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Element Sunday

55 Caesium


Symbol: Cs

Color:  silvery gold 

Phase: solid

Atomic Weight: 132.9054 u

Electronic Configuration: [Xe] 6s1

Melting Point: 301.59 K

Boiling Point: 944 K


a vial of caesium

Short Info

The name caesium (“cesium” in American English) comes from the Latin word caesius, which means “azure” and refers to the blue spectral lines that are found in the spectral analysis of caesium.

In 1861, caesium was discovered by German physicist Gustav Robert Kirchhoff and German chemist Robert Wilhelm Eberhard Bunsen in the spectral analysis of mineral water. They however could not obtain elementary caesium, which was first synthesized in 1881 by German chemist Carl Setterberg.

Caesium is a rare element, constituting about 6.5 ppm (0.00065 %) of the Earth’s crust. It is the rarest stabile and the most reactive of the alkali metals. In nature, it does not occur elementary. It often occurs accompanied with potassium and sodium, important minerals are pollucite and lepidolite.

Due to its high reactivity, scarcity and difficult production (about 20 tons per year), the applications of caesium are limited. It is mostly used for research applications. It is used for hot cathodes, MHD generators and for ion engines. Another interesting application is for definition of the second: since 1967, a second is defined as 9.192.631.770 times the period of a certain transition of caesium.

Caesium is a mononuclidic element, with the stabile Cs-133 as the only naturally occuring isotope. 38 Other isotopes are known, perhabs most importantly Cs-137: it has a half-life of 30.17 years and is used for radiation therapy of cancer. Unfortunately, large quantities of this radioactive isotope are produced in nuclear reactors and during the detonation of nuclear bombs and, when set free due to an accident or attack, can contaminate people and the environment.


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