586 WYCKOFF: forcks betweien atoms in solids 



vidual particles (atoms or molecules, depending upon the type 

 of solid)/*' Three limiting types of crystalline solids may be 

 said to exist: 



1. Molecule- forming compounds. — ^The atoms are held together 

 in molecules by valency forces. The molecules, in turn, are 

 held together in the solid by relatively weak stray fields. Com- 

 pounds giving soHds of this nature do not possess large doublets. 



2. Polar compounds. — ^The atoms are held together by electro- 

 static attractions. The chemical molecule has disappeared. 



3. Valency compounds. — The atoms are held together by val- 

 ency forces. The molecule is the entire crystal. 



Combinations of these classes and all transition stages be- 

 tween them are found. In sodium nitrate the nitrogen and 

 oxygen atoms are held together by valency forces to form the 

 nitrate ion. The combination between the sodium and the 

 nitrate group is polar. The silver and mercury (mercurous and 

 mercuric) halides are probably midway between (i) and (2). 

 In a crystal of silver iodide, because of the vigor with which 

 silver clings to the one electron, the electron may be considered 

 as placed part way between the silver and the iodine atoms. 

 The molecules of silver iodide are held together partly by the 

 stray fields and partly by the fields of the doublet of relatively 

 small moment. 



A brief discussion of certain phenomena which illustrate the 

 application of this point of view will now be given. Some of 

 these subjects have been considered already from similar points 

 of view.^^ They are given again either because they are especially 

 helpful to an understanding of the previous discussion or because 

 of their importance in the consideration of the structure of solids. 



Dissociating Solvents. — ^The hydrides of the electronegative 

 elements furnish the strongest dissociating solvents. Methane 

 has been dealt with in detail. The fields of its molecule are quite 



*" If the substance is cooled very rapidly it may be impossible for the particles 

 to arrange themselves in an orderly fashion. The solid is then amorphous. 



*^ E. C. C. Baly. Jom-n. Amer. Chem. Soc. 37: 979-993. 1915; also papers in 

 Trans. Chem. Soc. London. I. Langmuir. Journ. Amer. Chem. Soc. 38: 2221- 

 2295. 1916; 39: 1848-1906. 1917; 40: 1361-1403. 1918. W. D. Harkins, E. 

 C. H. Davies and G. L. Clark. Journ. Amer. Chem. Soc. 39: 541-596. 1917. 



