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.

Nearing Zero cartoon for caesium
Cartoon by Nick D Kim (, used by permission).

Table: basic information about and classifications of caesium.

Caesium: historical information

Caesium was discovered by Gustav Kirchhoff, Robert Bunsen at 1860 in 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: physical properties

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Caesium: orbital properties

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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.

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caesium atomic number