helium

Helium is a chemical element with the symbol He and atomic number 2. It is a noble gas and the second lightest element in the universe. Helium is produced by nuclear fusion in stars and accounts for a substantial fraction of the visible universe’s mass. On Earth it occurs only rarely in the atmosphere but is commercially recovered from natural gas deposits where it is dissolved under pressure.

Properties: Helium is colorless, odorless, tasteless, non-toxic, and chemically inert. It is monatomic and has the lowest boiling point of all elements, liquefying at 4.2 kelvin at 1 atm. It remains a gas under ordinary conditions and forms very few compounds.

Occurrence and production: Earth's atmosphere contains about 5 ppm by volume. Commercial helium is extracted from some natural gas fields where it coexists with other gases; the gas is purified and separated by cryogenic distillation and adsorption.

Applications: Helium is essential in cryogenics for cooling superconducting magnets in MRI and NMR, and in detectors; used as an inert shielding gas in welding; a leak-detection medium; and as a lifting gas for balloons and airships.

Safety and storage: Non-toxic and non-flammable, but extremely cold as a liquid and can cause asphyxiation in enclosed spaces. Stored in insulated dewars at low temperatures.

History: Helium was first observed in the solar spectrum in 1868 by Janssen and Lockyer. It was isolated on Earth in 1895 by Ramsay and Travers from a uranium ore. The name derives from Helios, the Greek sun.