CBSE Class 10 Answered

It is true that elements in the same group need to have the same valence electrons in the modern periodic table. Yes, according to this hydrogen should be placed in group 1. But the other group  1 elements are all metals and are known as alkali metals. These elements are eletropositive  in nature. The chemical properties of hydrogen are diferent from those of group 1 elements. Thus, in its outer electronic confiuguration it should be placed in group 1but has some different chemical properties. Since hydrogen has different chemical properties from those of group 1 elements it cannot be placed in group 1.

The number of valence electrons in Group 17 are 7 and that of hydrogen is 1. It resembles group 17 in a way that hydrogen also needs one electron to complete its oitermost shell and group 17 elements also need one electron to complete their octet.

So, it is neither palced in group 1 nor Group 17 but placed separately above the modern periodic table.