hydrogen
Pluralhydrogens (some meanings)
- 1The chemical element with atomic number 1 and symbol H, the lightest of all elements, with an atomic weight of 1.008.
Hydrogen sits at the top of the periodic table as element number one.
Most of the visible matter in the universe is hydrogen.
Note
The name records a chemical result. Antoine Lavoisier and Louis-Bernard Guyton de Morveau coined French hydrogène from Greek húdōr, "water", and gennáō, "I bring forth": the element that brings forth water. English builds the same way from hydro- and -gen. Ian McEwan's Enduring Love (1997) uses the mass noun on a cosmic scale, describing helium as "forged from hydrogen in the nuclear furnace of the stars."
- 2The element in its molecular form (H₂), a flammable gas that has no color or smell at room temperature.
Hydrogen ignites readily when mixed with air and exposed to a spark.
Engineers store hydrogen under high pressure to fuel the cells.
- 3A quantity or sample of the element or its molecular gas, as handled in practice.
The technician drew off a small amount of hydrogen for analysis.
Each cylinder holds enough hydrogen for a full week of testing.
- 4A single atom of hydrogen, the lightest chemical element, made of one proton and one electron.
In methane, four hydrogen atoms bond to a single carbon atom.
A single hydrogen loses its electron to become a bare proton.
Note
Here the word turns countable: a hydrogen is one atom, and the plural hydrogens counts them. It is easy to confuse with a single H₂ molecule, which can also be called a hydrogen, though the atom is only half of one. When the difference matters, saying hydrogen atom or hydrogen molecule, or writing H versus H₂, removes the doubt.
- 5A single molecule of the molecular species H₂, formed from two hydrogen atoms bonded together.
When light strikes it, each hydrogen can break apart into two reactive atoms.
Two atoms share a bond to form a single hydrogen.
- 6The isotope hydrogen-1 (protium), having one proton and no neutrons, as distinct from deuterium and tritium.
Most hydrogen on Earth is protium, with deuterium making up only a tiny fraction.
Unlike deuterium, ordinary hydrogen carries no neutron in its nucleus.
Similarprotium
Note
Isotope talk narrows the word. Set beside deuterium and tritium, plain hydrogen means the lightest form, protium, and formulas mark the contrast by letter: PdHₓ for palladium loaded with ordinary hydrogen, PdDₓ for palladium loaded with deuterium. Physicists generally treat hydrogen and deuterium as electronically the same, so the letter in the formula is what separates the two.
Practice
Can you tell the meanings of hydrogen apart? Answer 3 quick questions from this page, or write your own sentence and AI will check it.
Origin
From French hydrogène, coined by Louis-Bernard Guyton de Morveau and Antoine Lavoisier, from Ancient Greek húdōr ("water") plus gennáō ("I bring forth"), corresponding to hydro- plus -gen.