Rubidium (Rb)

Rubidium is a chemical element with the atomic number 37 in the periodic table. With an occurrence of 90 parts per million by weight, it’s the 25th most abundant chemical element found in Earth’s crust. As a member of the alkali group of metals, this soft, silvery-white chemical has one valence electron positioned in the s-orbital of the atom’s fifth energy level. 

Despite its natural plentifulness, rubidium extraction and processing require some of the most difficult and expensive processes, which limits the everyday use of this chemical element immensely. 

Chemical and Physical Properties of Rubidium

Atomic number37
Atomic weight (mass)85.4678 g.mol-1
UsesUsed as a catalyst, photocells, and vacuum and cathode-ray tubes.
DescriptionSoft, malleable, silvery-yellow metal.
GroupAlkali Metal
Crystal StructureCubic: Body centered
Covalent Radius2.16 Å
Atomic Radius2.98 Å
Atomic Volume55.9 cm³/mol
Name OriginLatin: rubidus (deep red); the color its salts impart to flames.
Discovered ByR. Bunsen, G. Kirchoff
Oxydation States1
ColorA silvery-white metallic element
Physical stateSolid at room temperature
Half-lifeFrom less than 1.5 microseconds to 4.923(22)×1010 years
Density1.53 at 20°C
Melting point39.30°C, 102.74°F, 312.45 K
Boiling point688°C, 1270°F, 961 K
Van der Waals radius0.243 nm
Ionic radius1.61 (+1) Å
Most characteristic isotope85Rb, 87Rb
Electronic shell[Kr] 5s1
The energy of the first ionization402.9 kJ.mol-1
The energy of the second ionization2633 kJ.mol-1
The energy of the third ionization3860 kJ.mol-1
Discovery dateIn 1861 by Gustav Kirchhoff and Robert Bunsen

With the periodic table symbol Rb, atomic number 37, atomic mass of 85.4678 g.mol-1, and electron configuration [Kr] 5s1, rubidium is a soft, silvery-white metal composed of two isotopes. It reaches its boiling point at 688°C, 1270°F, 961 K, while the melting point is achieved at 39.30°C, 102.74°F, 312.45 K. This member of the alkali metals family of elements in the periodic table has an electronegativity of 0.8 according to Pauling, whereas the atomic radius according to van der Waals is 0.243 nm. 

Rubidium metal is the second most electropositive of the stable alkali metals. Like the other alkali metals (lithium, sodium, potassium, cesium, and francium), this chemical is highly reactive with both air and water. When exposed to air, rubidium spontaneously ignites, while with H2O it creates an equally volatile reaction. 

As with the other alkalis, the water reaction with rubidium produces hydrogen gas and is vigorous enough to ignite the liberated hydrogen. Rubidium oxidizes in contact with air; due to this, rubidium must be kept either under dry mineral oil or in a vacuum or inert atmosphere.

How Was Rubidium Discovered?

In 1861, the German chemists Robert Bunsen and Gustav Kirchhoff dedicated their scientific interest to the analysis of the mineral lepidolite. In their Heidelberg laboratory, they were trying to employ the spectroscope (which they co-discovered only a year earlier) in the analysis of a lepidolite sample. The mineral that captured the scientific curiosity of Bunsen and Kirchhoff was obtained from Saxony, Germany.

In the first step of their scientific trial, the scientists used hydrochloroplatinic acid to precipitate potassium chloroplatinate from the mineral. This method produced yet another type of salt. This time, they used the Bunsen burner. In this way, the discoverers of rubidium observed a spectrum rich in new lines, among which two were strikingly clear and inclined to the red solar spectrum. 

By using the method of electrolysis, Bunsen also managed to isolate the first sample of pure rubidium metal and determine a larger portion of its chemical properties.

How Did Rubidium Get Its Name?

The name of element 37 originates from the Latin word ‘rubidus‘, which means ‘deepest red’. This term refers to the deep red lines displayed by rubidium in the spectrum, by which its discoverers Bunsen and Kirchhoff succeeded to detect the new element. 

Where Can You Find Rubidium?

Naturally, this chemical element occurs in igneous rocks, shale, sandstone, freshwater, marine and land plants, soil, and both land and marine animals. Furthermore, rubidium can be found in many minerals, such as zinnwaldite, carnallite, pollucite, leucite, and lepidolite. Potassium chlorides, potassium minerals, and brines also contain some rubidium quantities. 

The mines richest in rubidium are found on the island of Elba and in Manitoba, Canada. Element 37 is typically isolated as a byproduct of lithium extraction for commercial purposes. In cases where rubidium metal occurs along with other alkalis such as cesium, this chemical is extracted by an ion-exchange method, chemical reduction with sodium or calcium, or by the use of electrolysis.

Rubidium in Everyday Life

Despite being one of the most abundant elements in Earth’s crust, rubidium is a substance that is hard to extract and process. The complex extraction and the limited supply of this chemical add to its high price while limiting its everyday use:

  • Rubidium-87 radioactive isotope is often an atom of choice for making the Bose-Einstein condensates in dilute atomic gases;
  • Rubidium metal forms alloys with elements such as potassium, cesium, sodium, and gold. It’s also used in the production of gases, for condensation, as well as in thermoelectric generators;
  • Element 67 is one of the first choice substances in the manufacturing process of photocells, i.e. sensors that detect light, due to its physical and chemical properties;
  • In dentistry, rubidium is alloyed with gold and mercury for making dental amalgams;
  • Rubidium-87 is applied in the development of magnetometers with high sensitivity;
  • Rubidium carbonate is used for the production of optical glasses;
  • Element 37 is of great use in atomic clocks for high-precision timing;
  • Rubidium–strontium compound is popularly used in radioactive dating of minerals, rocks, and meteors; 
  • As a result of its easy ionization, rubidium is considered as a component of the ion engines used in the space-crafts, a working fluid for vapor turbines, and in thermoelectric generators;
  • The chemical industry used rubidium mainly as an agent for removing traces of oxygen from vacuum tubes (i.e as a getter in vacuum tubes), in vapor turbines, as well as to make special types of glass;
  • Rubidium salts are popularly applied in the production of ceramics and fireworks.

