Am.  Jour.  Pharm.) 
May,  1920.) 
Atoms  and  Chemical  Valence. 
319 
ture  of  the  theory  is  that  there  are  certain  groups  of  electrons,  such 
as  the  pair  in  the  first  shell  and  the  octet  in  the  second,  that  have  a 
remarkable  stability.  Those  atoms  in  which  all  the  electrons  form 
parts  of  such  stable  groups  {viz.,  the  inert  gases)  will  have  no  ten- 
dency to  change  the  arrangement  of  their  electrons  and  will  thus 
not  undergo  chemical  change.  Suppose,  however,  we  bring  to- 
gether an  atom  of  fluorine  (N  =  9)  ^  and  an  atom  of  sodium  (N  =  11). 
Ten  electrons  are  needed  for  the  stable  pair  in  the  first  shell  and  the 
octet  in  the  second  shell,  as  in  the  neon  atom„  The  sodium  atom 
has  one  more  electron  than  is  needed  to  give  this  stable  structure 
while  the  fluorine  atom  has  one  electron  too  few.  It  is  obvious 
then  that  the  extra  electron  of  the  sodium  atom  should  pass  over 
completely  to  the  fluorine  atom.  This  leaves  the  sodium  atom  with 
a  single  positive  charge  while  the  fluorine  becomes  negatively  charged. 
If  the  two  charged  atoms  or  ions^  were  alone  in  space  they  would  be 
drawn  together  by  the  electrostatic  force  and  would  move  as  a  unit 
and  thus  constitute  a  molecule.  However,  if  other  sodium  and 
fluorine  ions  are  brought  into  contact  with  the  "molecule"  they  will 
be  attracted  as  well  as  the  first  one  was.  There  will  result  (at  not 
too  high  temperature)  a  space  lattice  consisting  of  alternate  posi- 
tive and  negative  ions  and  the  "molecule"  of  sodium  fluoride  will 
have  disappeared.  Now  this  is  just  the  structure  which  we  find 
experimentally  for  sodium  fluoride  by  Bragg's  method  of  X-ray 
crystal  analysis.  There  are  no  bonds  linking  individual  pairs  of 
atoms  together.  The  salt  is  an  electrolytic  conductor  only  in  so 
far  as  its  ions  are  free  to  move.  In  the  molten  condition  or  when 
dissolved  in  water,  therefore,  it  becomes  a  good  conductor. 
The  case  of  magnesium  (N  =  12)  and  oxygen  (N  =  8)  is  simi- 
lar except  that  two  electrons  are  transferred  from  the  magnesium 
to  the  oxygen  atom.  The  resulting  ions  have  their  electrons  ar- 
ranged exactly  like  those  of  the  neon  atoms  and  the  ions  of  sodium 
^  We  will  denote  the  atomic  number  of  an  element  by  N. 
2  It  is  convenient  and  it  has  been  customary  with  many  physicists  to  speak 
of  a  charged  atom  or  molecule  as  an  ion,  irrespective  of  whether  or  not  the  parti- 
cle is  able  to  wander  under  the  influence  of  an  electric  field.  The  writer  has  used 
the  term  in  this  way  in  his  recent  publications.  This  practice  is  very  distasteful 
to  many  physical  chemists  and  is  apt  to  be  misunderstood  by  them.  Neverthe- 
less, it  seems  to  me  probable,  especially  in  view  of  the  recent  work  of  Milner 
and  Ghosh,  that  it  will  be  desirable  to  abandon  the  physical  chemists'  definition 
of  the  ion  and  to  apply  it  to  all  charged  atoms  or  molecules.  The  ion  which  wan- 
ders may  then  be  referred  to  as  a  "free  ion." 
