What is valency? Explain.

Valency is defined as the combining capacity of an element. It represents the number of electrons an atom needs to lose,gain,or share to achieve a stable octet configuration in its outermost shell.
The valency of representative elements is usually equal to the number of electrons in the outermost orbitals or equal to $8$ minus the number of outermost electrons as shown below:

Group	$1$	$2$	$13$	$14$	$15$	$16$	$17$	$18$
Number of valence electrons	$1$	$2$	$3$	$4$	$5$	$6$	$7$	$8$
Valency	$1$	$2$	$3$	$4$	$3$	$2$	$1$	$0$