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; see 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

Explanation

Hydrogen is the very lightest chemical element, and the first one listed on the periodic table. Some scientists believe that hydrogen will one day be used as fuel in cars and trucks. Hydrogen's atomic number is one, because it has a single proton in its nucleus. It's a light, flammable gas that's the most common element in the universe. Humans use hydrogen in many ways, including to make ammonia, fertilizers, and petroleum products. It's also used in rocket fuel, and some see its potential as a clean alternative to oil and gas. Hydrogen's Greek roots mean "water-producing," from the fact that when it's exposed to oxygen, hydrogen produces water.

Example Sentences

Hydrogen has often been touted as an environmentally-friendly future fuel because when burned the only by-product is water vapour.

From BBC • Apr. 21, 2026

Moeve—formerly known as Cepsa and owned by the United Arab Emirates’ investment company Mubadala and U.S. private-equity firm Carlyle—has kicked off construction of the Andalusian Green Hydrogen Valley development in southern Spain.

From The Wall Street Journal • Apr. 10, 2026

Hydrogen Hyperproducers are individuals who pass gas frequently.

From Science Daily • Mar. 14, 2026

They’ll also get a small plaque declaring them a “Prodigious Hydrogen Producer.”

From The Wall Street Journal • Feb. 20, 2026

Hydrogen gas had been leaking into the unit 3 building since the day before.

From "Meltdown" by Deirdre Langeland