1) The position of hydrogen in the periodic table is unique.
2) Its properties with both alkali (IA group) metals and halogen (VIIA group), because it can lose one electron like alkali metals as well as gain one electron like halogens.
3) It is therefore, to be placed at the top of both alkali metals and halogens.
4) Modern periodic table was based on electronic configuration. Hydrogen electronic configuration is resemble with Alkali metals hence it is placed at the top of alkali metals but not include in IA group (Alkali Metals).