Helium
. Helium (Greek helios, “sun”), 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. Shortly afterward it was identified as an element and named by the British chemist Sir Edward Frankland and the British astronomer Sir Joseph Norman Lockyer . The gas was first isolated from terrestrial sources in 1895 by the British chemist Sir William Ramsay , who discovered it in cleveite, a uranium-bearing mineral. In 1907 the British physicist Sir Ernest Rutherford showed that alpha particles are the nuclei of helium atoms, which later investigation confirmed. Helium has monatomic molecules, and 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 (-...