Question

Why is helium gas is inert

Answer 1

At the top of the noble gases is little helium (He), with a shell that is full with only two electrons. The fact that their outer shells are full means they are quite happy and don't need to react with other elements. In fact, they rarely combine with other elements. That non-reactivity is why they are called inert.

Answer 2

because it has stable electronic configuration it neither emits nor takes electron