Hydrogen is the first element in the periodic table but its position is ambiguous because of following reason:
(i) Since it has 1 valence electron in its s-subshell, it is placed in group-1 along with alkali metals.
(ii) It also shows properties of halogens of group-17 by picking up electron showing electronegative character like them.
(iii) In some yet another properties, hydrogen differs both from alkali metals and halogens.