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Helium, like the other noble gases, is chemically inert. Its single electron shell is filled, making possible reactions with other elements extremely difficult and the resulting compounds quite unstable. Helium is the second most abundant element in the universe, after hydrogen, however it is rare on earth. It is present in dry air at a concentration of 0.0005%. Its principal source is natural gas wells where the helium is extracted from the crude natural gas stream and purified. Helium can be stored and shipped either as a gas or a cryogenic liquid. Since helium is noncombustible and has a low specific gravity, it is preferred to hydrogen as the gas in lighter-than-air applications such as the filling of balloons and blimps. Helium is used in inert-gas arc welding for light metals such as aluminium and magnesium alloys that might otherwise oxidise (the helium protects heated parts from attack by air). Helium is also used in place of nitrogen as part of the synthetic atmosphere breathed by deep-sea divers, caisson workers, and others, because it reduces susceptibility to the bends.
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Helium is an inert, colourless,
odourless gaseous element. It has monatomic molecules and is the lightest of all
elements, except hydrogen. Helium solidifies at -272.2°C (-457.9°F) at pressures
above 19,000 torr (25 atmospheres), and boils at -268.9°C (-452.1°F) at atmospheric
pressure. It has a gaseous specific gravity of 0.138 and a density of 0.1664 g/litre
at 20°C (68°F).
