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hydrogen (hīˈdrəjən) [Gr.,=water forming], gaseous chemical element; symbol H; at. no. 1; interval in which at. wt. ranges 1.00784–1.00811; m.p. −259.14℃; b.p. −252.87℃; density 0.08988 grams per liter at STP; valence usually +1.

The Isotopes and Forms

Atmospheric hydrogen is a mixture of three isotopes. The most common is called protium (mass no. 1, atomic mass 1.007822); the protium nucleus (protium ion) is a proton. A second isotope of hydrogen is deuterium (mass no. 2, atomic mass 2.0140), the so-called heavy hydrogen, often represented in chemical formulas by the symbol D. The deuterium nucleus, or ion, is called the deuteron; it consists of a proton plus a neutron. The two isotopes are found in atmospheric hydrogen in the proportion of about 1 atom of deuterium to every 6,700 atoms of protium. Protium and deuterium differ slightly in their chemical and physical properties; for example, the boiling point of deuterium is about 3℃ lower than protium. The properties of compounds they form differ depending on the ratio of the two isotopes present.

Deuterium oxide (D2O), the so-called heavy water, is present in ordinary water; the concentration of deuterium oxide is increased by electrolysis of the water. The melting point (3.79℃), boiling point (101.4℃), and specific gravity (1.107 at 25℃) of deuterium oxide are higher than those of ordinary water. Deuterium oxide is used as a moderator in nuclear reactors. Deuterium is also of importance because of the wide use it has found in scientific research; for example, chemical reaction mechanisms have been studied by the use of deuterium atoms as tracers (i.e., deuterium is substituted for atoms of ordinary hydrogen in compounds), making it possible to follow the course of individual molecules in a reaction.

Tritium (mass no. 3, atomic mass 3.016), a third hydrogen isotope, is a radioactive gas with a half-life of about 121-4 years; it is often represented in chemical formulas by the symbol T. It is produced in nuclear reactors and occurs to a very limited extent in atmospheric hydrogen. It is used in the hydrogen bomb, in luminous paints, and as a tracer. The tritium nucleus, or ion, is called the triton; it consists of a proton plus two neutrons. Tritium oxide (T2O) has a melting point (4.49℃) higher than that of deuterium oxide.

Besides being a mixture of three isotopes, hydrogen is a mixture of two forms, an ortho form and a para form, which differ in their electronic and nuclear spins. At room temperature atmospheric hydrogen is about 3-4 ortho-hydrogen and 1-4 para-hydrogen. The two forms differ slightly in their physical properties.


Under ordinary conditions hydrogen is a colorless, odorless, tasteless gas that is only slightly soluble in water; it is the least dense gas known. It is the first element in Group 1 of the periodic table. Ordinary hydrogen gas is made up of diatomic molecules (H2) that react with oxygen to form water (H2O) and hydrogen peroxide (H2O2), usually as a result of combustion. A jet of hydrogen burns in air with a very hot blue flame. The flame produced by a mixture of oxygen and hydrogen gases (as in the oxyhydrogen blowpipe) is extremely hot and is used in welding and to melt quartz and certain glasses. Hydrogen gas must be used with caution because it is highly flammable; it forms easily ignited explosive mixtures with oxygen or with air (because of the oxygen in the air). At high temperatures hydrogen is a chemically active mixture of monohydrogen (atomic hydrogen) and the normal diatomic hydrogen (see allotropy).

Hydrogen has a great affinity for oxygen and is a powerful reducing agent (see oxidation and reduction). It reacts with nitrogen to form ammonia. With the halogens it forms compounds (hydrogen halides) that are strongly acidic in water solution. With sulfur it forms hydrogen sulfide (H2S), a colorless gas with an odor like rotten eggs; with sulfur and oxygen it forms sulfuric acid. It combines with several metals to form metal hydrides such as calcium hydride. Combined with carbon (and usually other elements) it is a constituent of a great many organic compounds, such as hydrocarbons, carbohydrates, fats, oils, proteins, and organic acids and bases.

It is theoretically possible for hydrogen to exhibit the properties of a metal, such as electrical conductivity. Although researchers have been able to squeeze hydrogen into liquid and crystalline solid states through applications of intense heat, cold, and pressure, the metallic form has proved elusive. In 1996, by compressing liquid hydrogen to nearly 2 million atmospheres pressure and a temperature of 4,400K, a team at the Lawrence Livermore National Laboratory reported that they detected metallic hydrogen for a millionth of a second (the creation of the form was not the purpose of their experiment). Subsequent claims of producing metallic hydrogen have been disputed. There is no practical application for metallic hydrogen, but proof of the existence of a metallic form may have implications for theories of how the magnetic fields of Jupiter and Saturn are produced.

Sources and Commercial Preparation

While hydrogen is only about one part per million in the atmosphere, it is the most abundant element in the universe. It is believed that hydrogen makes up about three quarters of the mass of the universe, or over 90% of the molecules. It is found in the sun and in other stars, where it is the major fuel in the fusion reactions (see nucleosynthesis) from which stars derive their energy.

