Define valency by taking examples of nitrogen and boron.

The valency may be defined as the combining capacity of the atoms of elements, or their tendency to form molecules with atoms of the same or different elements.

The electrons present in the outermost shell of an atom are known as the valence electrons. All atoms of various elements have tendency to have a fully-filled outermost shell with 8 electrons or octet. This is achieved through reaction with atoms of other elements, by sharing, gaining or losing electrons.

Example: NITROGEN: Atomic number of nitrogen is 7. Hence its electronic configuration is 2, 5. Hence to complete its octet it will take three electrons (5 + 3 = 8). Therefore its valency is 3 and negative (8 – 5).

BORON: Atomic number of boron is 5. Hence its electronic configuration is 2,3. Hence to complete its octet it will give three electrons Therefore its valency is 3 and positive.