﻿Em 
  ray 
  required 
  to 
  Ionize 
  an 
  Ati 
  

  

  185 
  

  

  same 
  conditions 
  and 
  with 
  the 
  same 
  apparatus, 
  so 
  that 
  the 
  

   relative 
  values 
  should 
  have 
  a 
  high 
  degree 
  of 
  accuracy. 
  

  

  From 
  the 
  fact 
  that 
  the 
  a-particle 
  on 
  acquiring 
  two 
  electrons 
  

   becomes 
  an 
  atom 
  of 
  helium, 
  and 
  from 
  the 
  many 
  experimental 
  

   results 
  which 
  agree 
  with 
  the 
  conception 
  that 
  the 
  number 
  of 
  

   electrons 
  in 
  the 
  molecule 
  is 
  about 
  half 
  the 
  molecular 
  weight, 
  

   we 
  may 
  conclude 
  that 
  the 
  helium 
  atom 
  contains 
  two 
  electrons 
  

   only. 
  This 
  view 
  is 
  further 
  supported 
  by 
  Bohr's* 
  deductions 
  

   concerning 
  the 
  absorption 
  of 
  a-particles 
  in 
  matter. 
  

  

  From 
  the 
  chemical 
  similarity 
  of 
  the 
  gases 
  of 
  the 
  helium 
  group 
  

   we 
  may 
  venture 
  to 
  assume 
  that 
  in 
  each 
  case 
  two 
  electrons 
  

   only 
  are 
  present 
  in 
  the 
  outer 
  ring, 
  the 
  remainder 
  forming 
  

   rings 
  with 
  such 
  small 
  radii 
  that 
  their 
  effect 
  on 
  the 
  energy 
  

   required 
  to 
  remove 
  one 
  of 
  the 
  outer 
  electrons 
  is 
  balanced 
  

   by 
  a 
  portion 
  (n 
  — 
  2)e 
  of 
  the 
  charge 
  of 
  the 
  positive 
  nucleus. 
  

  

  Accordingly, 
  for 
  our 
  purposes 
  the 
  atoms 
  of 
  the 
  inert 
  

   gases 
  can 
  all 
  be 
  represented 
  by 
  a 
  central 
  charge 
  2e, 
  and 
  two 
  

   electrons 
  at 
  opposite 
  ends 
  of 
  a 
  diameter, 
  2a. 
  

  

  The 
  energy 
  required 
  to 
  remove 
  one 
  of 
  these 
  electrons 
  

  

  from 
  the 
  atom 
  is 
  given 
  bv 
  

  

  - 
  -e. 
  , 
  =1-5 
  

   a 
  la 
  

  

  This 
  corresponds 
  with 
  the 
  energy 
  acquired 
  by 
  falling 
  through 
  

   a 
  difference 
  of 
  potential 
  E, 
  if 
  

  

  Ee=P5- 
  (1) 
  

  

  The 
  value 
  of 
  the 
  atomic 
  radius 
  of 
  helium 
  is 
  given 
  by 
  Jeans 
  

   (• 
  Dynamical 
  Theory 
  of 
  Gases. 
  ? 
  p. 
  3-iO) 
  as 
  '90 
  x 
  10" 
  8 
  ; 
  where 
  

   Avogadro's 
  number 
  has 
  been 
  taken 
  as 
  4 
  x 
  10 
  19 
  . 
  On 
  using 
  

   the 
  value 
  2*75 
  x 
  10 
  19 
  deduced 
  bv 
  Rutherford, 
  which 
  agrees 
  

   well 
  with 
  Planck's 
  value 
  2*75 
  x 
  10 
  19 
  , 
  we 
  get 
  1*09 
  X 
  10" 
  8 
  for 
  

   the 
  atomic 
  radius 
  of 
  helium. 
  

  

  

  

  

  E 
  in 
  

  

  volts. 
  

  

  

  Kelalive 
  

   Badii. 
  

  

  Absolute 
  

   Kadii. 
  

  

  Calculated. 
  

  

  Observed. 
  

  

  He 
  

  

  1 
  

  

  rooxio 
  -8 
  

  

  19-4 
  

  

  205 
  

  

  V 
  

  

  119 
  

   1-68 
  

  

  1-298 
  

   L-83 
  

  

  16*3 
  

   11-6 
  

  

  16 
  

   12 
  

  

  A 
  

  

  ; 
  Kr 
  

  

  1-91 
  

  

  i" 
  s 
  

  

  1015 
  

  

  — 
  

  

  Xe 
  

  

  2*25 
  

  

  245 
  

  

  

  — 
  

  

  The 
  numbers 
  in 
  the 
  second 
  column 
  are 
  the 
  ratios 
  of 
  the 
  

   * 
  Bohr, 
  Phil. 
  Mag. 
  January 
  1913, 
  pp. 
  10 
  31. 
  

  

  