Helium is a colorless, odorless, tasteless, non-toxic, monatomic gas that can form diatomic molecules only at very low temperatures. It is the first noble gas in the periodic table and has the lowest melting and boiling points of all elements. Helium has no reactivity with other gases and it cannot form chemical bonds. Helium can be found in large quantities in the atmosphere but it also plays an important role as a propellant and coolant in rockets and in the interior of spacecraft, as a replacement for hydrogen in balloons (blimps), as a cooling medium for high-powered magnets in magnetic resonance imaging (MRI) machines, in gas-cooled nuclear reactors, and in superconducting wires in particle accelerators like CERN’s Large Hadron Collider, and in quantum experiments such as those conducted by Russian physicist Pyotr Kapitsa and John F. Allen in 1937, where helium forms a special state called a superfluid.
Helium has the highest density of all liquids, including water and steam, but it remains a liquid to very low temperatures because of its extremely low viscosity. It is used to fill airships and serve as a substitute for oxygen in scuba diving tanks because it prevents decompression sickness, and mixes of helium and nitrogen are used in industrial gas leak detection using helium mass spectrometers. Helium is also a critical ingredient in some commercial metal fabrication applications such as arc welding and cryogenic refrigeration, and it provides an inert environment for growing silicon and germanium single crystals.