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The lightest chemical element: hydrogen
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HYDROGEN is a colourless, odourless, tasteless and flammable gaseous substance.
The hydrogen atom has a nucleus (核) consisting of proton（质子）bearing one unit of positive electrical charge; an electron（电子）, bearing one unit of negative electrical charge, is associated with this nucleus.
Hydrogen is by far the most abundant element in the universe, accounting for about 75 per cent of the mass of all matter. Collected by gravitational forces in stars, hydrogen is converted into helium (氦) by nuclear fusion.
Hydrogen is present in all animal and vegetable substances in the form of compounds in which it is combined with carbon (碳) and other elements. It consititutes nearly 11 per cent of the mass of water. 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 carbon monoxide (一氧化碳). In 1776 the English chemist Henry Cavendish showed that hydrogen, which was then called "inflammable air", was distinct from other combustible gases (易燃气体). In 1776 J Warltire noticed that water was formed when hydrogen burned. In 1781 Cavendish confirmed this observation, and the French chemist Antoine Lavoisier subsequently coined the name hydrogen (Greek: maker of water).
The largest amounts of hydrogen are currently used in synthesizing ammonia（氨水）and methanol（甲醇）, in desulfurizing（使脱硫）petroleum fuels, and for producing stable products from certain oil-refinery by-products.
Liquid hydrogen is used in the laboratory to produce extremely low temperatures. Liquid hydrogen is of great importance in space-exploration programmes as a rocket fuel with oxygen. The deuterium（重氢）isotype（同位型）of hydrogen is the key component of the nuclear bomb.
Hydrogen is the lightest chemical element, has the highest heat conductivity, and has the highest co-efficient of diffusion of all the gases.(SD-Agencies)
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