Define vale

The capacity of an atom of an element to form chemical bonds is known as valency of an element.

An atomic number of chlorine is 17 so its electronic configuration is 2, 8, 7.

Valency of non-metal chlorine = 8 – the number of valence electrons in its atom

= 8 – 7

= 1

Atomic number of magnesium is 12 so its electronic configuration is 2, 8, 2

Valency of magnesium = number of valence electrons in its atom = 2.