In Mendeleev’s periodic table, hydrogen has been placed in group I since like alkali metals, hydrogen also combines with halogens, oxygen and sulphur to form compounds having similar formulae. This means that hydrogen resembles alkali metals in some of the properties. Hydrogen also resembles halogens in some of the properties. So, hydrogen could also be placed in group VII of halogen elements. Thus, Mendeleev’s periodic law could not assign a correct position to hydrogen in the periodic table.