/see"zee euhm/, n.
a rare, highly reactive, soft, metallic element of the alkali metal group, used chiefly in photoelectric cells. Symbol: Cs; at. wt.: 132.905; at. no.: 55; sp. gr.: 1.9 at 20°C; melts at 28.5°C.
Also, caesium.
[1930-35; < NL, special use of L caesium, neut. of caesius bluish-grey; see -IUM]

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Chemical element, one of the alkali metals, chemical symbol Cs, atomic number 55.

The first element discovered by spectroscopy (1860), it is silvery white, liquid at warm room temperature (melting at 83 °F [28.4 °C]), and very soft when solid. About half as abundant as lead, it occurs in minute quantities as ores. It reacts explosively with cold water and is used to scavenge traces of oxygen and other gases in electron tubes. Other uses are as a catalyst and in photoelectric cells, ion propulsion systems, atomic clocks, and plasma for thermoelectric conversion. Cesium salts have various specialty applications, including in mineral waters.

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also spelled  caesium 
 chemical element of Group 1 (also called Group Ia) of the periodic table, the alkali metal group, and the first element to be discovered spectroscopically (1860), by German scientists Robert Bunsen (Bunsen, Robert Wilhelm) and Gustav Kirchhoff (Kirchhoff, Gustav Robert), who named it for the unique blue lines of its spectrum (Latin caesius, “sky-blue”).

      This silvery metal with a golden cast is the most reactive and one of the softest of all metals. It melts at 28.4 °C (83.1 °F), just above room temperature. It is about half as abundant as lead and 70 times as abundant as silver. Cesium occurs in minute quantities (7 parts per million) in Earth's (Earth) crust in the minerals pollucite, rhodizite, and lepidolite. Pollucite (Cs4Al4Si9O26∙H2O) is a cesium-rich mineral resembling quartz. It contains 40.1 percent cesium on a pure basis, and impure samples are ordinarily separated by hand-sorting methods to greater than 25 percent cesium. Large pollucite deposits have been found in Zimbabwe and in the lithium-bearing pegmatites at Bernic Lake, Manitoba, Can. Rhodizite is a rare mineral found in low concentrations in lepidolite and in salt brines and saline deposits.

      The primary difficulty associated with the production of pure cesium is that cesium is always found together with rubidium in nature and is also mixed with other alkali metals. Because cesium and rubidium are very similar chemically, their separation presented numerous problems before the advent of ion-exchange methods and ion-specific complexing agents such as crown ethers. Once pure salts have been prepared, it is a straightforward task to convert them to the free metal.

      Cesium can be isolated by electrolysis of a molten cesium cyanide/barium cyanide mixture and by other methods, such as reduction of its salts (salt) with sodium metal, followed by fractional distillation. Cesium reacts explosively with cold water; it readily combines with oxygen, so it is used in vacuum tubes as a “getter” to clear out the traces of oxygen and other gases trapped in the tube when sealed. The very pure gas-free cesium needed as a “getter” for oxygen in vacuum tubes can be produced as needed by heating cesium azide (CsN3) in a vacuum. Because cesium is strongly photoelectric (easily loses electrons when struck by light), it is used in photoelectric cells (photoelectric cell), photomultiplier tubes, scintillation counters, and spectrophotometers. It is also used in infrared lamps. Because the cesium atom can be ionized thermally and the positively charged ions accelerated to great speeds, cesium systems could provide extraordinarily high exhaust velocities for plasma propulsion engines for deep-space exploration.

      Cesium metal is produced in rather limited amounts because of its relatively high cost. Cesium has application in thermionic power converters that generate electricity directly within nuclear reactors or from the heat produced by radioactive decay. Another potential application of cesium metal is in the production of low-melting NaKCs eutectic alloy.

 Atomic cesium is employed in the world's time standard, the cesium clock. The microwave spectral line emitted by the isotope cesium-133 has a frequency of 9,192,631,770 hertz (cycles per second). This provides the fundamental unit of time. Cesium clocks are so stable and accurate that they are reliable to 1 second in 1.4 million years. Primary standard cesium clocks, such as NIST-F1 in Boulder, Colo., are about as large as a railroad flatcar. Commercial secondary standards are suitcase-sized.

      Naturally occurring cesium consists entirely of the nonradioactive isotope cesium-133; a large number of radioactive isotopes from cesium-123 to cesium-144 have been prepared. Cesium-137 is useful in medical and industrial radiology because of its long half-life of 30.17 years. However, as a major component of nuclear fallout and a waste product left over from the production of plutonium and other enriched nuclear fuels, it presents an environmental hazard. Removal of radioactive cesium from contaminated soil at nuclear-weapon-production sites, such as Oak Ridge National Laboratory in Oak Ridge, Tenn., and the U.S. Department of Energy's Hanford site near Richland, Wash., is a major cleanup effort.

      Cesium is difficult to handle because it reacts spontaneously in air. If a metal sample has a large enough surface area, it can burn to form superoxides. Cesium superoxide has a more reddish cast. Cs2O2 can be formed by oxidation of the metal with the required amount of oxygen, but other reactions of cesium with oxygen are much more complex.

      Cesium is the most electropositive and most alkaline element, and thus, more easily than all other elements, it loses its single valence electron and forms ionic bonds with nearly all the inorganic and organic anions (anion). The anion Cs has also been prepared. Cesium hydroxide (CsOH), containing the hydroxide anion (OH), is the strongest base known, attacking even glass. Some cesium salts are used in making mineral waters. Cesium forms a number of mercury amalgams. Because of the increased specific volume of cesium, as compared with the lighter alkali metals, there is a lesser tendency for it to form alloy systems with other metals.

      Rubidium and cesium are miscible in all proportions and have complete solid solubility; a melting-point minimum of 9 °C (48 °F) is reached.

James L. Dye
atomic number
atomic weight
melting point
28.44 °C (83.19 °F)
boiling point
671 °C (1,240 °F)
specific gravity
1.873 (at 20 °C, or 68 °F)
oxidation states
+1, -1 (rare)
electronic config.
2-8-18-18-8-1 or [Xe]6s1

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Universalium. 2010.

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  • cesium — n. the chemical element of atomic number 55. It is a univalent element, the most electropositive metal. Symbol Cs; atomic weight 132.905. IT has a melting point of 28.4[deg] C. Syn: caesium, Cs. [WordNet 1.5] …   The Collaborative International Dictionary of English

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