Question

why does inert gases placed in zero group?

Answer 1

Inert gases (“noble gases”) already have their full complement of electrons in their outer shell. They do not need to bond to fill their outer shell of electrons. ... That is because elements of inert gases have an octet structure which has fully paired electrons.

Answer 2

In the composition of gases nitrogen has the highest  percentage of gas at 78%, followed by oxygen gas which is at 21% followed by carbon dioxide gas which is at 0.03% and lastly are the inert gases which stands at 0.97%, the inert gases are place at zero since when rounded of they will give used zero and they are inactive gases in the atmosphere,