I 
^"'■^May'!''r92":l         ^^toms  and  Chemical  Valence.  317 
In  the  first  place,  the  properties  of  Hthium,  berylhum,  etc., 
show  that  in  these  elements  also  the  first  two  electrons  are  held  firmly 
while  the  remainder  are  easily  detached.  Thus,  lithium  readily 
forms  a  univalent  positive  ion  by  the  detachment  of  one  of  the  three 
electrons  in  its  neutral  atom.  The  divalence  and  other  proper- 
ties of  beryllium  prove  that  there  is  little  or  no  tendency  for  a  second 
stable  pair  of  electrons  to  surround  the  first  pair. 
In  the  second  place,  the  absence  of  irregularities  in  the  observed 
K  and  L  series  of  the  X-ray  spectra  of  the  various  elements  proves 
that  there  are  no  sudden  changes  in  the  number  of  electrons  in  the 
innermost  layers  of  electrons  about  the  nucleus.  From  these  two 
sets  of  facts,  as  well  as  from  other  evidence,  we  may  take  it  as  a 
fundamental  principle  that  the  arrangement  of  the  inner  electrons 
undergo  no  change  as  we  pass  from  elements  of  smaller  to  those  of 
higher  atomic  number. 
The  properties  of  neon  indicate  that  its  atoms  are  more  stable  than 
those  of  any  other  element  except  helium.  Since  the  atomic  num- 
ber is  10,  and  the  first  2  electrons  form  a  stable  pair  about  the  nucleus 
as  in  the  helium  atom,  it  follows  directly  that  the  other  eight  elec- 
trons arrange  themselves  in  a  second  layer  or  shell  possessing  a  very 
high  stability.  If  these  8  electrons  revolved  about  the  nucleus  in  a 
single  circular  orbit  or  ring,  as  would  be  suggested  by  Bohr's  theory, 
there  is  no  apparent  reason  why  there  should  by  any  very  great 
difference  in  stability  between  rings  having  7,  8  or  9  electrons. 
On  the  other  hand,  we  readily  see  that  the  geometrical  symmetry  of 
the  arrangement  of  the  8  electrons  at  (or  rotating  about)  the  8  cor- 
ners of  a  cube  would  not  only  account  for  a  high  degree  of  sta- 
bility but  for  the  fact  that  an  arrangement  of  7  or  9  electrons  would 
have  no  such  stability.  Chemical  considerations  and  Born  and 
Lande's  work  on  compressibility  also  lead  us  to  this  spatial  arrange- 
ment of  the  electrons.  We  shall  refer  to  the  stable  group  of  8  elec- 
trons by  the  term  octet.  From  the  principles  already  enunciated 
it  is  clear  that  in  the  atoms  of  all  the  elements  above  neon  the  inner 
electrons  are  arranged  in  the  same  way  as  those  of  neon. 
From  the  atomic  numbers  of  the  inert  gases  we  are  thus  able  to 
determine  the  number  of  electrons  in  the  various  layers  or  shells 
of  electrons  which  exist  in  the  atoms.  The  results  are  summarized 
in  Table  i. 
Thus  the  xenon  atom  with  an  atomic  number  54  contains  54 
electrons  arranged  as  follows:    Close  to  the  nucleus  are  two  elec- 
