How do we find valency of an element?

Valency of an element gives us an idea of the number of electrons required to completely fill the outer shell. It is also a measure of number of electrons involved in bond formation. The simplest way to find the valency of an element is to write its electronic configuration, for which the atomic number of the element should be known. From the electronic configuration, we can see the number of electrons that are required to fill the outer shell. This can be done either by gaining electrons (as in the case of non-metals) or losing electrons (as in the case of metals). Thus, the valency of the non-metals is negative and for metals, it is positive.

For example, sodium has an atomic number of 11 and hence it's electronic configuration is `1s^2, 2s^2, 2p^6, 3s^1` . It can lose 1 electron from 3s orbital and achieve fully filled configuration and hence has a valency of +1. On the other hand, chlorine (a non-metal) has a valency of -1.

Hope this helps.