WYCKOFF: FORCES BETWEEN ATOMS IN SOUDS 585 



atom by four iron atoms. ^'■* The diamond, carborundum, certain 

 oxides and sulfides, and presumably nitrides and carbides, are 

 compounds of this kind. The elements which go to make these 

 substances are not strongly electropositive or electronegative, so 

 that no actual electron transfer takes place. Completely closed 

 groups are not formed and each valence unit corresponds to two 

 bonds. The atoms in such a crystal are held together by valency 

 forces. The chemical molecule does not appear; the entire crys- 

 tal behaves as a single chemical individual. 



It is improbable that liquids of this sort can ^xist. Certainly 

 such compounds cannot be vaporized without undergoing pro- 

 found changes. It is possible that at elevated temperatures the 

 electronegative element may be able to relieve the other of 

 electrons. Such a substance, when existing in the liquid state, 

 would belong to the class previously described, the polar type 

 of compounds. 



Metals belong to the polar type. Just as the close proximity 

 of other atoms in the case of sodium chloride is sufficient to 

 break up the fields binding one particular sodium atom and one 

 particular chlorine atom, so in the metallic state the presence 

 of other atoms breaks up the fields holding certain electrons to 

 the metal ion. A metal may be considered as a compound of 

 metal ion and electrons entirely similar to a liquid of the polar 

 type, which is a compound of metal ion and negative ion. The 

 peculiar properties characteristic of metals are due to the fact 

 that the electrons as a result of their minute size are readily 

 able to pass between the atoms. This mobility makes the mech- 

 anism within the metal resemble that within a fused electrolyte. 

 The atoms in intermetallic compounds, which presumably exist 

 as such only in the solid state, are held together by the same 

 forces that hold metal ions together in the pure metal. If a 

 certain grouping of atoms offers an especially marked condensa- 

 tion and drawing-in of the fields of force, that grouping will 

 appear as one of these compounds. 



Classification of crystalline solids. — -The crystalline state fur- 

 nishes the greatest condensation of the fields about the indi- 



»» W. H. Bragg. Phil. Mag. (6) 30: 305. 1915. S. Nishikawa. Proc. Tokyo 

 Math. Phys. Soc. 8: 199. 1915. 



