Which element has atomic number 1 but is not an alkali metal?

Hydrogen has atomic number 1 but is not an alkali metal.

Hydrogen’s nucleus contains one proton, giving it atomic number 1. Its neutral atom also has one electron, with the electron configuration 1s¹. That arrangement places hydrogen above group 1, the column containing the alkali metals, but hydrogen’s chemistry is fundamentally different from lithium, sodium, and potassium.

At standard conditions, hydrogen is a colorless, highly flammable, nonmetallic gas that normally exists as diatomic H₂ molecules. Alkali metals are reactive solid metals that readily lose their outer electron to form +1 ions. Hydrogen can also lose its electron, but it commonly forms covalent bonds and can sometimes gain an electron, so it does not fit neatly into the alkali-metal family.

Hydrogen is also the lightest and most abundant element in the universe, making up roughly 75% of ordinary matter by mass. On Earth, it is commonly bound in water and organic compounds rather than found as free hydrogen gas.