helium

Colourless, odourless, gaseous, non-metallic element, atomic number 2, relative atomic mass 4.0026. It is grouped with the noble gases (rare gases) in Group 0 of the periodic table. Helium is nonreactive because of its full outer shell of electrons and forms no compounds. It is the second most abundant element (after hydrogen) in the universe, and has the lowest boiling (−268.9°C/−452°F) and melting points (−272.2°C/−458°F) of all the elements. It is present in small quantities in the Earth's atmosphere from gases issuing from radioactive elements (from alpha decay) in the Earth's crust; after hydrogen it is the second-lightest element.

Helium is a component of most stars, including the Sun, where the nuclear-fusion process converts hydrogen into helium with the production of heat and light. It is obtained by the compression and fractionation of naturally occurring gases. It is used for inflating balloons and as a dilutant for oxygen in deep-sea breathing systems. Liquid helium is used extensively in low-temperature physics (cryogenics).

Helium was originally discovered in 1868 in the spectrum of the Sun. It was found on Earth in 1895.