Helium

I

Introduction

Helium (Greek, helios, “Sun”), symbol He, inert, colourless, odourless 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.

The French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the Sun during an eclipse in 1868. Shortly afterwards it was identified as an element and named by the British 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, 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.

II

Properties and Occurrence

Helium has monatomic molecules, and is the lightest of all gases except hydrogen. Helium solidifies at -272.2° C (-457.9° F) at pressures above 25 atmospheres; helium boils at -268.9° C (-452.0° F) and has a density of 0.1664 g/litre at 20° C (68° F). 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. Molecules of compounds with neon, another noble gas, and with hydrogen have been detected, however, and other compounds have been suggested. Because of helium's abundance in the universe, the existence of such reactions, however rare, could be of importance in cosmology.

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 vapour 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 and pores and will even creep up the sides and over the lip of a container. Helium 3, the lighter helium isotope of mass 3, which has an even lower boiling point than ordinary helium, exhibits markedly different properties when liquefied. See Superfluidity.

Helium is the second most abundant element in the universe, after hydrogen. 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 per cent helium, is the major commercial source of helium.

III

Uses

Because it is noncombustible, helium is preferred to hydrogen as the lifting gas in lighter-than-air balloons; it has 92 per cent of the lifting power of hydrogen, although it weighs twice as much. Helium is used to pressurize and stiffen the structure of rockets before take-off 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 aluminium 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 tumours, by stabilizing or even shrinking the tumours. 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.