55
Cesium
Atomic Number55
Atomic Mass132.91 u
CategoryAlkali Metal
State at Room TempSolid
Year Discovered1860
About Cesium
The international definition of a second is based on the cesium-133 atom. Most reactive of all stable metals. Explodes on contact with water. Cesium atomic clocks are accurate to one second in 100 million years. Used in oil well drilling and photoelectric cells. Named for its blue spectral lines, caesius means sky blue in Latin.
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