584 WYCKOFF: FORCES BETWEEN ATOMS IN SOLIDS 



of this type. Sodium nitrate and calcium carbonate^^ are exam- 

 ples. In these cases the nitrate and carbonate ions act as single 

 units. Three factors are of importance in determining the 

 crystal form of such substances : 



1. The stable arrangement of points in space which corre- 

 spond in charge and number with the ions of the substance. 



2. The number and arrangement of the atoms making up the 

 ions. 



3. The volumes of the ions. 



The sodium chloride arrangement is the simplest possible 

 for the grouping of an equal number of positive and negative 

 particles of about equal volume. Sodium nitrate and calcium 

 carbonate have the same structure as sodium chloride, the 

 nitrate and carbonate ions replacing the chlorine ions. 



Valency compounds. — Magnetite (Fe^Fe'^o O4) is an example 

 of another general type of compound. In a crystal of magnetite 

 each divalent iron atom is surrounded by four oxygen atoms, 

 each trivalent iron atom by six oxygen atoms, and each oxygen 



it is natural to suppose that they will combine together to form a definite whole — ■ 

 the molecule. This is what happens in the case of gases. The first actual evidence 

 to show the existence of molecules came from the study of gases. This evidence 

 is furnished by the Gay-Lussac Law of Volumes. The splendid success of the 

 kinetic theory in describing the behavior of gases pointed in the same direction. 

 When gas molecules had been shown to exist, a tendency to apply the idea of mole- 

 cules to solids and liquids as well made its appearance. The extension of the gas 

 laws to the case of dilute solutions showed that in this case the material of the 

 dissolved substance is distributed throughout the solvent in a molecular condition, 

 that is, as single atoms or as groups of a few atoms together. Only in these two 

 cases, gases and dilute solutions, have we stue evidence of the existence of molecules. 

 Certain observations upon solutions of solids in solids would seem to indicate for 

 them a structure similar to that possessed by liquid solutions. The abnormal 

 behavior of some pure liquids finds its simplest explanation in the assumption of 

 a molecular structure. Because of this certainty of the existence of the molecule 

 in gases and in solutions, and because with organic solids and liquids it is quite im- 

 possible to imagine any other than a molecular composition, chemists in general 

 seem to have felt justified in concluding that all matter is molecular in structure. 

 It is important to note that this conclusion is only an inference, in no way justified 

 by experimental evidence, and that the recent evidence which has appeared, tend- 

 ing to show that certain kinds of solids and liquids are not made up in such a way 

 that each piece consists of a large number of chemical molecules, is merely de- 

 stroying some generally-held opinions and is not in any way contradictory to our 

 previously acquired knowledge. 



