Helium (He) is the element of the 8th group of the Mendeleev’s periodic table. It is classified as an inert gas; atomic number – 2; atomic weight – 4.0026. Natural helium consists of 3He and 4He stable isotopes. It was discovered in 1868 by the French astronomer Pierre Janssen and the English astronomer Norman Lockyer during the spectroscopic study of solar protuberances. For the first time ever helium was obtained on the Earth in 1895 by the English physicist William Ramsay from the cleveite mineral.
Helium is a colorless and odorless gas in normal conditions. Its density is 0.178 kilograms per cubic meter; boiling point – 268.93°С. Helium is the only element that does not transform into solid phase at normal pressure no matter how deeply it is refrigerated. Helium is a superfluid (capable to flow without viscosity). The minimum pressure required to transform helium from liquid into solid phase is 2.5 MPa, with a fusion point at −272.1°С. Thermal conductivity (at 0°С) is 2.1 x 10–2 W/mK. The helium molecule consists of one atom. 8.8 milliliters of helium dissolve in 1 liter of water at 20°С. Stable chemical compounds of helium have not been obtained.