Why is hydrogen placed in Group 1 but not classified as an alkali metal?

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A color-coded periodic table of the elements displays each element's symbol and atomic number, with categories such as alkali metals, noble gases, halogens, transition metals, and actinoid elements distinguished by different background colors. — alkali metal Modern version of the periodic table of the elements (printable).

Although hydrogen is placed in Group 1 of the periodic table because it possesses one electron in its outermost shell, it is not classified as an alkali metal.1

The primary reason for this distinction is that hydrogen is a gas at room temperature, whereas the alkali metals are metals. Furthermore, hydrogen is considered the most individualistic of all elements; it does not closely resemble the alkali metals or the halogens in its chemical properties. While it can exhibit an oxidation number of +1 in certain compounds—similar to alkali metals—it can also take on a -1 oxidation number, behaving more like a halogen. Because of these unique characteristics, hydrogen is the only element that cannot be conveniently categorized as a member of a specific group.12