Define valency by taking examples of silicon and oxygen.

The definite combining capacity of the atoms of each element, wherein electrons are lost, gained or shared to make the octet of electrons present in the outermost shell, is defined as valency.

(i) The electronic configuration of silicon [14] is 2, 8, 4. It has 4 electrons in its outermost shell. So, the valency of silicon = 8 – the number of electrons in the outermost shell = 8 - 4 = 4.

Hence, valency of silicon = 4.

(ii) The electronic configuration of oxygen [8] is 2, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of oxygen = 8 – the number of electrons in the outermost shell = 8 − 6 = 2.

Hence, valency of oxygen = 2.