Define valency by taking examples of silicon and oxygen.

(N/A) The valency of an element is defined as the combining capacity of that element.
The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom.
$1$. Valency of Silicon: The atomic number of silicon is $14$. Its electronic configuration is $2, 8, 4$. Since it has $4$ valence electrons,it shares these electrons to complete its octet. Therefore,the valency of silicon is $4$.
$2$. Valency of Oxygen: The atomic number of oxygen is $8$. Its electronic configuration is $2, 6$. To complete its octet,it needs $2$ more electrons. Therefore,the valency of oxygen is $2$.