![]() In 1967, acting on Einstein's proof that the speed of light is the most-constant dimension in the universe, the International System of Units used two specific wave counts from an emission spectrum of caesium-133 to co-define the second and the metre. 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. It has the largest atomic radius of all elements whose radii have been measured or calculated, at about 260 picometers. Caesium-137, a fission product, is 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. 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 cesium in American English) is a chemical element with the symbol Cs and atomic number 55.
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