Define the term valency. With reference to water and ammonia as compound respectively, state the valency of oxygen and nitrogen.

Valency in terms of hydrogen is defined as the combining or displacing capacity of a number of hydrogen atoms by one atom of an element or radical.

Water (H₂O) is composed when two atoms of hydrogen combine with one atom of oxygen. Hence valency of oxygen is 2.

In ammonia (NH₃), there is one atom of nitrogen which combine with 3 atoms of hydrogen. Hence valency of nitrogen is 3.