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History
Hydrogen was isolated and identified by the English chemist Henry Cavendish in 1776. It is the lightest element, consisting of a single proton, a single electron and, in its most common isotope, no neutrons, for an atomic weight of 1.00794. It is also by far the most abundant element in the universe, as most stars are made almost entirely of hydrogen plasma. Pure hydrogen (H2) is rare on Earth, but hydrogen is prevalent as a constituent of water, hydrocarbons and most other organic compounds, ammonia and acids. Because it is the simplest atom, study of hydrogen's atomic properties was instrumental in the development of quantum physics in the last century. Water with a high concentration of deuterium, an isotope of hydrogen that includes one neutron, is called "heavy water" and is used to cool and control the rate of fission in nuclear reactors.
Production
Hydrogen is isolated most commonly by chemical reactions involving water or acids or by electrolysis. When superheated steam is passed over a metal surface (such as iron, zinc or magnesium), it oxidizes the metal and releases hydrogen. When acids such as sulfuric acid or hydrochloric acid react with metals, hydrogen is released. Electrolysis is the splitting of a water molecule to produce hydrogen and oxygen, by passing electricity through a salt solution. Industrial production of hydrogen is generally done by a variation of the steam/iron interaction or by electrolysis; the latter yields the purest product.
Occurrence
Hydrogen is not a constituent of rocks, minerals and ores and is found in the Earth's crust as water or hydrocarbons (methane, natural gas, petroleum) permeating porous rock in aquifers or other underground deposits.
Uses
Hydrogen is used in the extraction of metals from oxide ores, as passing hydrogen over heated oxides strips the oxygen from the ore, leaving the pure metal behind. It is also used in the refining of petroleum and producing hydrocarbons from coal and to turn liquid animal or vegetable oils into solid saturated fats for the food industry. Hydrogen was once widely used in lighter-than-air craft such as balloons, blimps and zeppelins, but because it is highly combustible with air, it has been replaced in this role by the inert gas helium.
Future
Hydrogen is seen as a potential alternative fuel for cars and other vehicles, with hydrogen fuel cells already in use on some electric vehicles and studies and tests under way exploring the use of hydrogen in internal combustion engines. In addition, deuterium is seen as a possible fuel for fusion reactors, which are as yet only conceptual.