Question

why do you understand by valency of an atom?

Answer 1

Valency is defined as the combining capacity of an element.

Examples-valency of hydrogen is 1, and the valency of magnesium is 2

Some of the elements have more than one valency. They are said to have variable valencies.

Example-iron exhibits valency 2 when it combines with oxygen to form ferrous oxide($\mathrm{FeO}$) and it exhibits valency 3 when it combines with oxygen to form ferric oxide(${\mathrm{Fe}}_2{\mathrm{O}}_3$).