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Rubidium,  37Rb
General properties
Name, symbol Rubidium, Rb
Appearance Silvery-white
Rubidium in the periodic table


Atomic number 37
Standard atomic weight (Ar) 85.4678(3)
Group, block (alkali metals); s-block
Period period 5
Electron configuration [Kr] 5s1
per shell
2, 8, 18, 8, 1
Physical properties
Phase Solid
Melting point 312.45 K ​(39.30 °C, ​​102.74 °F)
Boiling point 961 K ​(688 °C, ​​1270 °F)
Density near r.t. 1.532 g/cm3
when liquid, at  1.46 g/cm3
Triple point 312.41 K, ​ kPa
Critical point 2093 K, 16 MPa (extrapolated)
Heat of fusion 2.19 kJ/mol
Heat of 69 kJ/mol
Molar heat capacity 31.06 J/(mol·K)
Atomic properties
Oxidation states +1, −1 ​(a strongly basic oxide)
Electronegativity Pauling scale: 0.82
energies 1st: 403 kJ/mol
2nd: 2632.1 kJ/mol
3rd: 3859.4 kJ/mol
Atomic radius empirical: 248 pm
Covalent radius 220±9 pm
Van der Waals radius 303 pm
Crystal structure ​Body-centered cubic (bcc)
Speed of sound thin rod 1300 m/s (at 20 °C)
Thermal expansion 90 µm/(m·K)
Thermal conductivity 58.2 W/(m·K)
Electrical resistivity 128·10-9 Ω·m (at 20 °C)
Magnetic ordering Paramagnetic
Young's modulus 2.4 GPa
Bulk modulus 2.5 GPa
Poisson ratio 0.3
Brinell hardness 0.216 MPa
CAS Registry Number 7440-17-7
Discovery Robert Bunsen and Gustav Kirchhoff (1861)
First isolation George de Hevesy
· references

Rubidium is an alkali metal with the chemical symbol Rb and an atomic number of 37. It is a relatively rare element due to its even distribution through the Earth's crust. It does not have any known ores with a concentration higher than 1%. The world production of rubidium compounds is between 2-4 tonnes per year.



Rubidium will react quickly and violently with water producing rubidium hydroxide and hydrogen. It also reacts violently with halogens in the presence of small amounts of water to form the corresponding halides.


Rubidium is a soft, silvery metal which will tarnish quickly in air. It has a relatively low melting point of 39°C. Like other alkali metals, it is quite soft and can easily be cut with a steel blade.


Rubidium metal is highly reactive and shipping regulations are very strict. It is offered by element dealers such as Metallium at very high prices(~$150/g). Rubidium salts are also available to the public on sites like eBay, priced at around $3/g.


Rubidium metal can be prepared by the dry distillation of a rubidium halide and lithium metal, yielding rubidium vapor due to its low boiling point. This low boiling point is responsible for shifting the equilibrium of this reaction; the vapor can then be condensed in a glass vessel. This is quite dangerous and the risk of explosion is very high. It must be done in an inert atmosphere to prevent the spontaneous oxidation and ignition of the rubidium vapor at the high reaction temperatures.




Rubidium metal is extremely dangerous as it will catch fire in moist air and explode on contact with water. Upon long term storage in mineral oil, it will react with air to form shock sensitive explosive peroxides, similar to potassium. In its ionic form, rubidium ions are extremely similar to potassium ions, and replace them throughout the body. Due to their similarity, this caused no ill effects in study participants with 100x the normal amount (0.36g) of rubidium in a human. Mice were able to survive with significant fractions of potassium in their bodies replaced with rubidium, but when the rubidium:potassium ratio approached 1:1, the mice died.


Rubidium can be stored in mineral oil, but since rubidium is much more reactive than sodium or potassium, it will oxidize much faster. It's best stored under inert atmosphere, such as argon. For long term storage, it should be sealed in a glass ampoule.


Since rubidium is expensive, it is best recycled if possible. However, rubidium ions are not toxic to the environment and moderate amounts may be discarded in municipal waste.


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