Question

How would you calculate the possible valency of an element from electronic configuration of its atom?

Answer 1

you should distribute the electrons using 2nsq. where n is shell no.
an atom get stable by complete it's outre shell gaining 8 electron.... if the electron in the last shell is 8 than valency is zero
if less than 4 than that is the valency
...if more than four electron than the no of electrons to fill the octat is valency.....

Answer 2

To find valency, see the no.of electron in outermost orbit/ shell. If no.of electron in outermost shell are less then the element can loose them and become electropositive. For instance, Sodium [Na] : 2,8,1 . Sodium has 1 electron in its outermost shell. It can loose this one electron instead of taking burden of another seven electrons on nucleus to become stable. So, Sodium looses electrons and Attain valency of Na+1.

For elements like Chlorine: 2,8,7. It has 7 electrons in its outermost shell. So if it  looses 7 electron then that would require a lot of energy. So, instead of loosing seven electron it gains one more electron to be stable. Valency of Cl: Cl-1