wyckoff: forces between atoms in solids 583 



A doublet of very large moment exists in this molecule and 

 sodium chloride vapor would be expected only at an elevated 

 temperature. In the solid and liquid states each sodium atom 

 (except those upon the surface), positively charged by the loss 

 of an electron, is surrounded by several chlorine atoms, all 

 negatively charged.^'' These other chlorine ions, each posses- 

 sing a pull upon the sodium ion, will more or less completely 

 tear apart the fields which in the gaseous state bind the sodium 

 atom to one particular chlorine atom. Sodium chloride be- 

 comes a body of ions held together mainly by the electrostatic 

 forces of attraction between opposite charges. A cross-section 

 of a crystal of sodium chloride would appear thus :^^ 



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The gaseous molecule disappears quite completely. A study 

 of the effect of a sodium chloride crystal upon X-rays leads to 

 belief in such a structure. ^^ 



Concerning the liquid state of sodium chloride we know prac- 

 tically nothing. It would seem most reasonable to assume a 

 structure similar to that possessed by the solid, with the added 

 fact of mobility; that is,^ an agglomeration of an equal number 

 of positive and negative ions. When such a solid sublimes, ions 

 should appear in the space above. ^^ In general all crystals made 

 up of strongly electropositive and electronegative elements are 



^ Sodium chloride possesses absorption bands in the extreme infra-red which are 

 produced by charged particles of atomic mass. 



^^ This structiure for sodium chloride and similar crystals has been suggested by 

 Stark, Prinz. d. Atomdynamik, III. p. 193. 



'^ W. H. and W. L. Bragg. X-rays and crystal structure. 



*8 The chemical molecule is in certain cases a natural consequence of the belief 

 in atoms. It is not, however, in aU cases a necessary consequence. If we are to 

 imagine two or three atoms, each with a certain tendency to react, coming together. 



