CBSE Class 12-science Answered

The main postulates of this theory are as follows:

1. Metals possess two types of valencies called

(a)Primary or principal valency

(b)Secondary valency

(a)Primary valencies are those in which a metal exhibits in the formation of its simple salts. Thus in the formation of PtCl₄, CoCl₃ and CuSO₄ , the primary valencies of Pt, Co and Cu are 4 , 3 and 2.Now primary valency is referred to as oxidation state.

(b)Secondary valencies are those which a metal atom or cation exercises towards the neutral molecules or negative groups (ligands) in the formation of its complex ions. The secondary valency is also referred as the coordination number. In [Pt(NH₃)₆]Cl₄ and [Co(NH₃)₆]Cl₃ , the secondary valencies are 6 for Pt⁺⁴ and 6 for Co⁺³ cations.

2. Every metal atom has a tendency to satisfy both its primary and secondary valencies. Primary valencies are satisfied by negative ions whereas secondary valencies are satisfied by negative ions or neutral molecules.

3. The ligands satisfying secondary valencies are always directed towards fixed positions in space thereby giving a definite geometry to the complex but the primary valencies are non-directional.