Question

what is valency and how to find valency

Answer 1

The valency of an element is the number of hydrogen atoms that can combine with or replace (either directly or indirectly) one atom of the element. ... For example, oxygen has six valence electrons, but its valency is 2. Some elements may have more than one combining power (or valency), while others have just one.

Answer 2

valency-In chemistry, the valence or valency of an element is a measure of its combining power with other atoms when it forms chemical compounds or molecules. 
The valency of an atom is equal to the number of electrons in the outer shell if that number is four or less. Otherwise, the valency is equal to eight minus the number of electrons in the outer shell. Once you know the number of electrons, you can easily calculate the valency.