A colorless, odorless and flammable gas.

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(H), a colourless, odourless, tasteless, flammable gaseous substance that is the simplest member of the family of chemical elements. The hydrogen atom has a nucleus consisting of a proton bearing one unit of positive electrical charge; an electron, bearing one unit of negative electrical charge, is associated with this nucleus.

Although on Earth hydrogen ranks ninth among the elements in abundance, making up 0.9 per-cent of the mass of the planet, it is by far the most abundant element in the universe, accounting
for about 75 percent of the mass of all matter. Collected by gravitational forces in stars, hydrogen is converted into helium by nuclear fusion, a process that supplies the energy of the stars, including the Sun. Hydrogen is present in all animal and vegetable substances in the form of compounds in which it is combined with carbon and other elements. In the form of hydrocarbons, it is a constituent of petroleum and coal. It also constitutes nearly 11 percent of the mass of seawater. The hydrogen content of the Earth's atmosphere remains low because of the continual escape of the gas into space.

Liquid hydrogen is used in the laboratory to produce extremely low temperatures and in bubble chambers for photographing the tracks of nuclear particles. Liquid hydrogen is of great importance in space-exploration programs as a rocket fuel with oxygen or fluorine as the oxidizer. The deuterium isotope of hydrogen is the key component of the thermonuclear bomb.

Hydrogen is the lightest chemical element, has the highest heat conductivity, and has the highest coefficient of diffusion of all the gases. Chemically, hydrogen resembles the elements of groups I and VII of the periodic classification. Under proper conditions, it combines directly with most of the lighter elements and with many of the heavier elements. In compounds with metals, the hydrogen atom acquires a second electron, forming the negatively charged hydride ion, H-; with nonmetals, it shares its electron to form covalently bonded molecules such as methane, ammonia, water, and hydrogen chloride. In certain cases, the covalent bond is easily broken, forming the
hydrogen ion, H+, and a negative ion from the remainder of the original molecule. The properties of most acids, particularly in aqueous solutions, arise from the presence of the hydrogen ion. For additional information about the major hydrogen compounds, see alcohol; ammonia; hydride; hydrocarbon.

Hydrogen reacts violently with fluorine, even at extremely low temperatures; with many other elements, hydrogen reacts upon heating or in the presence of catalysts.

Naturally occurring hydrogen consists of three isotopes: hydrogen-1, or protium, 99.985 percent; hydrogen-2, or deuterium (q.v.), 0.015 percent; and hydrogen-3, or tritium (q.v.), a minute trace. Tritium can be produced artificially; it is radioactive, having a half-life of 12.26 years.

Atomic number 1

Atomic weight 1.00797

Melting point -259.2° C (-434.6° F)

Boiling point -252.8° C (-422.8° F)

Density 0.08988 g/1 (0 C, 1 atm)

Oxidation states -1, 1

Electron config. 1s1