helium (He)

A prototype helium-filled Venus balloon in a JPL cleanroom. Image credit: NASA/JPL-Caltech

A colorless, odorless, unreactive gaseous element which is the second most abundant and second lightest element in the universe (after hydrogen). Helium is one of the noble gases (also known as noble gases). An ordinary helium atom consists of a nucleus of two protons and two neutrons surrounded by two electrons. Seven isotopes of helium are known.

The main source of helium is natural gas. It is used to inflate and provide lift for balloons and airships; in breathing mixtures for deep-sea divers; as a pressurizer for the fuel tanks of liquid-fueled rockets; in helium-neon lasers; as an inert gas shield for arc welding; as a protective gas in growing silicon and germanium crystals and producing titanium and zirconium; and as a superfluid in the form of helium II.

Forms of helium

Liquid helium ⁴He exists in two forms. Helium I, stable from 2.19K to 4.22K, is a normal liquid, used as refrigerant and in cryogenics. Below 2.18K, it becomes helium II, which is a superfluid displaying superconductivity, zero viscosity, the ability to flow over the side of a vessel in which it is placed, and other strange properties predicted by quantum mechanics.

³He does not form a superfluid. Solid helium can only be produced at pressures above 25 atm.

Discovery of helium

Jules Janssen obtained the first evidence of helium during the solar eclipse of 1868 when he detected a new line in the solar spectrum. J. Norman Lockyer and Edward Frankland suggested the name helium (from the Greek helios for Sun) for the new element. In 1895 William Ramsay discovered helium in the uranium mineral cleveite; it was independently discovered in cleveite by the Swedish chemists Cleve and Langlet at about the same time. In 1907, Ernest Rutherford and Thomas Royds demonstrated that alpha particles are helium nuclei.

Cosmic significance of helium

Most of the helium in the universe was produced immediately after the Big Bang, although an additional contribution has come from hydrogen burning inside main sequence stars. It occurs commonly in stars and in the atmospheres of gas giants. See also elements, cosmic abundance. The helium content of Earth's atmosphere is about 1 part in 200,000.

atomic number	2
relative atomic mass	4.0026
electron configuration	1s²
atomic radius	128 pm
density	0.176 g/dm³
melting point	-269.7°C (-453.5°F)
boiling point	-268.9°C (-452.0°F)

Related category

• INORGANIC CHEMISTRY

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