HELIUM (Greek helios, "sun"), symbol He, inert, colorless, odorless gaseous element. In group 18 (or VIIIa) of the periodic table. Helium is one of the noble gases. The atomic number of helium is 2.

Helium has the distinction of being the only element discovered outside of Earth prior to finding it within our planet.
The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay, who discovered it in cleveite, a uranium-bearing mineral.
In 1907 the British physicist Sir Ernest Rutherford showed that alpha particles are the nuclei of helium atoms, which later investigation confirmed.

Properties and Occurrence
Helium has monatomic molecules, and is the lightest of all gases except hydrogen. The atomic weight of helium is 4.003.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 most difficult of all gases to liquefy and is impossible to solidify at normal atmospheric pressures. These properties make liquid helium extremely useful as a refrigerant and for experimental work in producing and measuring temperatures close to absolute zero. Liquid helium can be cooled almost to absolute zero at normal pressure by rapid removal of the vapor above the liquid. It has no freezing point, and its viscosity is apparently zero; it passes readily through minute cracks and pores and will even creep up the sides and over the lip of a container.
      Helium is the second most abundant element in the universe, after hydrogen.  About 1 part per million of atmospheric helium consists of helium-3, now thought to be a product of the decay of tritium, a radioactive hydrogen isotope of mass 3. The common helium isotope, helium-4, probably comes from radioactive alpha emitters in rocks. Natural gas, which contains an average of 0.4 percent helium, is the major commercial source of helium.

Because it is noncombustible, helium is preferred to hydrogen as the lifting gas in lighter-than-air balloons; it has 92 percent of the lifting power of hydrogen, although it weighs twice as much. Helium is used to pressurize and stiffen the structure of rockets before takeoff and to pressurize the tanks of liquid hydrogen or other fuel in order to force fuel into the rocket engines. It is useful for this application because it remains a gas even at the low temperature of liquid hydrogen. A potential use of helium is as a heat-transfer medium in nuclear reactors because it remains chemically inert and nonradioactive under the conditions that exist within the reactors.
    Helium is used in inert-gas arc welding for light metals such as aluminum and magnesium alloys that might otherwise oxidize; the helium protects heated parts from attack by air. Helium is 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. This synthetic atmosphere is also used in medicine to relieve sufferers of respiratory difficulties because helium moves more easily than nitrogen through constricted respiratory passages. Helium is transported as a gas in small quantities, compressed in heavy steel cylinders. Larger amounts of helium can be shipped as a liquid in insulated containers, thus saving shipping costs.

"Helium," Microsoft(R) Encarta(R) 96 Encyclopedia. (c) 1993-1995 Microsoft Corporation. All rights reserved. (c) Funk & Wagnalls Corporation. All rights reserved.