﻿THE 
  NUCLEUS 
  OF 
  THE 
  ATOM 
  ^ 
  

  

  By 
  J. 
  A. 
  Cbowtheb 
  

   The 
  Universitij, 
  Reading, 
  England 
  

  

  Since 
  Kutherford 
  in 
  1911 
  first 
  propounded 
  his 
  nuclear 
  theory 
  of 
  

   the 
  atom, 
  the 
  evidence 
  which 
  has 
  been 
  accumulated 
  from 
  many 
  

   and 
  varied 
  phenomena 
  has 
  been 
  so 
  weighty, 
  and 
  so 
  consistently 
  

   in 
  favor 
  of 
  his 
  suggestions 
  that 
  there 
  is 
  now 
  a 
  very 
  general 
  agree- 
  

   ment 
  among 
  physicists 
  that 
  the 
  nuclear 
  hypothesis 
  embodies 
  the 
  es- 
  

   sential 
  truth 
  about 
  atomic 
  structure. 
  The 
  atom 
  of 
  any 
  substance 
  

   is 
  now 
  regarded 
  as 
  being 
  made 
  up 
  of 
  a 
  central 
  core, 
  or 
  nucleus, 
  of 
  

   almost 
  incredibly 
  small 
  dimensions 
  (in 
  which, 
  nevertheless, 
  prac- 
  

   tically 
  the 
  whole 
  of 
  the 
  mass 
  of 
  the 
  atom 
  resides), 
  surrounded 
  by 
  

   a 
  swarm 
  of 
  negative 
  electrons. 
  The 
  nucleus, 
  as 
  a 
  whole, 
  is 
  posi- 
  

   tively 
  charged 
  and 
  under 
  its 
  attractive 
  force 
  the 
  negative 
  electrons 
  

   describe 
  around 
  it 
  circular 
  or 
  elliptical 
  orbits, 
  in 
  much 
  the 
  same 
  

   way 
  as 
  the 
  planets 
  circulate 
  around 
  the 
  sun. 
  Since 
  the 
  normal 
  

   atom 
  is 
  electrically 
  neutral, 
  the 
  resultant 
  positive 
  charge 
  on 
  the 
  

   nucleus 
  is 
  equal 
  to 
  the 
  sum 
  of 
  the 
  charges 
  on 
  the 
  planetary 
  electrons. 
  

   If 
  we 
  take 
  the 
  charge 
  on 
  a 
  negative 
  electron 
  as 
  our 
  unit 
  of 
  charge 
  

   we 
  may 
  say 
  that 
  the 
  positive 
  charge 
  on 
  the 
  nucleus 
  of 
  an 
  atom 
  is 
  

   numerically 
  equal 
  to 
  the 
  number 
  of 
  planetary 
  electrons 
  it 
  contains. 
  

  

  The 
  problem 
  of 
  atomic 
  structure 
  thus 
  divides 
  into 
  two 
  parts 
  — 
  the 
  

   determination 
  of 
  the 
  arrangement 
  of 
  the 
  planetary 
  electrons, 
  and 
  the 
  

   structure 
  of 
  the 
  central 
  nucleus. 
  The 
  first 
  of 
  these 
  problems 
  has 
  

   been 
  attempted, 
  with 
  remarkable 
  success 
  by 
  Niels 
  Bohr, 
  and 
  the 
  

   general 
  outlines 
  of 
  his 
  solution 
  are 
  w^ell 
  known 
  to 
  most 
  readers. 
  The 
  

   second 
  problem 
  is 
  now 
  being 
  attacked 
  by 
  Sir 
  Ernest 
  Rutherford 
  

   and 
  his 
  pupils. 
  In 
  many 
  ways 
  it 
  is 
  the 
  more 
  difficult 
  problem 
  of 
  

   the 
  two. 
  The 
  motions 
  of 
  the 
  planetary 
  electrons 
  are 
  responsible 
  for 
  

   the 
  optical 
  and 
  X-ray 
  spectra 
  of 
  the 
  atom; 
  while 
  their 
  arrangement 
  

   determines 
  its 
  chemical 
  and 
  physical 
  properties. 
  There 
  is 
  thus 
  a 
  

   wealth 
  of 
  experimental 
  data 
  to 
  guide 
  the 
  adventurer 
  in 
  this 
  realm. 
  

   The 
  nucleus, 
  on 
  the 
  other 
  hand, 
  is 
  a 
  world 
  remote 
  and 
  inaccessible. 
  

   It 
  is 
  true 
  that 
  its 
  charge 
  determines 
  and 
  controls 
  the 
  electronic 
  orbits, 
  

   but 
  at 
  the 
  distances 
  at 
  which 
  these 
  electrons 
  revolve 
  the 
  evidence 
  shows 
  

   that 
  the 
  nucleus 
  acts 
  merely 
  as 
  a 
  point 
  charge, 
  and 
  reveals 
  nothing 
  of 
  

   its 
  structure. 
  The 
  only 
  properties 
  definitely 
  assignable 
  to 
  the 
  nucleus 
  

   are 
  mass, 
  and, 
  where 
  the 
  element 
  is 
  radioactive, 
  radioactivity. 
  These 
  

  

  * 
  Reprinted 
  by 
  permission 
  of 
  the 
  Editor, 
  Prof. 
  E. 
  Rignano, 
  from 
  Scientia, 
  International 
  

   Review 
  of 
  Scientific 
  Syutlicsis, 
  vol. 
  XXXIX, 
  No. 
  CL.XVII-3, 
  Mar. 
  1, 
  1926. 
  Publishers, 
  

   G. 
  E. 
  Stechert 
  and 
  Co., 
  New 
  York. 
  

  

  209 
  

  

  