Helium Helium, symbol He, inert, colorless, odorless gaseous element. In group 18 (or VIIIa) of the periodic table, helium is one of the noble gases. The atomic number of helium is 2. The French astronomer Pierre Janssen discovered helium in the spectrum of the corona of the sun during an eclipse in 1868. Helium is the lightest of all gases except hydrogen. Helium solidifies at -272.2° C (-457.9° F) at pressures above 19,000 torr (25 atmospheres); helium boils at -268.9° C (-452.0° F) and has a density of 0.1664 g/liter at 20° C (68° F). The atomic weight of helium is 4.003. Helium, like the other noble gases, is chemically inert. Its single electron shell is filled, making possible reactions with other elements extremely difficult and the resulting compounds quite unstable. Helium is the most difficult of all gases to liquefy....

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