Cesium, a silver-white metallic chemical element. Cesium is softer than talc, is ductile (can be drawn into wire), and has a low melting point. Chemically, cesium is the most reactive of the metallic elements. It can combine with most nonmetallic elements. Cesium reacts violently with water to form cesium hydroxide and hydrogen. This reaction generates enough heat to ignite the hydrogen produced. When exposed to air, cesium ignites and burns rapidly. It is therefore stored in kerosene or naphtha, or in airtight containers. Cesium readily emits electrons when heated or deposed to light or other radiation.

Cesium is used to remove oxygen and other gases from vacuum tubes, to convert heat into electricity in thermionic devices, and in the manufacture of photoelectric cells, infrared lamps, and spectrographic instruments. Cesium compounds are used in the manufacture of chemicals, mineral waters, optical crystals, and vacuum tubes. Cesium atoms vibrate so regularly that the rate is used in timekeeping as a standard for establishing the length of the second.

Cesium was discovered in 1860 by the German scientists Robert W. Bunsen and Gustav Kirchhoff. It is widely distributed in the earth's crust but in small quantities. The primary cesium ore is the mineral pollucite. Cesium compounds are obtained from the ore by various metallurgical processes. The metal is usually obtained by heating cesium chloride or cesium bromide with calcium in a vacuum.

Symbol: Cs. Atomic number: 55. Atomic weight: 132.9054. Specific gravity: 1.87. Melting point: 83.7 F. (28.7 C.). Boiling point: 1,252 F. (678 C.). Cesium has one stable isotope: Cs-133. Cesium is an alkali metal belonging to Group IA of the Periodic Table and has a valence of +1.