II.

Properties of Helium

Helium is the lightest of all gases except hydrogen and occurs as monatomic (one atom only) molecules. Helium becomes a liquid at -268.9°C (-451.8°F), in a phase called helium I; below -270.98°C (-455.476°F), it changes to a phase called helium II and becomes a superfluid. Helium solidifies at -272.2°C (-457.9°F) at pressures above 26 atmospheres; helium boils at -268.9°C (-452.0°F) and has a density of 0.1664 g/liter at 20°C (68°F) and one atmosphere. The atomic weight of helium is 4.0026.

The stable isotopes (nuclei with different mass numbers) of helium are helium-3 (2 protons + 1 neutron), a rare form, and helium-4 (2 protons + 2 neutrons), the most common form. The isotopes helium-5 through helium-10 are unstable and rapidly decay. These heavy isotopes are only known from certain nuclear reactions and from experiments with particle accelerators.

Helium, like the other noble gases, is almost chemically inert. Its single electron shell is filled, making possible reactions with other elements extremely difficult and the resulting compounds quite unstable. However, under extreme pressure some chemical reactions may occur. Molecules of compounds with neon, another noble gas, and with hydrogen have been detected, however, and other compounds have been suggested, such as a combination of hydrogen-helium-iron. 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 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 a phase called helium II, also called superfluid helium, a liquid with unique physical properties thought to result from quantum mechanical effects. 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.

When helium is cooled to near absolute zero under extreme pressure, it can behave like a solid. Some experiments indicate that at extremely low temperatures solid helium has unusual properties that suggest a possible new state of matter called supersolidity. Scientists are investigating the existence of supersolidity. Solid helium becomes stiffer in a supersolid state but also shows features of possible “perpetual flow” similar to superfluidity.