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  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  ancy 
  is 
  accounted 
  for 
  by 
  the 
  fact, 
  discussed 
  later, 
  that 
  when 
  elec- 
  

   trically 
  charged 
  particles 
  are 
  grouped 
  together 
  their 
  combined 
  mass 
  

   differs 
  slightly 
  from 
  the 
  sum 
  of 
  their 
  separate 
  masses. 
  We 
  may 
  con- 
  

   clude, 
  therefore, 
  that 
  all 
  atoms 
  are 
  built 
  up 
  of 
  hydrogen 
  atoms. 
  We 
  

   shall 
  see 
  later 
  that 
  the 
  hydrogen 
  atom 
  itself 
  consists 
  of 
  one 
  negative 
  

   electron 
  and 
  the 
  part 
  that 
  remains, 
  which 
  is 
  called 
  the 
  positive 
  elec- 
  

   tron. 
  The 
  positive 
  electron 
  carries 
  an 
  electric 
  charge 
  equal 
  to 
  that 
  of 
  

   a 
  negative 
  electron, 
  but 
  of 
  opposite 
  sign, 
  and 
  is 
  1,846 
  times 
  heavier. 
  

   Thus 
  we 
  go 
  a 
  step 
  further 
  and 
  conclude 
  that 
  all 
  atoms 
  are 
  built 
  up 
  

   of 
  positive 
  and 
  negative 
  electrons. 
  

  

  Why 
  was 
  not 
  this 
  simple 
  integral 
  relationship 
  between 
  atomic 
  

   weights 
  discovered 
  long 
  ago, 
  since 
  chemists 
  have 
  accurately 
  known 
  

   atomic 
  weights 
  for 
  many 
  years 
  ? 
  Simply 
  because 
  chemical 
  methods 
  

   of 
  determining 
  atomic 
  weights 
  measure 
  only 
  the 
  average 
  weight 
  of 
  a 
  

   great 
  number 
  of 
  atoms, 
  but 
  the 
  method 
  described 
  above 
  measures 
  the 
  

   weights 
  of 
  individual 
  atoms. 
  In 
  the 
  case 
  of 
  the 
  element 
  chlorine, 
  for 
  

   instance, 
  the 
  chemical 
  determinations 
  give 
  the 
  weight 
  equal 
  to 
  35.46 
  

   times 
  our 
  unit, 
  but 
  the 
  deflection 
  method 
  shows 
  that 
  there 
  are 
  three 
  

   different 
  kinds 
  of 
  chlorine 
  atoms, 
  of 
  weights 
  exactly 
  35, 
  37, 
  and 
  39, 
  

   which 
  are 
  chemically 
  inseparable 
  and 
  which 
  are 
  present 
  in 
  such 
  rela- 
  

   tive 
  proportions 
  as 
  to 
  make 
  the 
  average 
  atomic 
  weight 
  35.46. 
  These 
  

   different 
  kinds 
  of 
  chemically 
  similar 
  atoms, 
  with 
  different 
  masses, 
  are 
  

   called 
  isotopes. 
  It 
  has 
  been 
  found 
  that 
  isotopes 
  exist 
  in 
  a 
  large 
  

   number 
  of 
  the 
  chemical 
  elements, 
  but 
  that 
  the 
  weight 
  of 
  every 
  indi- 
  

   vidual 
  atom 
  or 
  isotope 
  is 
  an 
  exact 
  multiple 
  of 
  that 
  of 
  the 
  funda- 
  

   mental 
  unit. 
  

  

  If 
  positive 
  electrons, 
  or 
  the 
  massive 
  part 
  of 
  hydrogen 
  atoms, 
  are 
  

   parts 
  of 
  the 
  structure 
  of 
  all 
  atoms, 
  we 
  might 
  expect 
  to 
  be 
  able 
  to 
  

   break 
  up 
  heavier 
  atoms 
  into 
  hydrogen. 
  This 
  has 
  actually 
  been 
  done 
  

   by 
  Professor 
  Rutherford 
  in 
  the 
  case 
  of 
  nitrogen, 
  aluminium, 
  and 
  a 
  

   number 
  of 
  other 
  elements. 
  

  

  HOW 
  ELECTRONS 
  ARE 
  ARRANGED 
  IN 
  ATOMS. 
  

  

  Thus 
  we 
  have 
  both 
  direct 
  and 
  indirect 
  evidence 
  that 
  atoms 
  are 
  

   structures 
  built 
  out 
  of 
  positive 
  and 
  negative 
  electrons. 
  The 
  next 
  

   question 
  is, 
  "How 
  are 
  these 
  electrons 
  arranged 
  in 
  the 
  various 
  atoms? 
  " 
  

   A 
  good 
  deal 
  is 
  known 
  about 
  this 
  arrangement, 
  as 
  I 
  shall 
  proceed 
  to 
  

   indicate, 
  but 
  there 
  is 
  much 
  more 
  which 
  is 
  still 
  unknown. 
  

  

  The 
  nuclear 
  structure 
  of 
  atoms. 
  — 
  Radium 
  and 
  the 
  other 
  radio- 
  

   active 
  elements 
  owe 
  their 
  unusual 
  properties 
  to 
  the 
  fact 
  that 
  they 
  

   emit 
  positively 
  and 
  negatively 
  charged 
  particles, 
  called 
  a 
  and 
  (3 
  

   particles, 
  respectively, 
  with 
  tremendous 
  velocities. 
  By 
  the 
  bending 
  

   of 
  their 
  paths 
  in 
  electric 
  and 
  magnetic 
  fields, 
  or 
  by 
  other 
  methods, 
  it 
  

   is 
  found 
  that 
  the 
  (J 
  particles 
  are 
  negative 
  electrons 
  which 
  have 
  veloci- 
  

   ties 
  as 
  large 
  as 
  97 
  per 
  cent 
  of 
  the 
  velocity 
  of 
  light, 
  or 
  about 
  180,000 
  

  

  