Rubidium (Rb)

Stable isotopes of rubidium available from ISOFLEX

Isotope Z(p) N(n) Atomic Mass Natural Abundance Enrichment Level Chemical Form
Rb-85  37 48 84.911792 72.16% ≥99.50% Chloride
Rb-87 37 50 86.909186 27.84% >98.00% Chloride
Rb-87 37 50 86.909186 27.84% >98.00% Metal

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Rubidium was discovered spectroscopically in 1861 by Robert Bunsen and Gustav Kirchhoff. The origin of its name is the Latin word rubidius, meaning “dark red” or “deepest red,” referring to the element's bright red spectroscopic lines.

A soft, silvery white solid with body-centered cubic crystals, rubidium is ductile and very light and is easily oxidized in air. The liquid metal vaporizes, producing a blue vapor. It is soluble in acids and alcohol and reacts violently in water to form rubidium hydroxide. Rubidium is also a highly reactive metal, with most of its reactions similar to those of sodium or potassium. The metal ignites spontaneously in air, forming oxides, and is coated rapidly with a gray-blue oxide film. The reaction with dilute mineral acids can proceed with explosive violence, releasing hydrogen. Rubidium combines with hydrogen and nitrogen, forming the hydride RbH and the nitride Rb3N.

Rubidium metal and its salts have very few commercial applications. They are used in research involving magnetohydrodynamics and thermoionic experiments. Rubidium is also used in photocells and in vacuum tubes. The beta-emitter isotope Rubidium-87 is used to determine the age of some rocks and minerals. Radioisotopes of rubidium have been used as radioactive tracers to trace the flow of blood in the body. Rubidium salts are used in pharmaceuticals as soporifics and sedatives, as well as for treating epilepsy.

Properties of Rubidium

Name Rubidium
Symbol Rb
Atomic number 37
Atomic weight 85.468
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
Melting point 39.3 °C
Boiling point 686 °C
Vaporization point 689 °C
Thermal conductivity 58.2 W/(m·K) at 298.2 °K
Electrical resistivity 12.5 µΩ·cm at 20 °C
Electronegativity 0.8
Specific heat 0.36 kJ/kg K
Heat of vaporization 72 kJ·mol-1
Heat of fusion 2.19 kJ·mol-1
Density of liquid 1.47 g/cm3 at 39.3 °C
Density of solid 1.53 g/cm3
Electron configuration [Kr]5s1
Atomic radius 2.43 Å
Ionic radius Rb+: 1.48 Å
Atomic volume 55.9 cm3/g-atom at 20 ºC
Ionization potential 4.177 V
Oxidation state +1 
Mohs hardness scale 0.3

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