a colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known elements. Symbol: H; atomic weight: 1.00797; atomic number: 1; density: 0.0899 g/l at 0°C and 760 mm pressure.

1785–95; < French hydrogène. See hydro-1, -gen

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World English Dictionary
hydrogen (ˈhaɪdrɪdʒən)
a.  deuterium See also tritium 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
 b.  (as modifier): hydrogen bomb
[C18: from French hydrogène, from hydro- + -gen; so called because its combustion produces water]

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Word Origin & History

1791, from Fr. hydrogène, coined 1787 by G. de Morveau from Gk. hydr-, stem of hydros "water" + Fr. -gène "producing." So called because it forms water when exposed to oxygen. Hydrogen bomb first recorded 1947; shortened form H-bomb is from 1950.
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American Heritage
Medical Dictionary

hydrogen hy·dro·gen (hī'drə-jən)
Symbol H
A colorless, highly flammable gaseous element, the most abundant in the universe, used in ammonia and methanol synthesis, in the hydrogenation of organic materials, and as a reducing atmosphere. Atomic number 1; atomic weight 1.00797; melting point -259.34°C; boiling point -252.9°C; density at 0°C 0.08988 gram per liter; valence 1.

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American Heritage
Science Dictionary
hydrogen   (hī'drə-jən)  Pronunciation Key 
Symbol H
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 H2. 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. See Periodic Table. See Note at oxygen.
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American Heritage
Cultural Dictionary

hydrogen definition

The lightest chemical element; its symbol is H. Hydrogen normally consists of a single electron in orbit around a nucleus made up of a single proton. It is usually found as a gas and has several uses as a fuel.

Note: Hydrogen atoms are combined to form helium atoms in fusion reactions in stars and in hydrogen bombs, which release huge amounts of energy. Hydrogen also burns rapidly, producing water as it combines with oxygen (see H<sub>8</sub>O and oxidation).
Note: For a time, hydrogen was frequently used to fill blimps and dirigibles because of its extremely low weight. In 1937, however, the hydrogen in the dirigible Hindenburg caught fire, and many of the passengers and crew were killed. Since that time, helium has been widely preferred to hydrogen for use in airships; it is not as buoyant (see buoyancy) or cheap as hydrogen, but, being an inert gas, it does not burn.
Note: Because there is so much hydrogen in stars, it is by far the most abundant element in the universe.
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Example sentences
Hydrogen peroxide is a liquid commonly used to fight germs.
Over time, this hydrogen leaks out of the steel and into the chamber.
The hydrogen could then power hydrogen fuel cells, a type of battery that emits
  only water.
Some support hydrogen as a source of energy for the transportation sector.
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