Cesium

Cesium, chemical element, symbol Cs; for physical constants see Periodic Table. Cesium was discovered spectroscopically by Robert Bunsen and Gustav Kirchhoff in 1860. It is a metal belonging to the alkaline group. Elemental cesium is silvery-white, soft, and ductile; it is one of the three metallic elements that is liquid at room temperature. Cesium is used in ion propulsion systems and in atomic clocks, as well as in optical instruments and in glass and ceramic production.