Samarium is a chemical element, a very hard, silvery white metal of the lanthanide series in Group IIIB on the periodic table. Its symbol is Sm; its atomic number, 62; and its atomic weight, 150.4 (average weight of its seven natural isotopes). Three natural samarium isotopes are radioactive. Samarium was discovered in 1879 by Lecoq de Boisbaudran, who isolated it from samarskite. The metal ignites in air at 150 deg C (302 deg F). Because one of its isotopes has a high cross section of neutron absorption, samarium is used in the control rods of nuclear reactors. An alloy of samarium with cobalt is used to make a magnetic material with the highest resistance to demagnetization of any known material.