Helium

MELTING POINT: −272.2°C—does not solidify under its own vapor
BOILING POINT: −268.93°C
DENSITY: 0.1785 g/L
MOST COMMON IONS: None

Helium, a colorless gas at room temperature, is the first element in the noble gas group, and forms few compounds. It is rare in the atmosphere (1 part in 200,000) and recovered on Earth principally by its separation from natural gas obtained in underground wells. Named for the Sun (in Greek, helios), helium is a component of the production of energy as well as the basis of the science and technology of cryogenics. Its presence at the surface of the Sun was first confirmed by amateur British astronomer Joseph Norman Lockyer (1868), who observed characteristic lines in the optical spectrum of the Sun, at whose surface helium is produced via the energy-releasing fusion of hydrogen and deuterium nuclei.

Because it is such a light, nonreactive element, helium condenses (at atmospheric pressure) only at 4.2 kelvins. Furthermore, because of quantum mechanical effects, helium solidifies (under the application of 25.3 bars of external pressure) only at the lowest temperatures. Liquefied in large compression refrigerators, helium is used to cool cryogenic equipment, in particular the superconducting magnets used in medical magnetic resonance imaging (MRI). At 2.17 kelvins liquid helium transforms into an unusual quantum phase, called a superfluid, which has no viscosity and exhibits bizarre flow properties, such as its creeping out of containers.

The gas is also used to fill balloons, in gas discharge lamps, and as an additive in the breathing gases of astronauts and scuba divers. The rarer stable isotope of helium (³He) is produced by the decay of radioactive tritium, and is used in resonance imaging and in the attainment of very low temperatures, about 0.010 kelvin, via a process known as dilution refrigeration.

Helium

Bibliography