Rubidium (37)

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 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, cesium, sodium, and potassium. It colors a flame yellowish-violet.

•Name: Rubidium
•Symbol: Rb
•Atomic number: 37
•Atomic weight: 85.4678
•Standard state: solid at 298 K
•CAS Registry ID: 7440-17-7
•Group in periodic table: 1
•Group name: Alkali metal
•Period in periodic table: 5
•Block in periodic table: s-block
•Color: silvery white
•Classification: Metallic

Historical information

Rubidium was discovered by Robert Bunsen, Gustav Kirchhoff at 1861 in Germany. Origin of name is 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 (pretty!).

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.

Physical properties

•Melting point: 312.46 [or 39.31 °C (102.76 °F)] K
•Boiling point: 961 [or 688 °C (1270 °F)] K
•Density of solid: 1532 kg m-3

Orbital properties

•Ground state electron configuration: [Kr].5s1
•Shell structure: 2.8.18.8.1
•Term symbol: 2S1/2

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.