Caesium: the essentials

Caesium is known as cesium in the USA.

The metal is characterised by a spectrum containing two bright lines in the blue (accounting for its name). It is silvery gold, soft, and ductile. It is the most electropositive and most alkaline element. Caesium, gallium, and mercury are the only three metals that are liquid at or around room temperature. Caesium reacts explosively with cold water, and reacts with ice at temperatures above -116°C. Caesium hydroxide is a strong base and attacks glass.

Science and Ink cartoon for caesium
Cartoon by Nick D Kim ([Science and Ink], used by permission).

Image adapted with permission from Prof James Marshall's (U. North Texas, USA) Walking Tour of the elements CD.

Caesium: historical information

Caesium was discovered by Gustav Kirchhoff, Robert Bunsen in 1860 at Germany. Origin of name: from the Latin word "caesius" meaning "sky blue" or "heavenly blue".

Caesium (cesium in USA) was discovered spectroscopically by Robert Wilhelm Bunsen and Gustav Robert Kirchhoff in 1860 in samples of mineral water from Durkheim. Their identification was based upon two bright blue lines in the spectrum. The name caesium (from the Latin "caesius" - heavenly blue) was coined for its bright blue spectroscopic lines.

Caesium salts were isolated by Bunsen by precipitation from these spring waters - along with salts of other Group 1 elements. He was able to separate them and isolated the chloride and the carbonate. He was unable to isolate caesium metal, this wasachieved by Setterberg.

Caesium around us Read more »

Caesium (cesium in USA) has no biological role. However it is capable of replacing potassium in the body to some extent because of its chemical similarity. Ingestion of any caesium compounds is therefore to be avoided. Because of this similarity, the isotopes 134Cs and 137Cs (present in the biosphere in small amounts as a result of radiation leaks) are very toxic. Rats fed caesium in place of potassium in their diet die.

Caesium (cesium in USA) is not a common element - about the 45th most abundant in the earth's crust. Caesium occurs in pollucite (a hydrated caesium silicate of aluminium - 2Cs2O.2Al2O3.9SiO2.H2O). Pollucite is found in Norh America, Italy, Kazakstan, and Sweden. Caesium is also found in rhodizite, a borate mineral containing aluminium, caesium, beryllium, and sodium. This is found in the Urals and in Madgascar. Caesium is also contained in some potassium ores.

Abundances for caesium in a number of different environments. More abundance data »
Location ppb by weight ppb by atoms Links
Universe 0.8 0.01 Chemical elements abundance by weight in the universe on a miniature periodic table spark table
Crustal rocks 1900 290 Chemical elements abundance by weight in the earth's crust on a miniature periodic table spark table
Human 20 ppb by weight 1 atoms relative to C = 1000000 Chemical elements abundance by weight in humans on a miniature periodic table spark table

Physical properties Read more »

Heat properties Read more »

Crystal structure Read more »

The solid state structure of caesium is: bcc (body-centred cubic).

Caesium: orbital properties Read more »

Caesium atoms have 55 electrons and the shell structure is The ground state electronic configuration of neutral Caesium is [Xe].6s1 and the term symbol of Caesium is 2S1/2.


Isolation: caesium (cesium in USA) would not normally be made in the laboratory as it is available commercially. All syntheses require an electrolytic step as it is so difficult to add an electron to the poorly electronegative caesium ion Cs+.

Caesium is not made by the same method as sodium as might have been expected. This is because the caesium metal, once formed by electrolysis of liquid caesium chloride (CsCl), is too soluble in the molten salt.

cathode: Cs+(l) + e- → Cs (l)

anode: Cl-(l) → 1/2Cl2 (g) + e-

Instead, it is made by the reaction of metallic sodium with hot molten caesium chloride.

Na + CsCl ⇌ Cs + NaCl

This is an equilbrium reaction and under these conditions the caesium is highly volatile and removed from the system in a form relatively free from sodium impurities, allowing the reaction to proceed. It can be purified by distillation.

Caesium isotopes Read more »

Table. Stables isotopes of caesium.
Isotope Mass
(atom %)
spin (I)
moment (μ/μN)
133Cs 132.905429 (7) 100 7/2 2.582024

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