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Hydrogen is the lightest and most abundant chemical element in the universe. All heavier elements are built from hydrogen as well as helium. There are three isotopes of hydrogen. Protium, by far the most common at more than 99.98 percent of all hydrogen, has one proton and one electron. It is unique among the stable isotopes of all elements in that it has no neutrons. Deuterium, the other stable isotope, contains one proton and one neutron in its nucleus. Tritium, which contains one proton and two neutrons, is radioactive and decays with a half-life of about 12.32 years. At room temperature hydrogen is a colorless, odorless and tasteless gas. It is highly flammable in the presence of even small concentrations of oxygen, with which it combines to form water. It becomes a liquid at very low temperatures, and it can also be converted into a metal when subjected to extreme pressure. In fact, it is thought that larger planets, such as Jupiter and Saturn, contain large amounts of metallic hydrogen. Hydrogen is estimated to account for roughly 75 percent of the universe in terms of mass, and much more in terms of the number of atoms. However, it accounts for only a small fraction of the mass of Earth, and it is mostly locked in compounds, mainly water; only trace amounts of gaseous hydrogen exist in Earth's atmosphere. Hydrogen is a major component of organic molecules, and it is necessary for all organisms, which obtain it from water. Hydrogen is used in various industrial processes, particularly in the processing of fossil fuels and in the production of ammonia. There is also growing interest in using it as a fuel because of its lack of toxic emissions (it only emits water vapor). The main way that it is produced is from hydrocarbons, although it an also be produced by the electrolysis of water. Although hydrogen gas had been artificially produced centuries earlier, its discovery is usually credited to the British scientist Henry Cavendish in 1766. The word was created in 1783 by the French chemist Antoine Lavoisier, who combined the ancient Greek words for water and forming. Created November 3, 2006. |