Define valency by taking examples of silicon and oxygen.

Valency is the combined capacity of an atom.

Atomic number of oxygen = 8

Atomic number of silicon = 14 K L M

Electronic configuration of oxygen = 2 6 

Electronic configuration of silicon =2 8 4

In the atoms of oxygen, the valence electrons are 6 (i.e., electrons in the outermost shell). To fill the orbit, 2 electrons are required. In the atom of silicon, the valence electrons are 4. To fill this orbit 4 electrons are required.

Hence, the combining capacity of oxygen is 2 and silicon is 4.

i.e., Valency of oxygen = 2

Valency of silicon = 4.

[Extra information: The rules for the writing of the distribution of electrons in various shells for the first eighteen elements are given below:

(i) The maximum number of electrons present in a shell is given by the formula-2 n2

∵ n = orbit number i.e., 1, 2, 3

∵ The maximum number of electrons in different shells are:

K shell n = 1 2n2 => 2(1)2 = 2

L shell n = 2 2n2 => 2(2)2 = 8

M shell n = 3 2n2 => 2(3)2 = 18

N shell n = 4 2n2 => 2(4)2 = 32

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8.

(iii) Electrons are not accommodated in a given shell unless the inner shells are filled.]