TYPJES OF VALENCE 393 



Postulate 3 expresses merely a strong tendency so that in general the 

 charges of individual atoms are not necessarily zero. When the atomic 

 charges depart from zero, however, they do so only as the result of a defi- 

 nite force or action which opposes the tendency of Postulate 3. We shall see 

 that Postulates i and 3 are often in conflict and in such cases the tendency 

 of Postulate i may prevail against that of Postulate 3. 



We may now classify chemical compounds according to the types of 

 valence exhibited by their atoms and will consider the application of 

 Postulate 3 to each class of compound. There are 3 general subdivisions to 

 consider : 



(i) Complete Compounds, (2) Incomplete Compounds, and (3) Ex- 

 ceptional Cases. 



I. Complete Compounds. — All electrons are in complete layers of 

 2, 8, 18 and 32 electrons, in accordance with Postulate i. Since 2t/c in 

 Equation 4 can never be negative, 2z/e must always be either zero or 

 negative. Therefore atoms having negative valences must always be present 

 in a complete compound. Thus electropositive elements do not form com- 

 plete compounds with each other. 



a. Compounds Without Covalence. — 2z^c = o. Equation 4 becomes 

 St^e = o, 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 consist of positively and negatively charged ions. The 

 charge v on each ion of complete compouds of this type is uniquely de- 

 termined by the values of e for the elements forming the ion. This is a case 

 where Postulates i and 3 are in conflict. The tendency of Postulate 3 by 

 itself would make each atom electrically neutral, but this would leave the 

 sheaths of the atoms incomplete and so fail to satisfy the tendency of 

 Postulate I. The result is a kind of compromise by which Postulate i may 

 be satisfied by the formation of complete compounds provided this can take 

 place without the charges on the ions becoming too large. 



Although Postulate 3 does not definitely fix the charges of the in- 

 dividual atoms in the compounds we are considering, yet it does determine 

 the distribution of these ions in space. This is a factor of prime importance 

 in the crystal structure, in the electrolytic conductivity of substances when 

 in the liquid state, and in other properties. It is also the cause of an interest- 

 ing efifect 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 AICI3, 

 PCI.5, SFe, etc. Postulate 3 requires that the negative halogen 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 



