HELIUM (Greek helios, "sun"), symbol He, inert, colorless, odorlessgaseous element. In group 18 (or VIIIa) of the periodic table. Heliumis one of the noble gases. The atomic number of helium is 2.
Helium has the distinction of being the only element discovered outsideof Earth prior to finding it within our planet.
The gas was first isolated from terrestrial sources in 1895 by theBritish chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearingmineral.
In 1907 the British physicist Sir Ernest Rutherford showed that alphaparticles are the nuclei of helium atoms, which later investigation confirmed.
Properties and Occurrence
Helium has monatomic molecules, and is the lightest of all gases excepthydrogen. The atomic weight of helium is 4.003.Helium, like theother noble gases, is chemically inert. Its single electron shell isfilled, making possible reactions with other elements extremely difficultand the resulting compounds quite unstable.
Helium is the most difficult of all gases toliquefy and is impossible to solidify at normal atmospheric pressures.These properties make liquid helium extremely useful as a refrigerant andfor experimental work in producing and measuring temperatures close toabsolute zero. Liquid helium can be cooled almost to absolute zero atnormal pressure by rapid removal of the vapor above the liquid. Ithas no freezing point, and its viscosity is apparently zero; it passesreadily through minute cracks and pores and will even creep up the sidesand over the lip of a container.
Helium is the second most abundantelement in the universe, after hydrogen. About 1 part per millionof atmospheric helium consists of helium-3, now thought to be a productof the decay of tritium, a radioactive hydrogen isotope of mass 3. Thecommon helium isotope, helium-4, probably comes from radioactive alphaemitters in rocks. Natural gas, which contains an average of 0.4 percenthelium, is the major commercial source of helium.
Uses
Because it is noncombustible, helium is preferred to hydrogen as thelifting gas in lighter-than-air balloons; it has 92 percent of the liftingpower of hydrogen, although it weighs twice as much. Helium is used topressurize and stiffen the structure of rockets before takeoff and to pressurizethe tanks of liquid hydrogen or other fuel in order to force fuel intothe rocket engines. It is useful for this application because it remainsa gas even at the low temperature of liquid hydrogen. A potential use ofhelium is as a heat-transfer medium in nuclear reactors because it remainschemically inert and nonradioactive under the conditions that exist withinthe reactors.
Helium is used in inert-gas arc welding for lightmetals such as aluminum and magnesium alloys that might otherwise oxidize;the helium protects heated parts from attack by air. Helium is used inplace of nitrogen as part of the synthetic atmosphere breathed by deep-seadivers, caisson workers, and others, because it reduces susceptibilityto the bends. This synthetic atmosphere is also used in medicine to relievesufferers of respiratory difficulties because helium moves more easilythan nitrogen through constricted respiratory passages. Helium is transportedas a gas in small quantities, compressed in heavy steel cylinders. Largeramounts of helium can be shipped as a liquid in insulated containers, thussaving shipping costs.
"Helium," Microsoft(R) Encarta(R) 96 Encyclopedia. (c) 1993-1995 MicrosoftCorporation. All rights reserved. (c) Funk & Wagnalls Corporation.All rights reserved.
