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Atomic symbol Cs
Atomic number 55
Classification alkali metal
Atomic mass 132.9 amu
Other Information
Date of discovery 1860
Name of discoverer Bunsen, R.W. and Kirchoff, G.
Name origin From the Latin Caesius, meaning "sky blue"
Uses removing air traces in vacuum tubes
Obtained from lepidolite, pollucite

Cesium is an element[1] in the alkali metal class of the periodic table. It is so chemically active that it is never found free (in elemental form) in nature, and catches fire on mere exposure to air and almost explodes on contact with water. Cesium is a key component in atomic clocks.[2]

It was discovered by Robert Bunsen and Gustav Kirchoff, using their newly invented spectroscope, and was the first element to be discovered spectroscopically. It (along with rubidium) was identified by a previously unseen blue spectral line in the analysis of mineral water from a German spa. The name comes from that color.


  1. Wile, Dr. Jay L. Exploring Creation With Physical Science. Apologia Educational Ministries, Inc. 1999, 2000
  2. http://physicsworld.com/cws/article/news/25595