hydrogen

American  

[hahy-druh-juhn] / ˈhaɪ drə dʒən /

noun

hydrogens plural

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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Other Word Forms

Noun Inflected Forms

• hydrogens
  plural

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 peroxide will get rid of algae blooms if enough is used, he said, but it has to be mixed into the water appropriately.

From The Wall Street Journal • Jun. 18, 2026

Hydrogen is the most abundant element in the universe and is widely viewed as an important clean energy source.

From Science Daily • Jun. 2, 2026

Hydrogen already plays a major role in the global economy, which is valued at roughly $135 billion.

From Science Daily • May 20, 2026

Hydrogen sulfide smells like rotten eggs and can be produced by a chemical reaction and by some organic material, such as sewage.

From The Wall Street Journal • Apr. 22, 2026

The impetus was the publication of a highly sensationalized book, The Hydrogen Bomb: The Men, The Menace, The Mechanism, by journalists James Shepley and Clay Blair Jr., which accepted Teller’s version of events as gospel.

From "Big Science" by Michael Hiltzik