The first small-scale applications for caesium were as a ' getter' in vacuum tubes and in photoelectric cells. The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, a fission product, are extracted from waste produced by nuclear reactors. It has only one stable isotope, caesium-133. It is the least electronegative element, with a value of 0.79 on the Pauling scale. The most reactive of all metals, it is pyrophoric and reacts with water even at −116 ☌ (−177 ☏). Caesium has physical and chemical properties similar to those of rubidium and potassium.
It is a soft, silvery-golden alkali metal with a melting point of 28.5 ☌ (83.3 ☏), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium ( IUPAC spelling ) ( also spelled cesium in American English) is a chemical element with the symbol Cs and atomic number 55.
