3i8 
Atoms  and  Chemical  Valence. 
Am.  Jour.  Pharm. 
May,  1920. 
Table  i — Distribution  of  Electron  in  the  Various  Shells. 
Inert  Gas 
Number  of  Corresponding  to 
Shell.                                                 Electrons.  Completed  Layer. 
ist  shell                                       2=2Xi2  He  2 
2nd  shell,  ist  layer                       8  =  2  X  2^  Ne  10 
2nd  shell,  2nd  layer                      8  =  2X2^  AriS 
3rd  shell,  ist  layer                      18  =  2  X  3^  Kr  36 
3rd  shell,  2nd  layer                     18  =  2X3^  Xe  54 
4th  shell,  I  St  layer                     32  =  2  X  4^  Nt  86 
Irons  which  constitute  the  first  shell.  This  is  surrounded  by  the 
second  shell  which  contains  two  "layers"  of  8  electrons  each.  The 
third  shell,  which  in  the  xenon  atom  is  the  outside  shell,  contains 
18  electrons. 
An  examination  of  the  numbers  of  electrons  in  the  layers  (Table  i , 
2nd  column)  shows  that  they  bear  a  simple  mathematical  relation 
to  each  other,  namely,  that  they  are  proportional  to  the  squares 
of  the  successive  integers  i,  2,  3  and  4.  This  is  to  be  looked  upon 
as  perhaps  the  most  fundamental  fact  underlying  the  periodic  ar- 
rangement of  the  elements.  It  is  significant  that  in  Bohr's  theory 
these  same  numbers,  i,  4,  9,  16,  etc.,  play  a  prominent  part.  Thus 
the  energies  of  the  electron  in  the  various  "stationary  states"  are 
proportional  to  i,  V4,  V9,  Vi6»  etc.,  and  the  diameters  of  the  various 
possible  orbits  in  Bohr's  theory  are  proportional  to  i,  4,  9,  16,  etc. 
In  Bohr's  theory  the  various  stationary  states  correspond  to  differ- 
ent number  of  quanta  (Planck's  quantum  theory),  the  innermost 
orbit  corresponding  to  one  quantum,  the  second  orbit  to  two  quanta, 
etc.  We  should  thus  consider  (Table  i)  that  the  electrons  in  the 
ist  shell  are  monoquantic,  those  in  both  layers  of  the  2nd  shell  are 
diquantic,  etc.  It  is  interesting  that  Born  and  Lande,  from  quite 
other  evidence,  have  concluded  that  the  outermost  electrons  of  the 
chlorine  atom  (2nd  layer  of  the  2nd  shell)  are  diquantic  instead  of 
triquantic,  as  was  at  first  assumed. 
The  foregoing  theory  of  the  arrangement  of  electrons  in  atoms 
explains  the  general  features  of  the  entire  periodic  system  of  the 
elements  and  is  particularly  successful  in  accounting  for  the  posi- 
tion and  the  properties  of  the  so-called  8th  group  and  the  rare  earth 
elements.  It  also  serves  to  correlate  the  magnetic  properties  of  the 
elements. 
Let  us  now  consider  the  bearing  of  this  theory  of  atomic  struc- 
ture on  the  phenomena  of  chemical  valence.    The  outstanding  fea- 
