590 WYCKOFF: FORCKS BETWEEN ATOMS IN SOLIDS 



1. The degree of association of the solvent (which might be 

 taken as a measure of the potential reactivity), 



2. The moments of the doublets within the solid, 



3. The stray fields within the solid. 



In solids possessing multiply-charged ions the increase in the 

 intensity of the binding fields would, where a condensing together 

 of the fields is possible, be greater than the increase in the mo- 

 ments of the doublets. There would thus be a smaller tendency 

 to dissolve. This tendency to condense together, which would 

 ordinarily be found occurring with a double bonding, depends 

 upon both the number and arrangement of the atoms within 

 the solid. Calcium carbonate is only slightly soluble in water 

 while sodium nitrate, ^^ possessing the same structural arrange- 

 ment, dissolves readily. If the ion possesses about it con- 

 siderable fields, it may be able to form with the molecules of 

 the solvent more or less short-lived compounds. 



This point of view suggests for ionization a mechanism some- 

 what different from that usually accepted. A salt does not 

 usually become ionized at the moment of solution; it is already 

 ionized in the solid state. When a crystal of sodium chloride 

 is added to water, ions, not molecules, are torn from the solid 

 by the process of solution. These ions may, and will when there 

 are enough of them present, combine temporarily to form sodium 

 chloride molecules. The process of solution and ionization is 

 not molecule — > molecule — > ions, but ions — >■ ions — > 



solid solution solution solid solution 



molecule. 



solution 



Molecular Complexes. — Molecular complexes are formed by 

 the interaction of the stray fields of simple compounds. Molec- 

 ular compounds are formed when two molecules hold together. 

 Complex ions result from the entangling of a simple ion by the 

 fields of a "neutral" molecule. 



Hydrates are typical solid molecular compounds. In order 

 that such a compound shall be formed, it is essential that both 



^' The fact that the carbonate group, being a weak anion, is unable to draw the 

 extra calcium electrons near to it with the production of the large doublets present 

 in sodium nitrate is also of influence in reducing the solubility. 



