Why is hydrogen unusual among the elements in Group 1?

Although hydrogen is placed in Group 1 of the periodic table, it is not closely similar to the alkali metals or the halogens in its chemical properties.1

Hydrogen is considered the most individualistic of all elements because it does not resemble other elements in the way that members of a group typically resemble one another. Its unique status arises from its behavior in compounds: it can be assigned an oxidation number of +1, similar to Group 1 elements, or -1, similar to Group 17 elements. Because of this dual nature, hydrogen is the only element that cannot be conveniently classified as a member of a specific group.1