Valency is the combining power of an element. It is also called valency. The number of hydrogen atom or chlorine atom or twice the number of oxygen atoms with which one atom of the element combines is defined as the valence of the element

Valency of an element can be variable

Example: compounds of copper- CuCl (copper(I)chloride)

compounds of copper- CuCl₂ (copper(II)chloride)

Hence, Valency of an element can be variable