394 PHENOMENA, ATOMS, AND MOLECULES 



that these constitute molecules of considerable stability and inertness 

 towards outside influences. The volatility of these substances and the 

 absence of electrolytic conductivity are due to this cause. 



Typical examples of complete compounds without covalence are : 



Salts. — When the atomic charges are small as in NaCl, BaBr2, K2S, etc., 

 the salts are fairly readily fusible, soluble in liquids of high dielectric con- 

 stant, good electrolytic conductors when molten or in solution and very 

 difficultly volatile. With larger charges as in MgO, BN, AI2O3, etc., the 

 strong forces give great infusibility, insolubility, hardness, etc., to the 

 substance. Such compounds are exceptionally good electric insulators at 

 moderate temperatures but are electrolytic conductors when molten. 



Silicates, glasses, slags, complex sulfides, and most minerals, etc., are 

 compounds which usually contain several electropositive elements. In the 

 molten, and often in the solid condition, they are electrolytic conductors 

 and are usually soluble in one another. The valence relation 1,Ve = o gives 

 us no information in regard to the structure ; for example, we can not write 

 structural formulas for such compounds. The definite composition of many 

 solid minerals, etc., is largely due to the regularities of the space lattices 

 of their crystals. 



Volatile halogen compounds such as AIF3, PCI5, SFe and struc- 

 turally related complex ions such as SiFe"" in the compound K2SiF6. 

 Such high electrovalences as -{-5 for phosphorus, and -\-6 for sulfur 

 can occur only when the tendency of Postulate 3 is counteracted by 

 a particularly strong opposing tendency. In the case cited above it is the 

 exceptionally great affinity of the halogen atoms for electrons that causes 

 the action. The halogen atoms have this property in marked degree because 

 they have larger charges on their kernels than other atoms and therefore 

 exert a greater attraction on electrons (Coulomb's law). The fluorine atom 

 has a greater affinity for electrons than the other halogen atoms since the 

 radius of the atom is less and the force (by Coulomb's law) acting on the 

 electron is greater. 



b. Compounds Without Electropositive Atoms. — In these compounds 

 the electrovalence of every atom must be negative, for if the electrovalence 

 of any element is zero (inert gases) it can form no compounds. If we let 

 Vn represent the numerical value of the negative valence we obtain from 

 Equation 4 



Hvc = 2z/«. (6) 



Since Vn = s — e, the value of Vn is fixed for any particular atom. For 

 any given group of atoms, we can find ^Vc from (6) but we can not find 

 the values of Vc for the individual atoms, in this way. 



