(a) State the periodic law on which Mendeleev's periodic table was based. Why and how was this periodic law changed ? (b) Explain why, the noble gases are placed in a separate group.

(a) According to Mendeleev's periodic law: The properties of elements are a periodic function of their atomic masses. It was the discovery of atomic number which led to a change in Mendeleev's periodic law which was based on atomic mass.

(b) Noble gas are chemically inert or unreactive so they are placed in different group.