why is it that hydrogen can be placed in the first group as well as the 17th group?

Hydrogen can be placed in Group 1 because its configuration resembles that of Group 1 elements.Group 1 elements have general outer electronic configuration of ns¹ and hydrogen has an electronic configuration of 1s¹.

Hydrogen resembles halogens that is Group 17 elements because it requires one electron to complete its outermost shell. Its outermost shell is K shell which can have a maximum of two electrons. Group 17 elements also need one electron to  attain 8 electrons in their outermost shell.

It also has some properties which neither resembles Group 1 nor group 17. Hence, it is placed on the top of the modern periodic table separately.