Define valency by taking examples of silicon and oxygen.

Explanation

Valency. An atom of each element has a definite combining capacity, called its valency.
An outermost-shell, which had eight electrons was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell. This was done by sharing, gaining or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency. For example, hydrogen/lithium/sodium atoms contain one electron each in their outermost shell, therefore each one of them can lse one electron. So, they are said to have valency of one. Valency of magnesium and aluminium is two and three, respectively, because magnesium has two electrons in its outermost shell and aluminium has three electrons in its outermost shell. Similarly silicon has 4 electrns in its outermost shell. Therefore, its valency is four.
If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way. For example, the fluorine atom has 7 electrons in the outermost shell, and its valency could be 7. But it is easier for fluorine to gain one electron instead of losing seven electrons. Hence, its valency is determined by subtracting seven electrons from the octet and this gives you a valency of one for fluorine. Oxygen has six electrons in its outermost shell. Therefore, its valency is 8 - 6 = 2.