July 22, 1921] 



SCIENCE 



63 



in complete layers of 2, 8, 18 and 32 electrons, 

 in accordance with Postulate 1. Since S^^c 

 in Equation 4 can never be negative, S^e 

 must always be either zero or negative. There- 

 fore atoms having negative valences must 

 always be present in a complete compound. 

 Thus electropositive elements do not form 

 complete compounds with each other. 

 ■ a. Compounds Without Covalence. — 

 2t)(, =^ 0. Equation 4 becomes St'e = 0, so 

 that the sum of the negative valences in the 

 compound must be the same as the sum of the 

 positive valences. Since the residual charge v 

 for each atom must equal Vg + Vc it is evident 

 that compounds without covalence must con- 

 sist of positively and negatively charged ions. 

 The charge v on each ion of complete com- 

 pounds of this type is uniquely determined 

 by the values of e for the elements forming 

 the ion. This is a case where Postulates 1 

 and 3 are in conflict. The tendency of Postu- 

 late 3 by itself would make each atom elec- 

 trically neutral, but this would leave the 

 sheaths of the atoms incomplete and so fail 

 to satisfy the tendency of Postulate 1. The 

 result is a kind of compromise by which Postu- 

 late 1 may be satisfied by the formation of 

 complete compounds provided this can tahe 

 place without the charges on the ions becoming 

 too large. 



Although Postulate 3 does not definitely fix 

 the charges of the individual atoms in the 

 compounds we are considering, yet it does de- 

 termine the distribution of these ions in space. 

 This is a factor of prime importance in the 

 crystal structure, in the electrolytic conduc- 

 tivity of substances when in the liquid state, 

 and in other properties. It is also the cause 

 of an interesting effect observed when the 

 number of ions of one sign is much greater 

 than that of the other sign, as for example in 

 such compounds as AlCl^, PCl^, SF,., etc. 

 Postulate 3 requires that the negative halo- 

 gen atoms in these compounds shall surround 

 the most strongly positive atoms. The ions 

 thus form groups having strong internal and 

 weak external fields of force so that these con- 

 stitute 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, BaBr„, K„S, etc., the salts are 

 fairly readily fusible, soluble in liquids of 

 high dielectric constant, good electrolytic con- 

 ductors when molten or in solution and very 

 difiiciiltly volatile. With larger charges as in 

 MgO, BN", Al^Oj, etc., the strong forces give 

 great infusibility, insolubility, hardness, etc., 

 to the substance. Such compounds are ex- 

 ceptionally good electric insulators at moder- 

 ate temperatures but are electrolytic conduc- 

 tors when molten. 



Silicates, glasses, slags, complex sulfides, and 

 most minerals, etc., are compounds which usu- 

 ally contain several electropositive elements. 

 In the molten, and often in the solid condition, 

 they are electrolytic conductors and are usu- 

 ally soluble in one another. The valence rela- 

 tion S'f e = gives us no information in re- 

 gard to the structure; for example, we can not 

 write structural formulas for such compounds. 

 The definite composition of many solid min- 

 erals, etc., is largely due to the regularities 

 of the space lattices of their crystals. 



Volatile halogen compounds such as AlFj, 

 PClj, SF,,, and structurally related complex 

 ions such as SiFg— in the compound KjSiFg. 

 Such high electrovalences as -|- 5 for phos- 

 phorus, and + 6 for sulfur can occur only 

 when the tendency of Postulate 3 is counter- 

 acted by a particularly strong opposing ten- 

 dency. In the case cited above it is the ex- 

 ceptionally 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 (Cou- 

 lomb's law). The fluorine atom has a greater 

 afiinity 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 



