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==CHAPTER  XI - THE  SEPARATION  OF  ISOTOPES==
==CHAPTER  XI - THE  SEPARATION  OF  ISOTOPES==
[[Aston 1922/Chapter 11]]
[[Aston 1922/Chapter 11]]
===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==
==APPENDIX  I==

Revision as of 18:14, 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/Chapter 8

CHAPTER IX - ISOTOPES AND ATOMIC NUMBERS

Aston 1922/Chapter 9

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

Aston 1922/Chapter 11

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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