Why do you think the noble gases are placed in a separate group?

Explanation

In the Mendeleev's periodic table, the elements have been arranged in the different groups on the basis of A valency. For example, the elements placed in group I (IA and IB) have valency equal to one. Same is the case with the elements placed in other groups. Since the noble gas elements He, Ne, Ar, Kr, Xe, and Rn have zero valency, they could not find a place in Mendeleev's periodic table. These have been placed in a separate group called zero group in the periodic table. Please note that the noble gas elements were not a part of the Mendeleev's periodic table. They were added later on.