ISOMORPHISM, ISOSTENISM AND COVALENCE 257 



of available data : MgF.-NaaO ; KaS-CaCl^ ; NaClOa-CaSOs ; KHSO3 

 — SrHPOs; NaoSaOe— Ca2P206 ; NagSaO?— CasPaOT ; etc. 



The theory of isosterism makes it possible to derive certain relation- 

 ships in a very simple manner. Thus since argon is an isostere of the 

 potassium ion and methane is an isostere of the ammonium ion, it follows 

 that the potassium and ammonium ions must have similar properties be- 

 cause argon and methane are nearly alike in physical properties. Similarly, 

 the relation between argon and nitrogen enables us to trace an equally 

 close relationship between the cyanogen and the chlorine ions. From the 

 similarity in the solubilities of nitrates and perchlorates we may also con- 

 clude that carbonates and sulfates should be closely related. 



The experimental data discussed prove that the crystalline form of 

 substances depends on the structure as given by the octet theory, thus 

 indicating that this theory gives a true picture of the constitutions of 

 crystalline solids. In particular, the experimental results justify the follow- 

 ing conclusions : 



1. The covalence of sodium, potassium, chlorine in chlorides, is zero. 



2. The covalence of the central atom is 4 in nitrates, carbonates, sul- 

 fates, perchlorates, phosphates, permanganates, chromates, selenates, 

 arsenates, borofluorides, etc. 



3. Carbonates and sulfites are not isomorphous, the covalence of the 

 central atom being 4 and 3, respectively. 



4. Nitrates and chlorates are not isomorphous, the covalence of the 

 chlorine being 3 in chlorates. 



5. The applicability of the octet theory to complex inorganic compounds 

 receives further confirmation by its ability to explain such cases of iso- 

 morphism as between Na2BeF4 and MnCl2.4H20 ; K2SO4 and ZnT2.4NH3, 

 KoSnC]4.2H20 and KsFeClg.HaO, NaAlSisOg and CaAloSisOg, etc. 



