(So named (Latin caesius, "bluish-gray") in 1860 by Robert Wilhelm Bunsen because of the blue line in its spectrum) A soft, silver-white, ductile, metallic chemical element, one of the alkali metals and the most electropositive of all the elements. It ignites in air, reacts vigorously with water, and is used in photoelectric cells.

Symbol: Cs
Atomic number: 55
Atomic weight: 132.91
Density (at room temperature and pressure): 1.873 g/cc
Melting point: 28.44°C
Boiling point: 671°C
Valence: +1
Ground state electron configuration: [Xe]6s1

See also: cesium-137