Question

why 18th group elements are called inert gases?

Answer 1

Generally inert group gases are fulfilled.
and they don't react with any substance..
so it is called inert

Answer 2

18 gr elements are known as inert or noble gases as thier octet are completely filled or last shell is filled ... Moreover that's why they doesn't react with other elements....
Ex..
He - 2(electronic configuration)
And shell filled..

Hope this helps ☺️