Rubidium: the essentials

Rubidium can be liquid at ambient temperature, but only on a hot day given that its melting point is about 40°C. It is a soft, silvery-white metallic element of the alkali metals group (Group 1). It is one of the most most electropositive and alkaline elements. It ignites spontaneously in air and reacts violently with water, setting fire to the liberated hydrogen. As so with all the other alkali metals, it forms amalgams with mercury. It alloys with gold, caesium, sodium, and potassium. It colours a flame yellowish-violet.

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

Table: basic information about and classifications of rubidium.

Rubidium: historical information

Rubidium was discovered by Robert Bunsen, Gustav Kirchhoff at 1861 in Germany. Origin of name: from the Latin word "rubidius" meaning "dark red" or "deepest red".

Rubidium was discovered in 1861 spectroscopically by Robert Bunsen and Gustav Kirchoff as an impurity associated with samples of the mineral lepidolite (a form of mica). The name rubidium (from the Latin "rubidus" - dark red) was coined for its bright red spectroscopic lines.

Rubidium salts were isolated by Bunsen by precipitation from spring waters - along with salts of other Group 1 elements. He was able to separate them and isolated the chloride and the carbonate. He isolated rubidium metal by reducing rubidium hydrogen tartrate with carbon.

Rubidium: physical properties

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Rubidium: orbital properties

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Isolation: rubidium 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 rubidium ion Rb+.

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

cathode: Rb+(l) + e- → Rb (l)

anode: Cl-(l) → 1/2Cl2 (g) + e-

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

Na + RbCl ⇌ Rb + NaCl

This is an equilibrium reaction and under these conditions the rubidium is highly volatile and removed from the system in a form relatively free from sodium impurities, allowing the reaction to proceed.

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rubidium atomic number