Helium, a chemical element that is a colorless, odorless, and tasteless gas at most temperatures and pressures. It is the second lightest of the elements; only hydrogen is lighter. Helium is one of the noble, or inert, gases, and does not form stable compounds. Helium is abundant in stars, but is relatively rare on earth. It was discovered in 1868 as a component of the sun, but was not found on earth until 1895, when the British chemist Sir William Ramsay isolated it.
The basic source of helium in the stars is fusion, a nuclear reaction in which hydrogen atoms unite to form helium atoms. Helium found on earth is thought to be formed by the natural decay of radioactive substances in rocks. The alpha particles emitted during the decay process are helium nuclei, which quickly gain the two electrons needed to become stable helium atoms.
Most of the world's helium comes from certain natural-gas wells in Texas, Oklahoma. Kansas, New Mexico, and Wyoming. The helium is separated from the natural gas by cooling the gas until all the constituents except helium have been liquefied.
Helium has numerous uses in manufacturing and research. It serves as a pressurizing agent in the liquid-fuel tanks of rockets and missiles. Helium is used as an inert atmosphere for arc welding and metallurgical processes, and as a coolant and protective atmosphere in nuclear reactors. Helium-oxygen mixtures are breathed by divers to prevent the toxic effects of nitrogen (contained in ordinary air) and pure oxygen at great depths. Because such mixtures also make breathing easier, they are given to asthmatic patients and others with respiratory ailments. A once-important use was that of filling airships; it is still used for gas balloons. Helium has about 92 per cent of hydrogen's lifting power, without the fire hazard of hydrogen.
Liquid helium, with the lowest boiling point of any substance, is the principal refrigerant used in cryogenics, the study and use of ultralow temperatures. Liquid helium is especially useful in research dealing with superconductivity (the tendency of certain materials to lose their electrical resistance at very low temperatures).
Just below its boiling point, liquid helium behaves much like other liquids with low boiling points. In this state, it is known as helium I. However, when chilled several degrees below its boiling point, liquid helium enters a second state and is known as helium II. Helium II has some very unusual properties, among which are the lowest viscosity (resistance to flowing) of any liquid; the ability to flow uphill; and the capability of penetrating tiny passages where no other liquid can flow.
Symbol: He. Atomic number: 2. Atomic weight: 4.002602. Boiling point: -452.0 F. (-268.9 C.). Melting point (at about 26 atmospheres pressure): -453.5 F. (-269.7 C.). Specific gravity: gas, 0.138 (air = 1); liquid, 0.126 (water =1). Five isotopes are known, of which He-3 and He-4 are stable. Helium belongs to Group 0 (Inert Gases) of the Periodic Table and has a valence of 0.