ISOMORPHISM, ISOSTENISM AND COVALENCE 245 



having different electric charges-. Thus we should not expect sodium salts 

 to resemble neon, even though the sodium ion is an isostere of the neon 

 atom — the electric force around the ion is sufficient to account for the 

 differences in physical properties. 



There is, however, another way in which the actual isosterism of co- 

 molecules with different charges can be tested from experimental data. 

 It is evident that if any two substances are very much alike in physical 

 properties, then any isoelectric isosteres of these substances should show 

 similarly close relationships with one another. For example, in Types 3 and 

 8 of Table I, we find argon and nitrogen resemble each other closely. There- 

 fore the chlorine ion, isosteric with argon, should have a close resemblance 

 to the cyanogen ion which is isosteric with nitrogen. The striking similarity 

 of chlorides and cyanides is thus directly correlated with that between argon 

 and nitrogen. 



In an exactly similar manner the close relationship between potassium 

 salts and ammonium salts can be deriv-ed from the similarity between the 

 physical properties of argon and methane. For from Table I (Types 3 and 

 9) the potassium ion is isosteric with argon, while the ammonium ion is an 

 isostere -of methane. Of course the potassium ion and the ammonium ion 

 are not isosteres of one another. The resemblance between potassium and 

 ammonium salts is of a much lower order than that between truly isosteric 

 substances such as nitrous oxide and carbon dioxide, or between salts of 

 cyanic and hydronitric acids. The octet theory indicates in fact that the 

 potassium ion is cubic in form, while the ammonium ion, like methane, 

 must have a tetrahedral symmetry. This conclusion is in accord with the 

 crystal structures of potassium chloride and ammonium chloride. Bragg * 

 has found that in potassium chloride each potassium ion is surrounded by 

 6 equidistant chlorine ions, arranged just as if the crystal were built up 

 of cubical potassium and chlorine ions with their faces in contact ; but finds 

 that ammonium chloride, although it crystallizes in the isometric system, is 

 in no sense isomorphous with the other alkaline halides. Each ammonium 

 ion is surrounded by 8 equidistant chlorine ions arranged like the corners 

 of a cube about its center. This indicates that the tetrahedral ammonium 

 ions force the chlorine ions to arrange themselves symmetrically with re- 

 spect to the 4 faces or corners of the tetrahedron, while the cubical potas- 

 sium ions permit the simpler cubic packing. Ammonium and potas- 

 sium sulfates, however, are isomorphous, for the larger volume of the 

 sulfate ion causes its influence to predominate over the slight differences 

 between the forces around the two positive ions. 



* W. H. Bragg and W. L. Bragg, "X-Rays and Crystal Structure," London, 1916, 

 pp. 95 and 158. 



