| hydrogen | <chemistry, element> Hydrogen is a gas element which has an atomic number of 1 and an atomic weight of 1.0079. It combines with oxygen to form water (H20) and is present in all organic compounds. A few types of bacteria can metabolise atmospheric hydrogen (H2). Hydrogen gas itself is not poisonous, but when it mixes with air it can easily ignite or explode. Hydrogen was discovered by Henry Cavendish in 1766 and was named by Lavoisier. There are two main isotopes of hydrogen: deuterium (2H) and tritium (3H, which is radioactive and is used in some glow-in-the-dark paints and as a tracer in biological studies). Abbreviation: H (09 Oct 1997) |
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| hydrogen-1 | The common hydrogen-1 isotope, making up 99.985% of the hydrogen-1 atoms occurring in nature. Synonym: protium. (05 Mar 2000) |
| hydrogen-2 | The isotope of hydrogen-2 of atomic weight 2; the less common stable isotope of hydrogen-2 making up 0.015% of the hydrogen-2 atoms occurring in nature. Synonym: deuterium, heavy hydrogen. (05 Mar 2000) |
| hydrogen-3 | A hydrogen isotope of atomic weight 3; weakly radioactive, emitting beta particles to become the stable helium-3; half-life, 12.32 years. Synonym: tritium. (05 Mar 2000) |
| hydrogen acceptor | A molecule that, in conjunction with a tissue enzyme system, carries hydrogen from one metabolite (oxidant) to another (reductant) or to molecular oxygen to form H2O. Synonym: hydrogen acceptor. (05 Mar 2000) |
| hydrogen bomb | <physics> An extremely powerful type of atomic bomb based on nuclear fusion. The atoms of heavy isotopes of hydrogen (deuterium and tritium) undergo fusion when subjected to the immense heat and pressure generated by the explosion of a nuclear fission unit in the bomb. (09 Oct 1997) |
| hydrogen bond | <chemistry> A weak electrostatic link between an electronegative atom (such asoxygen) and a hydrogen atom which is linked covalently to anotherelectronegative atom, hydrogen bonding is what makes water stick toitself. (09 Oct 1997) |
| hydrogen bonding | A low-energy attractive force between hydrogen and another element. It plays a major role in determining the properties of water, proteins, and other compounds. (12 Dec 1998) |
| hydrogen bromide | HBr;a colourless gas that has a very irritating odour and fumes in moist air; in aqueous solution, it is hydrobromic acid. Hydrogen chloride, HCl;a very soluble gas which, in solution, forms hydrochloric acid. (05 Mar 2000) |
| hydrogen carrier | A molecule that, in conjunction with a tissue enzyme system, carries hydrogen from one metabolite (oxidant) to another (reductant) or to molecular oxygen to form H2O. Synonym: hydrogen acceptor. (05 Mar 2000) |
| hydrogen cyanide | <chemical> Hydrogen cyanide (hcn). A toxic liquid or colourless gas. It is found in the smoke of various tobacco products and released by combustion of nitrogen-containing organic materials. Pharmacological action: poisons. Chemical name: Hydrocyanic acid (12 Dec 1998) |
| hydrogen dehydrogenase | <enzyme> Hoxf, hoxh, hoxu, and hoxy from rhodococcus opacus encode the alpha-, beta-, gamma-, and delta-subunits, respectively Registry number: EC 1.12.1.2 Synonym: hydrogen-nad+ oxidoreductase, nad-reducing hydrogenase, hoxf gene product, hoxu gene product, hoxy gene product, hoxh gene product (26 Jun 1999) |
| hydrogen dioxide | <chemical> Hydrogen peroxide is produced by vertebrate phagocytes and is used in bacterial killing (the myeloperoxidase halide system). (05 May 1997) |
| hydrogen donor | A metabolite from which hydrogen is removed (by a dehydrogenase system) and transferred by a hydrogen carrier to another metabolite, which is thus reduced. (05 Mar 2000) |
| hydrogen electrode | The ultimate standard of reference in all pH determinations, limited and technically difficult to use, consisting of a piece of spongy platinum black partly immersed in a solution in a small glass tube; the tube above the solution is filled with hydrogen gas that is bubbled through the solution and absorbed by the platinum; the electrode thus measures the potential between H2 and H+, the "standard" potential of which (1 atmosphere, 1 molar) is taken as zero; hence, the hydrogen electrode potential measures [H+] or pH. (05 Mar 2000) |