Hydrogen occupies a unique position in Modern Periodic Table due to two reasons

(i) Both hydrogen and alkali metals have one electron in valence (outermost) shell so some of the properties of hydrogen are similar to those of alkali metals and hence it can be placed in 1^st group.

(ii) Both hydrogen and halogens have one electron less than the nearest inert gas configuration in outermost shell so some properties of hydrogen are similar to halogens and hence it can be placed in group 17.