Question

why elements of group 18 are called zero group?

Answer 1

The elements in group 18 have octetrule , means their last cell is already filled and they have no valency that's why it is called zero group.

Answer 2

The Valence shell of the elements of this group is completely filled. And the Valency of elements in this group is Zero. They are non reactive - also called as inert gas elements. All elements have attained octet expect helium ( which has attained duplet i.e., it contains 2 electrons in its outer most shell. )