On the periodic table, hydrogen is the only nonmetal in group 1. It is placed at the top of that column because its atom has one electron in its outer shell, like the alkali metals below it.
Hydrogen does not behave like a typical alkali metal. At ordinary conditions it is a gas, it commonly forms covalent bonds, and it can gain, share, or lose an electron depending on the reaction. The alkali metals, by contrast, are solid metals that usually form +1 ions.
This unusual position is why many periodic tables show hydrogen separately or with visual breaks. Some layouts place it above the alkali metals, while others emphasize its relationship with the halogens because hydrogen needs one electron to complete its shell.
Hydrogen’s atomic number is 1, and its most common isotope contains one proton and no neutrons. Its distinctive chemistry makes it a special case rather than a straightforward member of any one family.