Chemical element, one of the alkali metals, chemical symbol Cs, atomic number 55. The first element discovered by spectroscopy (1860), it is silvery white, liquid at warm room temperature (melting at 83 °F [28.4 °C]), and very soft when solid. About half as abundant as lead, it occurs in minute quantities as ores. It reacts explosively with cold water and is used to scavenge traces of oxygen and other gases in electron tubes. Other uses are as a catalyst and in photoelectric cells, ion propulsion systems, atomic clocks, and plasma for thermoelectric conversion. Cesium salts have various specialty applications, including in mineral waters.