(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 percent 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. It appears that the German-Swiss alchemist Paracelsus in the 16th century had handled hydrogen, since he found that a flammable gas was produced when a metal was dissolved in an acid. However, hydrogen was then confused with other flammable gases such as hydrocarbons and carbon monoxide. In 1766 the English chemist Henry Cavendish showed that hydrogen, which was then called inflammable air, or phlogiston, was distinct from other combustible gases by measuring its density and the amount of gas evolved from a given amount of acid and metal. In 1776 J. Warltire noticed that water was formed when hydrogen burned. In 1781 Cavendish confirmed this observation, and the French chemist Antoine-Laurent Lavoisier subsequently coined the name hydrogen (Greek: maker of water). The two heavier isotopes of hydrogen, deuterium and tritium, were discovered in 1931 and 1935, respectively. Hydrogen was used to fill balloons from 1783 until at least World War II, although, for passenger-carrying airships, helium has the advantage of nonflammability. The largest amounts of hydrogen are currently used in synthesizing ammonia and methanol, in desulfurizing petroleum fuels, and for producing stable, volatile products from certain oil-refinery by-products. Hydrogen also is used in the hydrogenation of organic compounds to make solvents and industrial chemicals, such as cyclohexane from benzene, and food products, such as margarine and shortenings from animal and vegetable fats and oils. Hydrogen reacts with chlorine or bromine to produce hydrogen chloride or hydrogen bromide. Tungsten and molybdenum are prepared by treating their oxides or salts with hydrogen. 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. The combustion of hydrogen with oxygen produces temperatures of about 2,600 C (4,700 F); even higher temperatures, above 3,400 C (6,150 F), result from the recombination of hydrogen atoms produced by the dissociation of hydrogen molecules by an electric arc or a heated tungsten filament. Small quantities of hydrogen usually are produced by the action of zinc upon sulfuric acid. Hydrogen is also produced by industrial processes that involve the action of steam or oxygen on hydrocarbons such as methane. The element also results as a by-product in various fermentations, in petroleum-refining operations, and in the manufacture of caustic alkalies and chlorine by electrolytic reactions. Hydrogen atoms are reactive and combine with one another in pairs, forming diatomic hydrogen molecules, represented by the formula H2. In the molecular form known as ortho-hydrogen (normal hydrogen), the nuclear spins of both protons are oriented in the same direction, while, in the form known as para-hydrogen, their spins are oriented in opposite directions. At ordinary temperatures, hydrogen consists of about 75 percent ortho-hydrogen and 25 percent para-hydrogen, but as the temperature is lowered, a growing proportion of the ortho-hydrogen molecules are converted to para-hydrogen ones. Similarly, at elevated temperatures para-hydrogen can be converted to normal hydrogen. 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 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 also associated with this nucleus. Under ordinary conditions, hydrogen gas is a loose aggregation of hydrogen molecules, each consisting of a pair of atoms, a diatomic molecule, H2. The earliest known important chemical property of hydrogen is that it burns with oxygen to form water, H2O; indeed, the name hydrogen is derived from Greek words meaning maker of water. Although hydrogen is the most abundant element in the Galaxy (10 times as abundant as helium, the next most widely occurring element), it makes up only about 0.14 percent of the Earth's crust by weight. It occurs, however, in vast quantities as part of the water in oceans, ice packs, rivers, lakes, and the atmosphere. As part of innumerable carbon compounds, hydrogen is present in all animal and vegetable tissue and in petroleum. Even though it is often said that there are more known compounds of carbon than of any other element, the fact is that, since hydrogen is contained in almost all carbon compounds and also forms a multitude of compounds with all other elements (except some of the noble gases), it is possible that hydrogen compounds are more numerous. Elementary hydrogen finds its principal industrial application in the manufacture of ammonia (a compound of hydrogen and nitrogen, NH3) and in the hydrogenation of carbon monoxide and organic compounds. Hydrogen has three known isotopes. The mass numbers of hydrogen's isotopes are 1, 2, and 3, the most abundant being the mass 1 isotope generally called hydrogen (symbol H, or 1H) but also known as protium. The mass 2 isotope, which has a nucleus of one proton and one neutron and has been named deuterium, or heavy hydrogen (symbol D, or 2H), constitutes 0.0156 percent of the ordinary mixture of hydrogen. Tritium (symbol T, or 3H), with one proton and two neutrons in each nucleus, is the mass 3 isotope and constitutes about 10-15 to 10-16 percent of hydrogen. The practice of giving distinct names to the hydrogen isotopes is justified by the fact that there are significant differences in their properties. Paracelsus, physician and alchemist, in the 16th century unknowingly experimented with hydrogen when he found that a flammable gas was evolved when a metal was dissolved in acid. The gas, however, was confused with other flammable gases, such as hydrocarbons and carbon monoxide. In 1766 Henry Cavendish, English chemist and physicist, showed that hydrogen, then