Hydrogen

Noun

1. The lightest chemical element (symbol H), with an atomic number of 1 and atomic weight of 1.008.

   "Our professor then reminded us that a hydrogen ion is merely a proton, and thus we should think about acid-base theory as we consider this problem further."

2. An atom of this element.

   "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.

   "A large tank of hydrogen constituted most of the bulk of that rocket."

4. A molecule of this molecular species.
5. Synonym of protium (“the lightest and most common isotope of hydrogen (sense 1; symbol H, H, or 11H), as contrasted with deuterium and tritium”).

   "Both hydrogen and deuterium are present in every glass of water that you drink, but the amount of deuterium is rather small."