What is a valency?

Question

Please tell me about what exactly valency is with a few examples.

Answer 1

Valency (or valence) is a measure of the combining power of atoms or groups of atoms with other atoms when it forms chemical compounds or molecules. The valency of an element is determined by the number of outer shell (valence) electrons. The valency of polyatomic ions is the charge on the ion.  Some examples might be:

Sodium (Na). Group I and has 1 valence electron.  Valency is 1+

Chloride (Cl). Group XVII and has 7 valence electrons so will want to gain 1.  Valency is 1-.

Aluminum (Al). Group III and has 3 valence electrons.  Valency is 3+

SO₄^2- (polyatomic anion).  Valency is the charge = 2-