(a) In order to achieve an octet in the outermost shell, atoms react. This is done by gaining, sharing or losing electrons. The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives the combining capacity of the element which is called the valency of an atom. The valency of atom (a) is 0. The outermost shell of this atom is complete and has 8 electrons. Since, the outermost shell of this atom is complete, it does not have to lose or gain any electron. This results in zero valency.

(b) The valency of this atom is 1. The outermost shell of this atom has seven electrons. It needs one more electron to complete its octet. In this case, this atom will gain an electron and will change into negative ion with valency 1.