hydrogen

noun5

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. •
   Synonym of protium (“the lightest and most common isotope of hydrogen (sense 1; symbol H, ¹H, 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.
4. •
   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.
5. •
   A molecule of this molecular species.