Ceasium OR Cesium, Symbol Cs. A soft silvery white metallic element belonging to group 1 (formerly IA) of the periodic table; a.n. 55; r.a.m. 132.905; r.d. 1.88; m.p. 28.4°C; b.p. 678°C. It occurs in small amounts in a number of minerals, the main source being carnallite (KCl.MgCl₂.6H₂O). It is obtained by electrolysis of molten cesium cyanide. The natural isotope is cesium–133. There are 15 other radioactive isotopes. Cesium–137 (half-life 33 years) is used as a gamma source. As the heaviest alkali metal, cesium has the lowest ionization potential of all elements, hence its use in photoelectric cells, etc.