Question

Why He, Ne and Ar are called inert gases ?

Answer 1

He, Ne and Ar are called inert gases because their outermost shell is complete and they do not lose or gain electrons in their outermost shell, hence they have zero valency.

Answer 2

The reason they are inert gases is because they do not bond easily with other elements. They have a full valence of electrons so they don't need any to satisfy their own valence. They are also called noble gases because they are like this.