Question

Why no place was given to hydrogen in Mendeleev's periodic table?

Answer 1

It is so because hydrogen has 1 valence electron in its outermost shell.so according to modern periodic table, hydrogen can be arranged in halogen family i.e. group no. 17 but hydrogen does not possess the properties of halogen family. That's why hydrogen is given its position in the first group and first period of the modern periodic table.

Answer 2

Hydrogen had no fixed position in Mendeleev's Periodic Table because it resembles alkali metals by forming positive ions and resembles halogens by forming diatomic molecule.