Hydrogen is prepared commercially by catalytic reaction of steam with hydrocarbons, by the reaction of steam with hot coke (carbon), by the electrolysis of water, and by the reaction of mineral acids on metals. Millions of cubic feet of hydrogen gas are produced daily in the United States alone.


Hydrogen was formerly used for filling balloons, airships, and other lighter-than-air craft, a dangerous practice because of hydrogen's explosive flammability; there were disastrous fires, e.g., the immolation of the German airship Hindenburg at its mooring at Lakehurst, N.J., in 1937. Helium is preferable for use in lighter-than-air craft since it is not flammable. Hydrogen is used in the Haber process for the fixation of atmospheric nitrogen, in the production of methanol, and in hydrogenation of fats and oils. It is also important in low-temperature research. It can be liquefied under pressure and cooled; when the pressure is released, rapid evaporation takes place and some of the hydrogen solidifies.

Discovery of Hydrogen and Its Isotopes

Although hydrogen was prepared many years earlier, it was first recognized as a substance distinct from other flammable gases in 1766 by Henry Cavendish, who is credited with its discovery; it was named by A. L. Lavoisier in 1783. Deuterium was discovered by H. C. Urey, F. G. Brickwedde, and G. M. Murphy in 1932, although its existence had been suspected for some years. Deuterium oxide was also discovered by Urey and was first obtained in nearly pure form by G. N. Lewis. Tritium was synthesized by Ernest Rutherford, L. E. Oliphant, and Paul Harteck in 1935.
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(hÿ -drŏ-jĕn) Chemical symbol: H. The simplest chemical element, with an atomic number of one and the lowest density of all elements. In its most abundant form it consists of a proton orbited by a single electron. Two other forms, i.e. isotopes, exist: deuterium has a proton plus a neutron as its nucleus; tritium, which is radioactive (i.e. unstable), has a proton plus two neutrons.

Hydrogen is the most abundant element in the Universe: 91% by numbers of atoms, 70% by mass. All this hydrogen is primeval in origin, created in the earliest phase of the Universe (see Big Bang theory). The amount of deuterium formed relative to the amount of hydrogen would have been very sensitive to the density of matter at the time of formation: deuterium readily combines with an additional neutron to form tritium, which rapidly decays into an isotope of helium. Thus if the Universe was very dense in its first few minutes, most of the deuterium would have been converted to helium. Deuterium is not easy to detect but recent measurements of the ratio of deuterium to hydrogen give an upper limit of about 2 × 10–5 for interstellar gas.

Hydrogen can exist in atomic, molecular, and ionized forms. Ionized hydrogen, H+, more usually denoted H II, results when neutral atoms are stripped of their electrons. It occurs at high temperatures, as in the centers of stars and in H II regions. Nuclear fusion of hydrogen ions, i.e. protons, in the stellar core generates the energy of main-sequence stars by either the proton-proton chain reaction or the carbon cycle. Ionized hydrogen can also be produced by photoionization, as in emission nebulae, and may then be detected by its recombination line emission.

In addition to the positive ion, the negative ion, H, can occur when an electron attaches itself loosely to a neutral atom. A continuous spectrum of radiation is emitted in the process. The negative ion is not very stable and breaks up easily with the second electron escaping with any amount of energy: this results in a continuous absorption spectrum. These two processes, producing continuous emission and absorption of both light and infrared radiation, take place concurrently, as happens in the Sun's photosphere. The H2– ion has recently been discovered.

Neutral hydrogen, usually denoted H I, occurs throughout interstellar space as filaments and clouds (H I regions) of varying density. It is detected by means of the 21-cm hydrogen line emission. At low temperatures and when the hydrogen density is sufficiently high, pairs of hydrogen atoms can combine to form molecular hydrogen, H2, which exists in discrete molecular clouds. The molecular ion H3 + forms in such clouds when a molecule, H2, is ionized and then further reacts with H2. The molecular ion is thought to have an important role in interstellar chemistry, allowing protons to be transferred to oxygen, carbon, and other heavy atoms, thereby leading to the formation of many different interstellar molecules. Molecular hydrogen is also a major constituent of the atmosphere of the giant planets, with liquid hydrogen forming the bulk of the interiors of Jupiter and Saturn.

Hydrogen can be observed at a number of wavelengths including radio (21 cm), infrared, visible, and ultraviolet wavelengths (see hydrogen spectrum).

Collins Dictionary of Astronomy © Market House Books Ltd, 2006


The first chemical element, symbol H, in the periodic table, atomic number 1, atomic weight 1.00797; under ordinary conditions it is a colorless, odorless, tasteless gas composed of diatomic molecules, H2; used in manufacture of ammonia and methanol, for hydrofining, for desulfurization of petroleum products, and to reduce metallic oxide ores.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a. 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
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
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