What is the lightest chemical element?

The lightest chemical element is hydrogen. Its atoms contain one proton and, in the most common form, one electron, giving hydrogen an atomic number of 1 and an atomic weight of about 1.008 u.

Hydrogen is the most abundant element in the universe, making up much of the Sun and other stars. On Earth, it is usually bonded to other elements rather than found as a free gas; water, hydrocarbons, and acids all contain hydrogen. Under ordinary conditions, elemental hydrogen exists as H₂, a colorless, odorless, highly flammable gas.

Hydrogen was identified as a distinct substance in the 18th century, and the English chemist Henry Cavendish described its properties in 1766. The name comes from Greek roots meaning “water-former,” because burning hydrogen produces water.

A common mix-up is confusing lightest with smallest or least dense. Helium is also very light and is less reactive, but hydrogen has the lowest atomic mass of all elements. Hydrogen is also the first entry on the modern periodic table.