called flammable air, phlogiston, or the flammable principle, was distinct from other combustible gases because of its density and the amount of it that evolved from a given amount of acid and metal. In 1781 Cavendish confirmed previous observations that water was formed when hydrogen was burned, and Antoine-Laurent Lavoisier, the father of modern chemistry, coined the French word hydrogne from which the English form is derived. In 1929 Karl Friedrich Bonhoeffer, a German physical chemist, and Paul Harteck, an Austrian chemist, on the basis of earlier theoretical work, showed that ordinary hydrogen is a mixture of two kinds of molecules, ortho-hydrogen and para-hydrogen. Because of the simple structure of hydrogen, its properties can be theoretically calculated relatively easily. Hence hydrogen is often used as a theoretical model for more complex atoms, and the results are applied qualitatively to other atoms. Additional reading Adalbert Farkas, Orthohydrogen, Parahydrogen, and Heavy Hydrogen (1935), is a classic monograph by an early researcher in this area. Isidor Kirshenbaum, Physical Properties and Analysis of Heavy Water (1951), is a comprehensive treatise. Willard F. Libby, Collected Papers, vol. 1, Tritium and Radiocarbon (1981), contains classic articles on tritium dating. E. Anthony Evans, Tritium and Its Compounds, 2nd ed. (1974), is a compact source of information. R.P. Bell, The Proton in Chemistry, 2nd ed. (1973); and Howard K. Rae (ed.), Separation of Hydrogen Isotopes (1978), are also useful. William Lee Jolly The Editors of the Encyclopdia Britannica Figure 15: Transformations of hydrogen compounds in the atmosphere. hydrogen bond Figure 17: The linking of atoms in two peptide links by the hydrogen bonds they can form. The links may be part of the same polypeptide chain that has doubled back on itself, or they may belong to different chains. hydrogen chloride (HCl), a compound of the elements hydrogen and chlorine, a gas at room temperature and pressure. A solution of the gas in water is called hydrochloric acid. Hydrogen chloride may be formed by the direct combination of chlorine (Cl2) gas and hydrogen (H2) gas; the reaction is rapid at temperatures above 250 C (482 F). The reaction, represented by the equation H2 + Cl2 2HCl, is accompanied by evolution of heat and appears to be accelerated by moisture. Hydrogen chloride is commonly prepared both on a laboratory and on an industrial scale by the reaction of a chloride, generally that of sodium (NaCl), with sulfuric acid (H2SO4). It is also produced by the reaction of some chlorides (e.g., phosphorus trichloride, PCl3, or thionyl chloride, SOCl2) with water and as a by-product of the chlorination of many organic substances (e.g., methane, benzene). Hydrochloric acid is prepared by dissolving gaseous hydrogen chloride in water. Because of the corrosive nature of the acid, ceramic, glass, or sometimes tantalum apparatus is commonly used. Hydrochloric acid is usually marketed as a solution containing 2835 percent by weight hydrogen chloride, commonly known as concentrated hydrochloric acid. Anhydrous liquid hydrogen chloride is available, but because heavy and expensive containers are required to store it, the use of hydrogen chloride in this form is limited. Hydrogen chloride is a colourless gas of strong odour; it condenses at -85 C (-121 F) and freezes at -114 C (-173 F). The gas is very soluble in water: at 20 C (68 F) water will dissolve 477 times its own volume of hydrogen chloride. Because of its great solubility, the gas fumes in moist air. A water solution containing 20.24 percent by weight hydrogen chloride boils at 110 C (230 F) without change in composition (azeotropic mixture). In aqueous solution the compound is extensively dissociated into a hydronium ion (H3O+) and chloride ion (Cl-); in dilute solutions the dissociation is essentially complete. Thus, hydrochloric acid is a strong acid. Gaseous hydrogen chloride reacts with active metals and their oxides, hydroxides, and carbonates to produce chlorides. These reactions occur readily only in the presence of moisture. Completely dry hydrogen chloride is very unreactive. The reactions of hydrochloric acid are those of typical strong acids, such as: reactions with metals in which hydrogen gas is displaced, reactions with basic oxides and hydroxides that are neutralized with the formation of a chloride and water, and reactions with salts of weak acids in which the weak acid is displaced. Hydrochloric acid also enters into chemical reactions characteristic of the chloride ion, such as reactions with various inorganic and organic compounds in which hydrochloric acid is used as a chlorinating agent and reactions with metals and their oxides in which complex chloride-containing ions are formed (e.g., with platinum, PtCl62-, with copper, CuCl42-). The latter type of reaction accounts for the ease of solution of certain metals and metallic compounds in hydrochloric acid although they are slowly dissolved in other acids of equal strength (e.g., sulfuric or nitric acid). For this reason hydrochloric acid is used extensively in the industrial processing of metals and in the concentration of some ores. Hydrochloric acid is present in the digestive juices of the human stomach. Excessive secretion of the acid causes gastric ulcers; a marked deficiency of it impairs the digestive process and is sometimes the primary cause of deficiency anemias. Exposure to 0.1 percent by volume hydrogen chloride gas in the atmosphere may cause death in a few minutes. Concentrated hydrochloric acid causes burns and inflammation of the skin.
Meaning of HYDROGEN in English
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