CESIUM

also spelled Caesium (Cs), chemical element of Group Ia of the periodic table, the alkali metal group, first element to be discovered spectroscopically (1860), by Robert Bunsen and Gustav Kirchhoff, who named it for the unique blue lines of its spectrum (Latin caesius, sky-blue). This silvery white metal is the most reactive and one of the softest of all metals and is liquid at room temperature. It is about half as abundant as lead and 70 times as abundant as silver. Cesium occurs in minute quantities in the Earth's crust, as in the minerals pollucite and lepidolite. Cesium can be isolated by electrolysis of a molten cesium-barium-cyanide mixture and by other methods. Cesium reacts explosively with cold water; it readily combines with oxygen, so that it is used in electron tubes as a getter to clear out the traces of oxygen and other gases trapped in the tube when sealed. Because cesium is strongly photoelectric (easily loses electrons when struck by light), it is used extensively in photoelectric cells and in television cameras to form the electronic image, and it has been proposed for use in plasma propulsion engines for deep-space exploration. Cesium is also used in atomic clocks. Cesium has 32 isotopes. Naturally occurring cesium is entirely the nonradioactive isotope, cesium-133; 20 radioactive isotopes from cesium-123 to cesium-144 have been artificially prepared. Cesium-137 is useful in medical and industrial radiology because of its long half-life of 30 years. 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 electrovalent bonds with nearly all the inorganic and organic anions. Cesium hydroxide (CsOH), containing the hydroxyl radical (OH-), is the strongest base known, attacking even glass. Some cesium salts are used in making mineral waters. atomic number 55 atomic weight 132.90543 melting point 28.44 C boiling point 671 C specific gravity 1.873 (20 C) valence 1 electronic config. 2-8-18-18-8-1 or (Xe)6s1