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(sē`zēəm) [Lat.,=bluish gray], a metallic chemical element; symbol Cs; at. no. 55; at. wt. 132.90545; m.p. 28.4°C;; b.p. 669.3°C;; sp. gr. 1.873 at 20°C;; valence +1. Cesium is a ductile, soft-as-wax, silver-white metallic element. It is in Group 1 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. An alkali metalalkali metals,
metals found in Group 1 of the periodic table. Compared to other metals they are soft and have low melting points and densities. Alkali metals are powerful reducing agents and form univalent compounds.
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, it is the most alkaline of all elements. Cesium liquefies in a warm room; mercury and gallium are the only other metals with this property. Chemically cesium resembles rubidium and potassium. It is the most reactive metal and is never found uncombined in nature. Pure cesium can be prepared by electrolysis of fused cesium cyanide in an inert atmosphere; the pure metal must be kept under an inert liquid or gas or in a vacuum to protect it from air and water. Cesium reacts readily with oxygen; it is sometimes used to remove traces of the gas from vacuum tubes and from light bulbs. It reacts with ice; it reacts explosively with water to form cesium hydroxide, the strongest base known. Cesium reacts with the halogens to form a fluoride, chloride, bromide, and iodide. It also forms a sulfate, carbonate, nitrate, and cyanide. The chloride is used in photoelectric cells, in optical instruments, and in increasing the sensitivity of electron tubes. Cesium compounds are used in the production of glass and ceramics and as antishock agents in conjunction with drugs containing arsenic. Cesium-137, a waste product of nuclear reactors, is a radioactive isotope used in the treatment of cancer. Cesium is found in the mineral pollux, or pollucite, which occurs on the island of Elba, in SW Africa, in the United States in Maine and South Dakota, and in Manitoba, Canada. Commercially useful quantities of inexpensive cesium are now available as a byproduct of the production of lithium metal. Minute quantities of cesium chloride are found in mineral springs and in seawater. In 1860, R. W. Bunsen and G. R. Kirchoff discovered the element (the first to be discovered by the use of the spectroscopespectroscope,
optical instrument for producing spectral lines and measuring their wavelengths and intensities, used in spectral analysis (see spectrum). When a material is heated to incandescence it emits light that is characteristic of the atomic makeup of the material.
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) and named it for the two bright blue lines characteristic of its spectrum. It was first isolated by Carl Sefferburg in 1881 by electrolysis of its salts.



Cs, a chemical element in Group I of Mendeleev’s periodic system. Atomic number, 55; atomic weight, 132.9054. A silver-white alkali metal, cesium occurs naturally as the stable isotope 133Cs. Of the artificial cesium radioisotopes, whose mass numbers range from 123 to 142, 137Cs is the most stable, with a half-life of 33 years.

History. Cesium was discovered in 1860 by R. W. Bunsen and G. R. Kirchhoff through the spectral analysis of mineral waters from Dürkheim in Germany. It was named cesium (Latin caesius, “sky blue”) for its two bright lines in the blue region of the spectrum. Cesium metal was first isolated by the Swedish chemist C. Setterberg in 1882 by the electrolysis of a molten mixture of cesium cyanide and barium.

Distribution in nature. Cesium is a typical rare metal and dispersed element (seeDISPERSED ELEMENT and RARE METAL). Its average content in the earth’s crust (clarke) is 3.7 × 10–4 percent by weight. Its content in ultrabasic rocks is 1 × 10–5 percent, and in basic rocks 1 × 10–4 percent. Geochemically, cesium is closely related to granitic magma, forming concentrations in pegmatites along with Li, Be, Ta, and Nb, particularly in pegmatites rich in Na (albite) and Li (lepidolite). Two cesium minerals are known —pollucite (seePOLLUCITE) and avogadrite, (K,Cs)(BF)4—both extremely rare; the highest concentration of cesium is found in pollucite, which contains 26–32 percent Cs2O. A large number of cesium atoms isomorphically replace K and Rb in feldspars and micas. Admixtures of cesium are also encountered in beryl, carnallite, and volcanic glass. Low concentrations of cesium have also been found in some thermal waters. On the whole, cesium is a weak water migrant. The processes of isomorphism and sorption of the large cesium cations have played a major role in the geological history of cesium. Cesium is geochemically related to Rb, K, and, to some extent, Ba.

Physical and chemical properties. Cesium is a very soft metal, with a density of 1.90 g/cm3 (at 20°C), a melting point of 28.5°C, and a boiling point of 686°C. At ordinary temperatures, it has a body-centered cubic lattice [a = 6.045 angstroms (Å)]. The atomic radius is 2.60 Å; the ionic radius of Cs+ is 1.86 Å. Cesium has a specific heat of 0.218 kilojoule/kg · °K (kJ/kg · °K), or 0.052 cal/g · °C, a latent heat of fusion of 15.742 kJ/kg (3.766 cal/g), and a latent heat of vaporization of 610.28 kJ/kg (146.0 cal/g). The coefficient of linear thermal expansion (from 0° to 26°C) is 9.7 × 10–5, the thermal conductivity (at 28.5°C) is 18.42 W/m · °K (0.44 cal/cm · sec · °C), the electrical resistivity is 0.2 μohm · m (at 20°C), and the temperature coefficient of electrical resistivity is 0.005 (0°–30°C).

Cesium is a diamagnet, with a specific magnetic susceptibility of –0.1 × 10–6 (at 18°C). Liquid cesium has a dynamic viscosity of 0.6299 meganewton · sec/m2 (MN · sec/m2) at 43.4°C and 0.4065 MN · sec/m2 at 140.5°C. The surface tension of cesium is 6.75 × 10–2 N/m, or 67.5 dynes/cm (at 62°C), and the compressibility (at 20°C) is 7.05 MN/m2, or 70.5 kilograms-force/cm2(kgf/cm2). The ionization potential is 3.893 electron volts, the normal electrode potential is 2.923 volts, and the electron work function is 1.81 electron volts. The Brinell hardness is 0.15 MN/m2 (0.015 kgf/cm2).

