Question

Why can hydrogen be placed in group 1 and 17?

Answer 1

Hydrogen is a nonmetal and is placedabove group in the periodic table because it has ns1 electron configuration like the alkali metals. 

Answer 2

The reason why the hydrogen can be placed in both group 1 and group 17 is that it resembles Alkali metals in some of its properties like it can easily form cations so, it can be placed in Group 1 of periodic table but it also resembles Halogens in its properties like it forms $H^2$ which is true property of halogen that is why it can be placed in group 17 of periodic table. As hydrogen can accept as well has the tendency to reject electron to make itself stable. It is the smallest member of the family.