Periodic Table


Atomic number: 37
Atomic weight: 85.4678
Symbol: Rb
Group number: 1
Electronic configuration: [Kr].5s1


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

General information

Discoveror: Robert Bunsen, Gustav Kirchhoff
Date discovered: 1861
Discovered at: Germany
Meaning of name: From the Latin word "rubidius" meaning "deepest red"

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: 1532
Molar volume/cm3: 55.76

Radii /pm

Atomic: 265
Covalent (single bond):
Pauling radius for the ion [Rb]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius249.0no datano datano data


Both values are quoted on the Pauling scale.

Pauling: 0.82
Allred Rochow: 0.89

Crystal Structure

structure: bcc (body-centred cubic)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 312.46
boiling point: 961

Enthalpies /kJ mol-1

fusion: 2.19
vaporization: 69.2

single bond enthalpies:
Rb-F Rb-Cl Rb-Br Rb-I Rb-Rb
490 444 385 331 45.6

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
85Rb84.911794 (3)72.165 (20)
87Rb86.909187 (3)27.835 (20)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Rb.html