Question

why does inert gases placed in zero group?

Answer 1

hlo here is your ans inert gas ' inert' mean which don't react with the another . and inert gas He , Ne , Ar, ,xe etc in all this all the gases don't react with another atom these are stable and their octet is completely filled
It therefore makes a lot of sense to group all of these gases in “group zero”.

I hope that this helps you

Answer 2

Inert gas elements have their outer most shell completed or full. So they have no tendency to enter into any chemical reaction by giving or receiving electrons. Hence, inert gases are chemically inert and placed in zero group (Group-viii) in the periodic table. Inert gases with zero valency. Also because of their electronic configuration stability