Question

why non metals, halogen and noble gases are placed in separate group in periodic table

Answer 1

They are in a separate group because of a different number of electrons that they have

Answer 2

Here is your answer:

non metals, halogen and noble gases are placed in separate group in periodic table because they all the elements in their group have the same valency.

All of the groups have diff. valencies thus they are kept separated in the periodic table.

Eg: Halogen; F₂, Cl₂, Br₂, I₂ etc

Noble gases; Ne, He, Ar, Kr, Xe, Rn