Caesium Information
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Caesium Element
Caesium (IUPAC spelling[6])
(also spelled cesium in American English)[note 1] is a chemical element with
the symbol Cs and atomic number 55. It is a soft, silvery-golden 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.[note 2] Caesium
has physical and chemical properties similar to those of rubidium and
potassium. The most reactive of all metals, it is pyrophoric and reacts with
water even at −116 °C (−177 °F). It is the least electronegative element, with
a value of 0.79 on the Pauling scale. 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.
Some barium compounds that are
released during industrial processes dissolve easily in water and are found in
lakes, rivers, and streams. Because of their water-solubility these barium
compounds can spread over great distances. When fish and other aquatic
organisms absorb the barium compounds, barium will accumulate in their bodies.
Because it forms insoluble
salts with other common components of the environment, such as carbonate and
sulphate, barium is not mobile and poses little risk. Barium compounds that are
persistent usually remain in soil surfaces, or in the sediment of water soils.
Barium is found in most land soils at low levels. These levels may be higher at
hazardous waste sites.
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 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. 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.
Nonradioactive caesium compounds are only mildly toxic, but the pure metal's
tendency to react explosively with water means that caesium is considered a
hazardous material, and the radioisotopes present a significant health and
ecological hazard in the environment.
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