Cesium was the first chemical element identified through spectroscopy.
German physicists Robert Bunsen and Gustav Kirchhoff discovered cesium in 1860 while examining mineral-water samples with a spectroscope. They observed two previously unknown bright blue lines in the sample’s emission spectrum and named the element cesium, from the Latin caesius, meaning sky blue.
Cesium has the symbol Cs and atomic number 55. It is a soft, pale-gold alkali metal with a very low melting point of about 28.5°C. The element is highly reactive and is not found as a free metal in nature.
Cesium is also important in precision timekeeping. The international definition of the second is based on the frequency of radiation associated with the cesium-133 atom. Cesium should not be confused with rubidium, which was discovered by spectroscopy shortly afterward in 1861.