• Symbol: Cs
  • Atomic Number: 55
  • Relative Atomic Mass: 132.90545
  • Melting Point/K: 301.5
  • 1st Ionisation Energy (KJmol-1): 375.7
  • 2nd Ionisation Energy (KJmol-1): 2234
  • Atomic Radius (nm): 0.265
  • Ionic Radius (nm): 0.0170
  • Ground State Electron Configuration: [Xe]6s1
Image of Ceasium


Picture given by permission of Dr Mark Winter, and Dr James Marshall, Walking Tour of the elements CD
Caesium like rubidium was discovered by R. W Bunsen and G.R. Kirchoff. It was discovered in 1860 only a few months before that of Rubidium. Again its discovered was aided by the introduction of the spectroscope (1859). Caesium was the first element not to be discovered chemically. The name casium is derived from its line spectra. (Caesius, sky blue.)


Caesium was isolated from mineral spa waters. Around 40 tonnes of water had to be evaporated to obtain a few grams of caesium salt. Bunsen and Kirchoff were unable to isolate the pure metal, this was achieved by Carl Setterberg in 1882. Setterberg isolated Cesium by electrolysis of the cyanide in the presence of barium cyanide.

Caesium can now be isolated using a number of different methods.

1)It is made by reducing rubidium chloride with calcium at 750oC and Reduced pressure. Equations shown below

At Anode: Cl-(l) arrow 1/2Cl2 (g)

At Cathode: Cs+(l) + e- arrow Cs

2) Small amounts of cesium maybe recovered from cesium azide by heating it. (decomposition)

2CsN3arrow 2Cs + 3N2


Caesium may be found occuring in the minerals, pollucite (CsAlSi2O6). (hydrated silicate of aluminium and ceasium), lepidolite, and in other sources. The Bemic lake in Manitoba is estimated to contained over 300, 000 tonnes of pollucite, this is one of its richest sources. Caesiums natural isotope is ceasium 133 but over 15 more exist.


Author: Joshua Swann (document modification date: 23rd May 2002)