An outermost shell, which needs eight electrons to fulfill it's [[1]] is called that elements valency.

The tendency to react with atoms of the same or different elements to form molecules is an attempt to attain a fully-filled outermost shell. It means atoms react with other atoms in order to attain a fully-filled outermost shell. So, an outermost shell that has eight electrons is called an octet and the electrons needed to fulfill its octet are called its valency.