How the valency can be determined if the electronic configuration is known? (CBSE 2012, 2011)

Valency of an element can be determined if its electronic configuration. With the help of electronic configuration, the valence electrons are find out which helps in finding the no. of electrons required to obtain stable gas electronic configuration which is known as valency. Those elements which have valence electrons from 1 to 4, the valency of elements is equal to the number of valence electrons. Elements having valence electrons from 5 to 8, then the valency is given by no. of electrons required or to be donated to obtain stable gas electronic configuration.

Atomic number of element = 9

Electronic configuration of the element = 2, 7

No. of electrons required to obtain stable gas configuration = 1

So, valency= 1