Question

Why He,Ne and Ar are called inert gases??​

Answer 1

Inert gases are those which have stable configuration in their outer most shell.

He, Ne, Ar have their outermost shell complete and are stable.

Thus are called inert gases.

Answer 2

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..