Caesium

Caesium is the chemical element with atomic number 55. A soft, silvery-gold alkali metal with a melting point of 28 °C (83 °F), it is one of only five metals that are liquid at or near room temperature. It has physical and chemical properties similar to those of rubidium and potassium. The metal is extremely reactive and pyrophoric, reacting with water even at −116 °C (−177 °F). It is the least electronegative element with stable isotopes, of which it has only one, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, are extracted from waste produced by nuclear reactors. German chemists Robert Bunsen and Gustav Kirchhoff discovered caesium in 1860 by flame spectroscopy. The first small-scale applications for caesium have been as a "getter" in vacuum tubes and in photoelectric cells. In 1967, a specific frequency from the emission spectrum of caesium-133 was chosen for use in the definition of the second by the International System of Units. Since then, caesium has been widely used in atomic clocks. Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids


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