Define valency by taking examples of nitrogen and boron.

Explanation

Valency : The number of electrons present in outer most orbit of an atom is called its valency.
a) Atomic number of nitrogen is 7.
b) The distribution of electrons is 2, 5.
c) The outer most orbit has 5 electrons.
d) Hence its valency should be 5. But it is easier to nitrogen to gain 3 electrons than to loose 5 electrons for becoming octet.
e) Hence the valency of nitrogen is '3'.
Valency of Boron :
a) Atomic number of boron is 5.
b) The outer most orbit has 3 electrons.
d) Hence the valency of boron is 3.