of, 20

lonisation in discharge tube, 24

Ionium, 1, 7, 9, 18

,, atomic weight of, 18 Isobares, 12, 13, 97, 110 Isotopes, definition of, 12

diagrams of, 97

discovery of, 5

melting point of, 18

refractive index of, 18

separation of, 127

solubility of, 18

table of, 89, 141

James and Stewabt, 142 JoLY and Poole, 133

Keetman, 7

Kernel of atom, 98

Kibchoff, 116

Knipping and Franck, 68

kohlweiler, 116

Kratzer, 126

Kremers, Owens and Balke, 142

Krepelka and Bbaun, 141

,, and RiCHABDS, 141

Krypton, 70

,, anomalous atomic weight of, 114

Landaueb and Wendt, 70 Langmuib, 95, 96, 99 Lead, atomic weight of, 16

,, from carnotite, 124

,, from thorite, 17 

     isotopes  of,  14,  15


INDEX


15)


Lembert and Richards, 17, 121 Lewis-Langmuir atom, 95 LmDEMANN, 102, 124, 134, 139

,, and Aston, 131

Lines of first and second order, 61, 76 

     of  reference,  55,  64

Lithium, 86, 97, 146 LooMis, 125, 126

LUDLAM, 129

McAxpiNE and Willard, 142

Magnesimn, 80

Magnetic field of mass-spectrograph,

51 Marckwald, 7, 8 Mass, change of, 100 

     deduced  from  parabolas,  28

    deduced  from  mass -spectrum,

55 Mass-spectrograph, 43 Mass-spectrum, 47, 54 Measurement of lines on mass-

spectrum, 59 Meitner, 21

,, and Hahn, 8 Melting point of isotopes, 18 Mercury, 72, 80 

 parabolas  of,  30

        separation  of  the  isotopes

of, 134 Merton, 121, 123, 124, 125 Mesothorium, 8, 10 Meta-elements, 4

Metallic elements, mass-spectra of, 80 Meteoric nickel, 113 MiCHiELSON and Bruylants, 142 Microbalance for density, 35 MiLLIKAN, 22, 91

Molecular lines of second order, 75 Moles and Batuecas, 141 MOSELEY, 11, 93, 115 Mtjller, 142 Multiply charged rays, 30

Natural numbers and atomic weights,

111 Negatively charged rays, 29, 62 Negative mass-spectra, 62, 66 Neon, 1, 33, 64, 97 Neuberger, 21 Nickel, 79 

     meteoric,  113

Nitrogen, 67, 110 Nomenclature of isotopes, 61 Nucleus atom, 10, 92, 97, 125 

       structure  of,  101

Ntjttall and Geiger, 10, 13

Order, lines of first and second, 61 Owens, Balke and Kremers, 142 Oxygen, 63


Packing effect, 100 Paneth and Hevesy, 11 Parabola method of analysis, 25 Parsons and Baxter, 113 Perforated electrodes, 22, 24 Periodic law, 11, 12, 34 

       table  of  the  elements,  144,

145 Period of radio-elements, 13 Perrin, 104 Phosphonas, 77

Photochemical separation, 137 Photographic plates for positive rays,

25 Planck's quantum, 95 Planetary electrons, 92 Poole, 133 

     and  JoLY,  133

Positive ray paraljolas, 28 

       rays,  22

     separation    by,    136

Potassium, 87 Pressure diffusion, 131 Proton, the, 92 Protyle, 90, 118 Prout's hypothesis, 2, 90, 100


Radioactive isotopes, 7, 14 

       classification  of,

21 

 transformations,  13,  14,

15 Radium B and lead, 11 

       D  and  lead,  11

Ramsay, 115 

        and  Collie,  39
        and  Travers,  33

Ratner, 24 Rayleigh, 127 Reference lines, 55, 64 Refractive index of isotopes, 18 Reichenheim and Gehrcke, 80, 83,

88 Renz, 139

Resolving power of mass-spectro- graph, 60 Richards 17 

        and  Krepelka,  141
        and  Lembert,   17,   121
        and  Wads  WORTH,  17

Richardson, 85 Rossi and Russell, 9, 120 Rubidium, 87 Russell, U 

       and  Rossi,  9,  120

Rutherford, Sir E., 7, 9, 13, 92, 93, 102 

 and  Chadwick,  103

 and  Andrade,  11

Rydberg, 141


162


INDEX


SCHUTZENBERGER, 3

Screens, willemite, 25

Secondary rays, 29

Second order, lines of the, 61

Selenium, 77

Separation of isotopes, 127

Silicon, 72 

      fluoride,  74

Skaupy, 139

Slit system of mass-spectrograph, 49 Smith and Van Haagen, 72 SoDDY, 6, 8, 10, 11, 12, 13, 14, 16, 17, 35 

      and  Hyman,  17,  121

Sodium, 86 Solubility of isotopes, 18

SOMMERFEIiD, 95

Spectra of isotopes, 9, 121,

Spectrum lines, form of, 53

Spencer, 91

Starkweather and Baxter, 141

Stas, 91

Statistical relation of isotopes, 109

Stewart, 11, 12 

        and  James,  142

Sulphur, 76

Tani, Baxter and Chapin, 142 Tellurium, 77 Thermal diffusion, 129 Third order line of argon, 67 

      lines  of,  61

Thomson, G. P., 86, 88

Sir J. J., 1, 22, 29, 33, 62, 70, 72, 75, 84, 91, 129 Thorite, 17, 18 Thorium, 7, 9, 14, 15, 18, 120


Thorium, chain, 17, 18, 116

,, atomic weight of, 18

Tin, 78 Travers, 39 

       and  Ramsay,  33

Triatomic hydrogen, 70

Unitary theory of matter, 90 Uranium, 10, 120 ,, chain, 15

Valency electrons, 98

Van Haagen and Smith, 72

Wadsworth and Richards, 17 Watson, 33 

       and  Aston,  24,  35

Weatherell, Baxter and Holmes,

73, 141 Welsbach, 8

Wendt and Landaueb, 70 Wey and Classen, 142 Whole number rule, 90 WiEN, 22

WiLLARD and McAlpine, 142 Willemite screens, 25 Wilson and Baxter, 142 

       and  Harkins,  116

Xenon, 70

anomalous atomic weight of, 114

X-ray spectra of isotopes, 1 1

Zechmeister and Hevesy, 20 Zinc, 147

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