﻿NUCLEUS 
  OF 
  THE 
  ATOM 
  CEOWTHER 
  213 
  

  

  theory 
  would 
  lead 
  us 
  to 
  expect 
  that 
  the 
  mass 
  of 
  the 
  hydrogen 
  nucleus 
  

   would 
  be 
  somewhat 
  less 
  when 
  it 
  Avas 
  closely 
  packed 
  together 
  with 
  

   other 
  similar 
  particles 
  than 
  when 
  in 
  an 
  isolated 
  state. 
  It 
  is 
  generally 
  

   held 
  at 
  present 
  tliat 
  the 
  hydrogen 
  nucleus 
  is 
  the 
  positive 
  counterpart 
  

   of 
  the 
  negative 
  electron; 
  the 
  second 
  of 
  the 
  two 
  fundamental 
  units 
  

   from 
  which 
  all 
  matter 
  is 
  built. 
  For 
  this 
  reason 
  Sir 
  Ernest 
  Ruther- 
  

   ford 
  proposes 
  to 
  call 
  it 
  a 
  proton. 
  The 
  mass 
  of 
  any 
  atom 
  would 
  thus 
  

   be 
  that 
  of 
  tlie 
  protons 
  contained 
  in 
  its 
  nucleus. 
  The 
  u.raniuni 
  nucleus 
  

   must 
  therefore 
  contain 
  about 
  238 
  protons, 
  inclosed 
  in 
  a 
  volume 
  whose 
  

   radius 
  is 
  certainly 
  not 
  more 
  than 
  6X10-^^ 
  cm. 
  The 
  proton 
  is 
  clearly 
  

   then 
  a 
  very 
  minute 
  particle. 
  

  

  Assuming 
  that 
  Aston's 
  experiments 
  indicate 
  that 
  every 
  atomic 
  

   nucleus 
  is 
  built 
  up 
  of 
  an 
  integral 
  number 
  of 
  protons, 
  we 
  can 
  easily 
  

   determine 
  the 
  constituents 
  of 
  any 
  given 
  nucleus. 
  The 
  mass 
  of 
  the 
  

   nucleus, 
  that 
  is 
  to 
  all 
  intents 
  and 
  purposes 
  the 
  mass 
  of 
  the 
  atom, 
  tells 
  

   us 
  the 
  number 
  of 
  protons 
  contained 
  in 
  the 
  nucleus 
  ; 
  the 
  atomic 
  num- 
  

   ber 
  of 
  the 
  element 
  gives 
  us, 
  as 
  vre 
  have 
  already 
  seen, 
  the 
  resultant 
  

   positive 
  charge. 
  Except 
  in 
  the 
  case 
  of 
  hydrogen 
  the 
  atomic 
  number 
  

   is 
  less 
  than 
  the 
  atomic 
  weight. 
  Consequently 
  part 
  of 
  the 
  charge 
  on 
  

   the 
  protons 
  must 
  be 
  neutralized 
  by 
  a 
  negative 
  charge, 
  presumably 
  

   supplied 
  by 
  an 
  appropriate 
  number 
  of 
  negative 
  electrons. 
  The 
  helium 
  

   nucleus, 
  for 
  example 
  has 
  a 
  mass 
  of 
  four, 
  and 
  must, 
  therefore, 
  contain 
  

   four 
  protons. 
  Its 
  nuclear 
  charge 
  is, 
  however, 
  two, 
  and 
  consequently, 
  

   since 
  the 
  charge 
  on 
  the 
  proton 
  is 
  numerically 
  equal 
  to 
  that 
  on 
  an 
  

   electron, 
  the 
  helium 
  nucleus 
  must 
  also 
  contain 
  two 
  negative 
  electrons. 
  

   It 
  is 
  interesting 
  to 
  notice 
  that, 
  since 
  an 
  electron 
  has 
  a 
  diameter 
  of 
  

   about 
  4X10"^^ 
  cm., 
  and 
  the 
  volume 
  of 
  the 
  proton 
  is 
  negligible, 
  this 
  

   structure 
  would 
  have 
  dimensions 
  agreeing 
  quite 
  closely 
  with 
  those 
  

   suggested 
  by 
  Chadwick 
  and 
  Bieler. 
  The 
  presence 
  of 
  negative 
  elec- 
  

   trons 
  in 
  the 
  nuclei 
  of 
  the 
  radioactive 
  elements 
  is 
  proved 
  by 
  their 
  

   y8 
  ray 
  activity. 
  The 
  majority 
  of 
  the 
  high 
  speed 
  electrons 
  making 
  

   up 
  the 
  ft 
  radiation 
  undoubtedly 
  come 
  from 
  the 
  disintegrating 
  nucleus. 
  

   The 
  i:)resence 
  of 
  negative 
  electrons 
  in 
  the 
  nuclei 
  of 
  the 
  lighter 
  ele- 
  

   ments 
  is, 
  therefore, 
  not 
  surprising. 
  

  

  It 
  will 
  be 
  seen 
  that 
  on 
  these 
  supj^ositions 
  a 
  nucleus 
  with 
  a 
  given 
  

   nuclear 
  charge 
  can 
  be 
  built 
  up 
  in 
  many 
  different 
  ways. 
  Thus 
  the 
  

   lithium 
  nucleus, 
  with 
  a 
  resultant 
  charge 
  of 
  three, 
  might 
  equally 
  well 
  

   consist 
  of 
  six 
  protons 
  and 
  three 
  electrons, 
  or 
  of 
  seven 
  protons 
  and 
  

   four 
  electrons. 
  Since 
  the 
  chemical 
  and 
  physical 
  properties 
  of 
  an 
  ele- 
  

   ment 
  depend 
  only 
  on 
  the 
  arrangement 
  of 
  the 
  planetary 
  electrons, 
  

   which 
  in 
  turn 
  depends 
  only 
  on 
  the 
  resultant 
  nuclear 
  charge, 
  both 
  the 
  

   nuclei 
  would 
  give 
  rise 
  to 
  atoms 
  possessing 
  the 
  same 
  properties. 
  Both 
  

   kinds 
  of 
  atoms, 
  the 
  first 
  with 
  an 
  atomic 
  mass 
  six, 
  the 
  second 
  with 
  

   atomic 
  mass 
  Feven, 
  have 
  been 
  observed 
  by 
  Aston 
  in 
  lithium. 
  The 
  

   74906—28 
  15 
  

  

  