Helium (Greek helios,”sun”), symbol He, inert, colorless, odorless gas element. In group 18 of the periodic table, helium is one of the noble gases. The atomic number of helium is 2.Pierre Janssen discovered helium in the spectrum of the corona of the sun during an eclipse in 1868. Shortly after it was identified as an element and named by the chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer. The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay, who discovered it in cleveite.

In 1907 Sir Ernest Rutherford showed that alpha particles are the nuclei of helium atoms.IIPROPERTIES AND OCCURRENCE Helium has monatomic molecules, and is the lightest of all gases except hydrogen. Helium solidifies at -272.2 C; helium boils at -268.9 C. The atomic weight of helium is 4.

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0026.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.

Molecules of compounds with neon, another noble gas, and with hydrogen have been detected.Helium is the most difficult of all gases to liquefy and is impossible to solidify at atmospheric pressure. 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. At a temperature slightly above absolute zero, it is transformed into helium II, also called superfluid helium, a liquid with unique physical properties. It has no freezing point, and its viscosity is apparently zero; it passes readily through minute cracks.

Helium-3, the lighter helium isotope, which has an even lower boiling point than ordinary helium, exhibits different properties when liquefied.Helium is the second most abundant element in the universe, after hydrogen; however, it is rare on earth, primarily found mixed with natural gas trapped in underground pockets. Once helium is released it is so light it escapes the earth’s atmosphere and cannot be recovered. At sea level, helium occurs in the atmosphere in the proportion of 5.

4 parts per million. The proportion increases slightly at higher altitudes. 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.

By far the largest users of helium are agencies of the United States government.IIIUSES 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. In surgery, beams of ionized helium from synchrocyclotron sources are proving useful in treating eye tumors, by stabilizing or even shrinking the tumors.

Such beams are also used to shrink blood-vessel malformations in the brains of patients.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.

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