Cesium
. Cesium, symbol Cs, white, soft, chemically reactive metallic element. In group 1 (or Ia) of the periodic table , cesium is one of the alkali metals . The atomic number of cesium is 55. Cesium was discovered in 1860 by the German chemist Robert Wilhelm Bunsen and the German physicist Gustav Robert Kirchhoff through the use of a spectroscope (see Spectrum). Cesium ranks about 46th in natural abundance among the elements in crustal rocks. Cesium melts at about 28° C (about 82° F), boils at about 669° C (about 1236° F), and has a specific gravity of 1.88; its atomic weight is 132.905. The natural source yielding the greatest quantity of cesium is the rare mineral pollux (or pollucite). Ores of this mineral found on the island of Elba contain 34 percent of cesium oxide; American ores of pollux, found in Maine and South Dakota, contain 13 percent of the oxide. Cesium also occurs in lepidolite, carnallite , and certain feldspars . It is extracted by separating the cesium compound from the...