32b  Atoms  and  Chemical  Valence.        j^'"-  May^^g'S: 
and  fluorine.  Therefore,  the  crystalline  form  of  magnesium  oxide 
and  sodium  fluoride  should  be  identical,  and  this  prediction  of  the 
theory  has  been  confirmed  experimentally  by  Dr.  A.  W.  Hull  by 
the  X-ray  method.  Because  of  the  much  greater  forces  acting  be- 
tween the  ions  as  a  result  of  the  double  charges,  the  stability  of  the 
magnesium  oxide  is  much  higher  than  that  of  the  sodium  fluoride. 
This  is  manifested  by  the  high  melting  point,  low  conductiv^ity,  low 
solubility,  and  hardness  of  magnesium  oxide. 
Phosphorus  (N  =  15)  and  sulphur  (N  =  16)  have,  respectively, 
5  and  6  electrons  more  than  neon,  and  are  thus  capable  of  giving 
up  these  numbers  of  electrons.  If  these  elements  are  brought  into 
contact  with  an  excess  of  fluorine  (which  because  of  its  proximity  to 
neon  has  a  particularly  strong  tendency  to  take  electrons)  all  the 
extra  electrons  pass  to  fluorine  atoms.  Thus  a  sulphur  atom  will 
supply  electrons  to  6  fluorine  atoms  and  will  form  the  compound 
SFe.  The  force  acting  between  the  fluorine  ions  and  the  central 
sulphur  ion  is  still  electrostatic  in  nature  it  must  be  nearly  6  times 
greater  than  the.  force  between  sodium  and  fluorine  ions.  Further- 
more, the  6  fluorine  ions  would  surround  the  sulphur  ion  so  that  there 
would  be  little  stray  field  of  force.  Therefore,  we  should  not  ex- 
pect sulphur  fluoride  to  be  salt-like  in  character  but  to  consist  of 
very  stable  molecules  having  weak  external  fields  of  force  and, 
therefore,  readily  existing  in  the  form  of  a  gas.  As  a  matter  of  fact, 
this  extraordinary  substance  has  these  properties  developed  to  such 
a  degree  that  it  is  an  odorless  and  tasteless  gas  with  a  boiling  point 
of  — 62  deg.  Phosphorus  pentafluoride,  as  would  be  expected 
from  its  less  symmetrical  structure,  is  a  gas  having  greater  chemical 
activity. 
The  fluosilicate  ion  SiFe""  has  a  structure  exactly  like  that  of 
the  sulphur  fluoride  molecule,  since  the  number  and  arrangement 
of  the  electrons  are  the  same.  This  is  clear  if  we  consider  that  the 
atomic  number  of  silicon  is  14  while  that  of  sulphur  is  16.  Thus 
if  we  should  replace  the  nucleus  of  the  sulphur  atom  in  a  molecule 
of  sulphur  fluoride  by  the  nucleus  of  a  silicon  atom,  without  dis- 
turbing any  of  the  surrounding  electrons,  we  would  have  removed 
two  positive  charges  and  would  obtain  a  negative  ion  with  two 
negative  charges  of  the  formula  SiFe  .  In  the  presence  of  potas- 
sium ions  we  would  then  have  the  familiar  salt  potassium  fluosilicate. 
The  theory  is  thus  capable  of  explaining  typical  complex  salts.  In 
fact,  it  is  applicable  to  the  whole  field  of  inorganic  compounds 
