A soft ductile chemical element of the → alkali metal group; symbol Cs. It is found in several → silicate minerals, including pollucite. The metal oxidizes in air and reacts violently with water. → Atomic number 55; → atomic weight 132.9054; → melting point 28.4°C; → boiling point 669.3°C; → specific gravity 1.873 at 20°C; and → valence +1. Cesium has several radioactive isotopes, among which 134Cs with a half-life of 2.07 years and 137Cs with a half-life of 30.3 years. Cesium was discovered spectroscopically in 1860 by W. Bunsen and G. Kirchhoff in mineral water from Durkheim.
From L. caesius "bluish gray," which was the color of the cesium line in the spectroscope, + → -ium.
Fr.: horloge à cesium