hydrogen

noun

1. (uncountable)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.”

   “The ignis fatuus or Jack a lantern, ſo frequently alluded to by poets, is ſuppoſed to originate from the inflammable air, or Hydrogene, given up from moraſſes; […]”

   “But my Leech—a leech / Fit to suck blood, with lubricous round rings, / Capaciously expatiative, which make / His little body like a red balloon, / As full of blood as that of hydrogen, / Sucked from men's hearts; […]”

   “Water is the sole product of the combustion of hydrogen gas. For this important fact we are indebted to Mr. [Henry] Cavendish. He demonstrated it by burning oxygen and hydrogen gases in a dry glass vessel, when a quantity of pure water was generated exactly equal in weight to that of the gases which had disappeared.”

   “It was an enormous balloon filled with helium, that elemental gas forged from hydrogen in the nuclear furnace of the stars, first step along the way in the generation of multiplicity and variety of matter in the universe, including our selves and all our thoughts.”

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

   “Each carbon of the ring has one hydrogen attached except for the fourth one, which is bonded to a substituent.”

   “Water occupies an intermediate position and shows tendencies both to add and give up hydrogen, which are nearly balanced. Then, in terms of the Lewis theory, a free pair of electrons on one water molecule might be able to exert sufficient force on a hydrogen held by a pair of electrons on another water molecule to bind the two molecules together.”

3. (uncountable)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.”

   “[Jacques] Charles was the first to send up a hydrogen balloon, 12 feet in diameter, which rose 3,123 feet, disappeared in the clouds, and fell at the distance of 15 miles.”

   “As the tension between the Soviets and the United States increased from 1948 to 1950, the administration took a number of actions that [George Frost] Kennan felt narrowed the possibilities for constructive negotiations between the two nations. These actions included the decisions to build the hydrogen bomb, to maintain American troops in Japan after the formal occupation was over, to create an independent West German state, and to form the North Atlantic Treaty Organization (NATO).”

   “To this end the Department of Science and Technology has identified the hydrogen economy and related fuel cell technologies as a "Frontier Science and Technology Area" that could potentially change the innovation course of the country's natural resources, and yield multiple social and economic benefits.”

4. (countable)Molecular hydrogen (sense 1; symbol H₂), a colourless, odourless and flammable gas at room temperature.
5. (uncountable)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.”

   “Hydrogen is generally considered to be electronically the same as deuterium. […] Neutron diffraction experiments by Ferguson, Schindler, Tanaka, and Morita (ref. 4) revealed that, near 50K, some hydrogen had moved from interstitial octahedral sites (0, 0, 1/2; 1/2, 0, 0; …) of the palladium lattice to the tetrahedral sites (1/4, 1/4, 1/4; 3/4, 3/4; …) (O-T transitions).”