Define valency by taking the examples of silicon and oxygen.

Explanation

Valency of an element may be defined as
the combining capacity of its atom with atoms of ther elemetns in order to acquire 8 elctrons (2 in some exceptional cases )
Valecy of silicon (Si): Atomic numbr of the element is 14. It electonic distribution is , K(2),L(8),M(4As silicon atom has four valence electons , it can lose all of them .At the sametime ,it can also gain four electrons to have a complete octed. Therefore the valency of silicon is 4
Valecy of oxygen (O) : Atomic number of the element is 8 Its electronic distribution is ,K(2),L(6) As oxygen atom has six valence electrons , it needs two more electrons to complete its octet (8-6=-2) Therefore valency of oxygen is 2.