hydrogen

American  

[hahy-druh-juhn] / ˈhaɪ drə dʒən /

noun

1. a colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known elements. H; 1.00797; 1; density: 0.0899 grams/liter at 0°C and 760 millimeters pressure.

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hydrogen British  

/ ˈhaɪdrɪdʒən /

noun

1. 1. a flammable colourless gas that is the lightest and most abundant element in the universe. It occurs mainly in water and in most organic compounds and is used in the production of ammonia and other chemicals, in the hydrogenation of fats and oils, and in welding. Symbol: H; atomic no: 1; atomic wt: 1.00794; valency: 1; density: 0.08988 kg/m³; melting pt: –259.34°C; boiling pt: –252.87°C See also deuterium tritium

   2. ( as modifier )

      hydrogen bomb

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hydrogen Scientific  

/ hī′drə-jən /

1. The lightest and most abundant element in the universe, normally consisting of one proton and one electron. It occurs in water in combination with oxygen, in most organic compounds, and in small amounts in the atmosphere as a gaseous mixture of its three isotopes (protium, deuterium, and tritium) in the colorless, odorless compound H ₂. Hydrogen atoms are relatively electropositive and form hydrogen bonds with electronegative atoms. In the Sun and other stars, the conversion of hydrogen into helium by nuclear fusion produces heat and light. Hydrogen is used to make rocket fuel, synthetic ammonia, and methanol, to hydrogenate fats and oils, and to refine petroleum. The development of physical theories of electron orbitals in hydrogen was important in the development of quantum mechanics. Atomic number 1; atomic weight 1.00794; melting point −259.14°C; boiling point −252.8°C; density at 0°C 0.08987 gram per liter; valence 1.

2. See Periodic Table See Note at oxygen

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Etymology

Origin of hydrogen

First recorded in 1785–95; from the French word hydrogène; hydro- ¹, -gen

Compare meaning

How does hydrogen compare to similar and commonly confused words? Explore the most common comparisons:

• hydrogen vs. atomic bomb
• deuterium vs. hydrogen

Example Sentences

That causes the molecule to crack into hydrogen and ethylene, the basic building block of plastics such as polyethylene.

From The Wall Street Journal • Apr. 5, 2026

Stars like our Sun eventually run out of hydrogen fuel.

From Science Daily • Apr. 4, 2026

Helium is the second-most-common element in the universe after hydrogen, but it is rare on Earth, where it mainly exists in small concentrations in pockets of natural gas.

From The Wall Street Journal • Mar. 31, 2026

One type contains high amounts of aliphatic organic compounds, which are simple carbon-based molecules made of chains of carbon and hydrogen.

From Science Daily • Mar. 31, 2026

At 3:36, somewhere in the unit 1 building, a spark met the cloud of hydrogen gas.

From "Meltdown" by Deirdre Langeland