Valency is the combining capacity of an atom.

Explanation:
The combining capacity of an atom of any element is known as valency. Valency is generally positive integer ranging from 0 to 8.

Noble gases have zero valency.

Valency of Silicon
The valency of silicon is 4.
If we take its atomic number which is 14
Its electronic configuration comes out to be 1s²2s²2p²3s²3p².
Valency = 8 – outer shell electron
Silicons’ outer shell has 4 electrons
Thus Valency of silicon = 8 – 4 = 4

Valency of Oxygen
The valency of oxygen is 2
The atomic number of oxygen is 8
So, the electronic configuration is 1s²2s²2p²
Here the outer shell electrons are 6
Thus Valency of oxygen = 8 – 6 = 2