Define the term 'valency'. With reference to water & ammonia as compounds respectively state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2⁺. Give reasons.

Valency is the number of hydrogen atoms which can combine with [or displace] one atom of the element [or radical] forming a compound.

In case of water, two atoms of hydrogen combine with one atom of oxygen. Hence, valency of oxygen is 2.

In case of ammonia, three atoms of hydrogen combine with one atom of nitrogen. Hence, valency of nitrogen is 3.

Valency is also defined as the number of electrons an atom can donate or accept so as to achieve stable electronic configuration of the nearest noble gas. In case of magnesium [2, 8, 2], it donates two electrons to achieve a stable electronic configuration of the nearest noble gas [Neon (2,8)], hence its valency is 2⁺