hydrogen

1. The lightest chemical element (symbol H), with an atomic number of 1 and atomic weight of 1.008. (uncountable)
2. An atom of this element. (countable) · Each carbon of the ring has one hydrogen attached except for the fourth one, which is bonded to a substituent.
3. Molecular hydrogen (sense 1; symbol H₂), a colourless, odourless and flammable gas at room temperature. (uncountable) · A large tank of hydrogen constituted most of the bulk of that rocket.
4. A molecule of this molecular species. (countable)
5. Synonym of protium (“the lightest and most common isotope of hydrogen (sense 1; symbol H, ¹H, or 11H), as contrasted with deuterium and tritium”). (uncountable) · Both hydrogen and deuterium are present in every glass of water that you drink, but the amount of deuterium is rather small.

Plural hydrogens