The electron configuration of the cesium atom’s outer shell is 6s1, and cesium has an oxidation state of + 1 in its compounds.

Cesium is very reactive. In the air it instantaneously ignites with the formation of the peroxide Cs2O2 and the superoxide CsO2; in the case of insufficient oxygen, the oxide Cs2O is obtained. The ozonide CsO3 is also known. With water, halogens, carbon dioxide, sulfur, and carbon tetrachloride, cesium reacts explosively to yield the corresponding hydroxide CsOH, halides, oxides, sulfides, and CsCl. It reacts with hydrogen at temperatures of 200°–350°C and pressures of 5–10 MN/m2 (50–100 kgf/cm2) to yield cesium hydride. Above 300°C, it causes the decomposition of glass, quartz, and other substances and the corrosion of metals. Upon heating, it combines with phosphorus to yield Cs2P5, with silicon to yield CsSi, and with graphite to yield C8Cs and C24Cs. Alloys are formed in the reaction of cesium with alkali and alkaline-earth metals, as well as with Hg, Au, Bi, and Sb. With acetylene it yields cesium acetylide, Cs2C2. Most of the simple salts of cesium, especially, CsF, CsCl, Cs2CO3, Cs2SO4, and CsH2PO4, are readily soluble in water. CsMnO4, CsClO4, and Cs2Cr2O7 are only slightly soluble. Cesium is not a complex-forming element, although it is a component of many complexes as an outer sphere cation.

Production. Cesium is obtained directly from pollucite by vacuum-thermal reduction, in which calcium, magnesium, aluminum, and other metals serve as the reducing agents. Various compounds of cesium are also obtained from pollucite. The ore is initially concentrated by flotation or manual ore treatment, and the concentrate obtained is then decomposed either by acids, such as H2SO4 or HNO3, or by roasting with oxide-salt mixtures, for example, a CaO and CaCl2 mixture. Cesium is precipitated out in the form of CsAl(SO4)2 · 12H2O, Cs3[Sb2Cl9], or other slightly soluble compound from the pollucite decomposition products. Subsequently, the precipitates are converted into soluble salts, such as cesium sulfate, cesium chloride, and cesium iodide. The final step of the technological process is the preparation of very pure cesium compounds by crystallization from solutions of Cs[I(I)2], Cs3[Bi2I9], and Cs2[TeI6] and the sorption of impurities on oxidized activated charcoals.

The extensive purification of cesium metal is achieved by distillation. The production of cesium from wastes in the processing of nepheline and some micas seems promising, as is the production of cesium from underground waters in petroleum extraction. In this instance, cesium is removed by extraction and sorption techniques.

Cesium is stored in Pyrex glass bottles in the presence of argon gas or in hermetically sealed steel containers beneath a layer of anhydrous petroleum jelly or paraffin oil.

Uses. Cesium is used in the production of photoelectric-cell cathodes (antimony-cesium, bismuth-cesium, and oxygen-silver-cesium cathodes), electron vacuum tubes, photoelectron multipliers, and image converters. The use of “cesium plasma” in ion rocket engines appears promising, as is the use of cesium in magnetohydrodynamic generators and thermionic converters.

Cesium isotopes are also useful; 33Cs is used in quantum frequency standards, while 137Cs is used in radiology. The resonance frequency of the energy transition between sublevels of the ground state of 133Cs is the basis for the modern determination of the length of the second.


In organisms. Cesium is a constant chemical microcomponent of plant and animal organisms. Marine algae contain 0.01–0.1 μg of cesium per 1 g of dry weight, while terrestrial plants contain 0.05 to 0.2 μg per 1 g of dry weight. Animals obtain cesium from water and food. Arthropods contain 0.067 to 0.503 μg/g, reptiles contain 0.04 μg/g, and mammals contain 0.05 μg/g. In mammals cesium is concentrated in muscles, the heart, and the liver; the blood contains up to 2.8 μg of cesium per liter. Cesium has relatively low toxicity; its biological role in plant and animal organisms has not been conclusively established.

137Cesium.137Cs, a radioisotope of cesium that emits beta and gamma radiation, is a major radioactive polluter of the biosphere. It is found in radioactive fallout and various radioactive wastes, including the wastes of plants for the treatment of atomic power plant wastes. It is rapidly absorbed or adsorbed by the soil and bottom deposits. In water it is found primarily in the form of ions.

137Cs is found in plants and in animal and human organisms. The highest accumulations are found in freshwater algae and arctic terrestrial plants, especially lichens. In animals, 137Cs accumulates mainly in muscles and the liver. The highest accumulations are found in reindeer and North American waterfowl. In man, 137Cs is distributed relatively uniformly throughout the body and produces few ill effects.



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a variety of soft wheat. The smoky-gray spike is aristate and glabrous, with red awns and red grain. The wheat is cultivated on all continents. In the USSR the spring varieties Cesium 31 and Cesium 114 have been regionalized.


A chemical element, symbol Cs, atomic number 55, atomic weight 132.905.


(US), cesium
a ductile silvery-white element of the alkali metal group that is the most electropositive metal. It occurs in pollucite and lepidolite and is used in photocells. The radioisotope caesium-137, with a half-life of 30.2 years, is used in radiotherapy. Symbol: Cs; atomic no.: 55; atomic wt.: 132.90543; valency: 1; relative density: 1.873; melting pt.: 28.39±0.01°C; boiling pt.: 671°C
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