What is "caesium"

caesium \caesium\, Caesium \C[ae]"si*um\, n. [NL., from L. caesius bluish gray.] (Chem.) A rare alkaline metal found in mineral water; -- so called from the two characteristic blue lines in its spectrum. It was the first element discovered by spectrum analysis, and is the most strongly basic and electro-positive substance known. Symbol Cs. Atomic number 55. Atomic weight 132.6.

Syn: cesium, Cs

see cesium.

alt. A metallic chemical element (''symbol'' Cs) with an atomic number of 55. n. A metallic chemical element (''symbol'' Cs) with an atomic number of 55.

n. a soft silver-white ductile metallic element (liquid at normal temperatures); the most electropositive and alkaline metal [syn: cesium, Cs, atomic number 55]

Caesium or cesium is a chemical element with symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature. Caesium is an alkali metal and 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. It has only one stable isotope, caesium-133. Caesium is mined mostly from pollucite, while the radioisotopes, especially caesium-137, a fission product, are extracted from waste produced by nuclear reactors.

The German chemist Robert Bunsen and physicist Gustav Kirchhoff discovered caesium in 1860 by the newly developed method of flame spectroscopy. The first small-scale applications for caesium were as a " getter" in vacuum tubes and in photoelectric cells. In 1967, acting on Einstein's proof that the speed of light as 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 meter. Since then, caesium has been widely used in highly accurate atomic clocks.

Since the 1990s, the largest application of the element has been as caesium formate for drilling fluids, but it has a range of applications in the production of electricity, in electronics, and in chemistry. The radioactive isotope caesium-137 has a half-life of about 30 years and is used in medical applications, industrial gauges, and hydrology. Although the element is only mildly toxic, the metal is a hazardous material and the radioisotopes would present a significant health and ecological hazard in the environment.