What is valency of an element? How do you derive valency of Aluminium?

Valency of an element is defined as its combining capacity and it is measured by the number of hydrogen atoms with which, one atom of the element combines.One atom of aluminium combines with three atoms of chlorine.
∴ One atom of Al = 3 atoms of chlorine
Since the valence of chlorine is one,
Valence of Al = 1 × 3 = 3.