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

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Etymology

Origin of hydrogen

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

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Compare meaning

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

• hydrogen vs. atomic bomb
• deuterium vs. hydrogen

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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

See Examples For:

hydrogen hydrogens

These simulations attempt to recreate how galaxies formed, how gas moved through them, and how radiation eventually transformed the hydrogen filling intergalactic space.

From Science Daily ● Oct. 30, 2026

As more hydrogen became ionized, ultraviolet light could travel much farther through intergalactic space.

From Science Daily ● Oct. 30, 2026

When full, it contains 1,629,577 pounds of propellants, as well as liquid oxygen and liquid hydrogen, both of which are extremely cold materials.

From Los Angeles Times ● Oct. 28, 2026

“But Bloom creates energy from hydrogen gas, with faster speed — three to four months.”

From MarketWatch ● Oct. 26, 2026

As a cloud of gas collapses, it gets hotter and denser, and hydrogen atoms slam into one another with increasing force.

From "Zero: The Biography of a Dangerous Idea" by Charles Seife

But, at room temperature, the number of hydrogens attached to lanthanum fluctuates between 2 and 3, making it impossible to have efficient conduction.

From Science Daily ● Dec. 22, 2023

When two hydrogens and an oxygen share electrons via covalent bonds, a water molecule is formed.

From Textbooks ● Jun. 9, 2022

The mass spectrometer then works out how many carbons there are in the molecule, how many hydrogens, how many oxygens; and starts to build up the complete structure.

From BBC ● Jul. 7, 2015

Fructose and allulose have the same atomic components—six carbons, 12 hydrogens and six oxygens—but differ in the placement of one oxygen and two hydrogen atoms.

From Newsweek ● Feb. 22, 2015

The hydrogens were part of the hydrogen bonds that held together the three intertwined chains.

From "Double Helix" by James D. Watson

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