M'ay^^9'2o:}        ^toMs  Gfid  Chemical  Valence.  321 
covered  by  the  work  of  Werner,  and  helps  to  simpHfy  the  theory  of 
such  compounds.  There  is  no  time,  however,  to  go  into  this  sub- 
ject. 
The  simple  theory  of  atomic  structure  which  we  have  discussed 
thus  far  elxplains  perfectly  what  has  usually  been  called  "the  maxi- 
mum positive  and  negative  valence."  The  maximum  positive 
valence  represents  the  number  of  electrons  which  the  atom  possesses 
in  excess  of  the  number  needed  to  form  one  of  the  particularly  stable 
configurations  of  electrons.  On  the  other  hand,  the  maximum 
negative  valence  is  the  number  of  electrons  which  the  atom  must 
take  up  in  order  to  reach  one  of  these  stable  configurations. 
For  example,  magnesium  has  a  positive  valence  of  two,  since  its 
atomic  number  is  12,  while  that  of  neon  is  10.  Sulphur  has  a  posi- 
tive valence  of  6,  since  it  has  6  electrons  more  than  neon ;  but  it  has 
a  negative  valence  of  two  because  it  must  take  up  more  electrons 
before  it  can  assume  a  form  like  that  of  the  argon  atom. 
It  is  clear,  however,  that  this  theory  of  valence  is  not  yet  com- 
plete.^ It  is  not  applicable  to  those  cases  where  we  have  usually 
taken  valences  of  4  for  sulphur,  or  3  and  5  for  chlorine,  etc.  But 
more  especially  it  does  not  explain  the  structure  of  organic  com- 
pounds and  such  substances  as  Hj,  CI2,  O2,  N2H4,  PCI3,  etc. 
J.  J.  Thomson,  Stark,  Bohr,  and  others  had  suggested  that  a 
pair  of  electrons  held  in  common  by  two  adjacent  atoms  may  func- 
tion in  some  cases  as  chemical  bonds  between  the  atoms,  but  this 
idea  had  not  been  combined  with  the  conception  of  the  stable  groups 
of  electrons  or  octets.  G.  N.  Lewis,  in  an  important  paper  in  191 6, 
advanced  the  idea  that  the  stable  configurations  of  electrons  in 
atom  could  share  pairs  of  electrons  with  each  other  and  he  identified 
these  pairs  of  electrons  with  the  chemical  bond  of  organic  chemis- 
try. This  work  of  Lewis  has  been  the  basis  and  the  inspiration  of 
my  work  on  valence  and  atomic  structure. 
As  a  result  of  the  sharing  of  electrons  between  octets,  the  num- 
ber of  octets  that  can  be  formed  from  a  given  number  of  electrons  is 
increased.  For  example,  two  fluorine  atoms,  each  having  seven 
electrons  in  its  outside  shell,  would  not  be  able  to  form  octets  at  all 
except  by  sharing  electrons.  By  sharing  a  single  pair  of  electrons, 
however,  two  octets  holding  a  pair  in  common  required  only  14 
1  The  theories  of  Kossel,  Lacomblc,  Teudt,  etc.,  which  have  recently  been 
proposed  in  Germany,  have  not  advanced  beyond  this  point  and  are  therefore 
very  unsatisfactory  as  a  general  theory  of  valence. 
