Question

What are primary valencies and secondary valencies give examples.

Answer 1

Valency of an element can be defined as combining capacity of an atom. It can also be defined as the number of bonds that an atom can form as part of a compound is expressed as the valency of that element.

Complete answer:
Valence electrons are those electrons which are present in the outermost shell of an atom. Valency is dimensionless and has no sign. When two atoms react with each other their outer shell first comes into contact and therefore the outer shell of the electron is normally involved in a chemical reaction. The atoms exchange valence electrons in a chemical reaction to increase the stability of their valence shell.
The valency is basically of two types:
1. Primary valency: It can be explained as in coordination compounds the number of negative ions which are equivalent to the charge on the metal ion.
2. Secondary valency: Secondary valency is defined as the number of ions of molecules which are coordinated to the metal ion or the number of ligands attached or coordinated to metal ions.
The example of this is given by:
${{K}_{4}}[Fe{{(CN)}_{6}}]$, Primary valency in this coordination complex is given as
${{K}_{4}}[Fe{{(CN)}_{6}}]=4+x-6=x-2=2$
Whereas secondary valency number of ligands attached which is given by $CN$i.e. 6 $CN$ ligands are attached with $Fe$.

Note:
An atoms valence electron is equal to the number of electrons in the outer shell when that number is four or fewer. Then the valence electrons in the outer shell is equal to eight minus the number of electrons.