Helium, derived from the Greek word helios, meaning sun, is the lightest of the noble gases. It has an atomic mass of 4.0026 and an atomic radius of 0.49 Angstrom.

It was first identified as an element by the British chemist Sir Joseph Norman Lockyer. In 1895 Sir William Ramsay successfully isolated the gas.

Helium is a monatomic gas and is the second most abundant element in the universe. It has a melting point of 0.95K and a boiling point of 4.215K. At sea level, helium is roughly abundant in the air at 5 parts per million. Helium has a hexagonal crystal structure.

Helium can be changed into helium (II) at temperatures just above absolute zero, where it has no viscosity and therefore is a perfect example of a superfluid. Another interesting property of helium is its effect on a person's voice. Because helium is lighter than the usual oxygen/nitrogen blend, it changes the resonant frequency of the human vocal tract. This results in a faster vibration and a higher pitched sound.

Helium has many uses. For instance, it is used as a lifting gas in balloons and is used in place of nitrogen in the breathing tanks used by deep-sea divers. This is because helium is less soluble in the blood than nitrogen and so reduces the risk of the bends effect. It is of great importance in both medicine and industry. In particular, helium is very important in arc welding of metals such as aluminium. It protects heated parts from oxidation.

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