WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 59I 



constituents shall be possessed of fairly large outside fields. In 

 compounds where the binding electrons are held between the 

 electropositive and electronegative portions, the fields are more 

 condensed together and more stable than those of the extreme 

 electrolytes. Complexes formed by them are more stable than 

 those involving ionizing substances. The conditions for stability 

 of a hydrate can be shown by an example. In a solution of zinc 

 sulfate the zinc ions, and probably to a lesser extent the sulfate 

 ions, will be combined with varying numbers of water molecules, 

 that is, probably some ions will be anhydrous, some will have 

 one water molecule attached, etc. As the concentration of the 

 solution is increased, some of the zinc ions will hold to sulfate 

 ions forming hydrated as well as non-hydrated zinc sulfate 

 molecules. The non-hydrated molecule of zinc sulfate by rea- 

 son of the large doublet it contains will tend to become hydrated. 

 There will therefore be in the solution molecules of different 

 degrees of hydration which will in turn tend to acquire more 

 water molecules and also to associate together. When con- 

 centration has progressed far enough, this associating tendency 

 will outweigh the reverse action. That particular hydrate will 

 continue to grow which offers the greatest condensation of the 

 forces involved. This will be usually the highest hydrate within 

 which the forces are great enough to overcome the disrupting 

 effect of thermal agitation. 



Complex ions. — Complex ions result from the interaction be- 

 tween the fields of a neutral molecule and an ion. The ion becomes 

 imbedded within- the fields of the neutral part. A molecule, in 

 order to form the neutral part of a complex ion, must have the 

 following requisites : 



1. It must have fields strong enough to hold the ion. 



2. The attractions within the molecule must be such that it 

 is not dissociated into ions either as a solid or upon solution. 



Such molecules are furnished by compounds of a type inter- 

 mediate between the molecule-forming and the polar classes, 

 where the electron is held so strongly by the electropositive 

 element that it can pass only part of the way over to the negative 

 atom. The complex formed by the addition of potassium cyanide 



