Final answer:
Hydrogen is not classified as an alkali metal because it is a nonmetal with unique properties, including forming a variety of compounds and having the ability to both lose and gain electrons, unlike the purely metallic alkali metals.
Step-by-step explanation:
Hydrogen is not included in the alkali metal family because it is not a metal. While hydrogen is in Group 1 of the periodic table and has one electron in its outer shell similar to alkali metals, it is unique due to its properties as a nonmetal. The key characteristics of alkali metals include being highly reactive with water and being metals; however, hydrogen, despite some chemical reactivity akin to alkali metals, does not exhibit metallic properties, has comparatively low electronegativity, and also forms different types of compounds like ionic hydrides and covalent bonds depending on the electronegativity of the other element involved.
Moreover, hydrogen can act like a Group 17 element (halogens) since it can gain an electron to complete its valence shell. Its placement in the periodic table is somewhat ambiguous because it can lose an electron to form H+ or gain an electron to form H-.