Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 mineral, transforming the compound thus obtained into the cyanide, and electrolysis of the fused cyanide. Cesium can also be obtained by heating its hydroxides or carbonates with magnesium or aluminum and by heating its chlorides with calcium. Commercial cesium usually contains rubidium, with which it usually occurs in minerals and which resembles it so closely that no effort is made to separate them.

Like potassium, cesium oxidizes readily when exposed to air and is thus used to remove residual oxygen from radio vacuum tubes. Because of its property of emitting electrons when exposed to light, it is used in the photosensitive surface of the cathode of the photoelectric cell. The radioactive isotope cesium-137, which is produced by nuclear fission, is a useful by-product of atomic-energy plants. Cesium-137 emits more energy than radium and is used in medical and industrial research. See Isotopic Tracer.