Helium

Chemical element, chemical symbol He, atomic number 2. A noble gas, it is colourless, odourless, tasteless, completely unreactive, and nontoxic. First found by spectroscopy of the Sun's atmosphere in 1868, it is the second most abundant and second-lightest element in the universe (after hydrogen).

Helium makes up a tiny proportion of the atmosphere but as much as 7% of natural gas. It is the product of radioactive decay (&see; radioactivity) and is used in helium dating. It is used as an inert gas in welding, rocket propulsion, balloon flight, hyperbaric chambers, deep-sea diving (&see; nitrogen narcosis), gas chromatography, luminous signs, and cryogenics. Liquid helium, which exists only below −452 °F (−268.9 °C, about 4° C above absolute zero), is a “quantum fluid” (&see; fluid mechanics; quantum mechanics), with unique properties, including superfluidity, superconductivity, and near-zero viscosity.