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Element Fact
The liquid helium-4 possesses the most unusual physical properties that defy common sense. Below 2.2 K it becomes a superfluid that can flow, against gravity, up the wall of a container.

Rubidium, Rb

rubidium sample

Atomic number: 37
Atomic mass: 85.4678(3)
Natural abundance: 78 ppm
85Rb (84.911789) - 72.165%
87Rb (86.909183) - 27.835%

History and Production

From the Latin word rubidus, deepest red. It was discovered by R.W. Bunsen and G.R. Kirchhoff in 1861 from the mineral lepidolite by means of spectroscopy. The name rubidium was derived due to the deep red line shown in the spectra. It can be commercially recovered from lepidolite. It can also be extracted from potassium minerals. At present, there is little industrial use of the element. It is used as a getter in vacuum tubes and as a photocell component.

Physical Data

It is a soft, silvery-white metal. No rubidium mineral is found in nature, but it is usually associated with the lithium mineral lepidolite. Rubidium compound gives a yellowish violet color in flame.

Interatomic distance: 495 pm
Melting point: 39.31C
Boiling point: 688C
Thermal conductivity/Wm-1K-1: 58.2 (27C)
Density/kgm-3: 1532 (20C), 1475 (m.p.)

Standard Thermodynamic Data (atomic gas)

Enthalpy of formation: 80.9 kJ/mol
Gibbs free energy of formation: 53.1 kJ/mol
Entropy: 170.1 J/mol K
Heat capacity: 20.8 J/mol K

Electronic data

Electronic configuration: [Kr] 5s1
Term symbol: 2S1/2
Electron affinity: 46.8842 kJ/mol   Electronegativity (Pauline): 0.82
Ionization energy (first, second, third): 403.032, 2632.60, 3859.42 kJ/mol

Chemical properties

It is very reactive that ignites spontaneously in air and violently in water to give rubidium hydroxide.

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