Cesium

Cesium
Properties
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 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.

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.