Question

How can the valency of an element be determined if its electronc configuration is known? What will be the valency of an element of atomic number 9?

Answer 1

If the electronic configuration of an element is known then the last digit is considered as the valency of that element.  

For eg. the electronic configuration of sodium is 2,8,1. So, the valency of sodium is 1.

The valency of an element of atomic number 9 is 7. This is because its electronic configuration is 2,7.