How Dangerous Is Rubidium?

Since it resembles potassium in many aspects, this chemical element is considered a non-toxic substance. Rubidium ions are not naturally found in the body. However, when they are present in any organs, rubidium ions are treated by the body’s chemical mechanisms as if they were potassium.

On the other hand, rubidium metal triggers symptoms of moderate toxicity when it’s ingested, inhaled, or comes in contact with the skin. 

In such cases, the affected individual typically experiences skin irritation, ulcers, slurred speech, incoordination, cognitive problems, as well as gastrointestinal problems. If rubidium ignites, its flame may cause severe thermal burns.

Environmental Effects of Rubidium

Being widely spread in Earth’s crust, rubidium is not considered to be an environmental hazard. Since rubidium is also a member of the potassium family group, it can easily replace the electrolyte function of potassium due to its highly similar properties. This is especially the case with bacteria, algae, fungi, and the echinoderms (like starfish, sea urchin, or sea cucumber).

Isotopes of Rubidium

There are 36 isotopes of rubidium. Naturally occurring rubidium is made of the only stable isotope 85Rb (with 72.2% of natural occurrence) and the radioactive 87Rb isotope (with 27.8% of natural occurrence and a half-life of 4.92×1010 years). This radioactive isotope decays to form the strontium-87 isotope.

Rubidium isotopes mostly undergo a beta mode of decay, but proton emission, electron capture, and isomeric transition to elements Kr, Se, Rb, and Sr, are also observed. All radioisotopes of rubidium have been synthetically produced and are highly radioactive. 


[n 1]

ZNIsotopic mass (Da)

[n 2][n 3]


[n 4][n 5]



[n 6]



[n 7][n 8]

Spin and


[n 9][n 5]

Natural abundance (mole fraction)
Excitation energy[n 5]Normal proportionRange of variation
71Rb373470.96532(54)# p70Kr5/2−#  
72Rb373571.95908(54)#<1.5 μsp71Kr3+#  
73Rb373672.95056(16)#<30 nsp72Kr3/2−#  
74Rb373773.944265(4)64.76(3) msβ+74Kr(0+)  
75Rb373874.938570(8)19.0(12) sβ+75Kr(3/2−)  
76Rb373975.9350722(20)36.5(6) sβ+76Kr1(−)  
β+, α (3.8×10−7%)72Se
77Rb374076.930408(8)3.77(4) minβ+77Kr3/2−  
78Rb374177.928141(8)17.66(8) minβ+78Kr0(+)  
79Rb374278.923989(6)22.9(5) minβ+79Kr5/2+  
80Rb374379.922519(7)33.4(7) sβ+80Kr1+  
81Rb374480.918996(6)4.570(4) hβ+81Kr3/2−  
82Rb374581.9182086(30)1.273(2) minβ+82Kr1+  
83Rb374682.915110(6)86.2(1) dEC83Kr5/2−  
84Rb374783.914385(3)33.1(1) dβ+ (96.2%)84Kr2−  
β (3.8%)84Sr
85Rb[n 10]374884.911789738(12)Stable5/2−0.7217(2) 
86Rb374985.91116742(21)18.642(18) dβ (99.9948%)86Sr2−  
EC (.0052%)86Kr
87Rb[n 11][n 12][n 10]375086.909180527(13)4.923(22)×1010 yβ87Sr3/2−0.2783(2) 
88Rb375187.91131559(17)17.773(11) minβ88Sr2−  
89Rb375288.912278(6)15.15(12) minβ89Sr3/2−  
90Rb375389.914802(7)158(5) sβ90Sr0−  
91Rb375490.916537(9)58.4(4) sβ91Sr3/2(−)  
92Rb375591.919729(7)4.492(20) sβ (99.98%)92Sr0−  
β, n (.0107%)91Sr
93Rb375692.922042(8)5.84(2) sβ (98.65%)93Sr5/2−  
β, n (1.35%)92Sr
94Rb375793.926405(9)2.702(5) sβ (89.99%)94Sr3(−)  
β, n (10.01%)93Sr
95Rb375894.929303(23)377.5(8) msβ (91.27%)95Sr5/2−  
β, n (8.73%)94Sr
96Rb375995.93427(3)202.8(33) msβ (86.6%)96Sr2+  
β, n (13.4%)95Sr
97Rb376096.93735(3)169.9(7) msβ (74.3%)97Sr3/2+  
β, n (25.7%)96Sr
98Rb376197.94179(5)114(5) msβ(86.14%)98Sr(0,1)(−#)  
β, n (13.8%)97Sr
β, 2n (.051%)96Sr
99Rb376298.94538(13)50.3(7) msβ (84.1%)99Sr(5/2+)  
β, n (15.9%)98Sr
100Rb376399.94987(32)#51(8) msβ (94.25%)100Sr(3+)  
β, n (5.6%)99Sr
β, 2n (.15%)98Sr
101Rb3764100.95320(18)32(5) msβ (69%)101Sr(3/2+)#  
β, n (31%)100Sr
102Rb3765101.95887(54)#37(5) msβ (82%)102Sr   
β, n (18%)101Sr
103Rb[2]3766 26 msβ103Sr   
104Rb[3]3767 35# ms (>550 ns)β?104Sr   

Source: Wikipedia

List of Rubidium Compounds 

ELement 37 adopts an oxidation state of +1 in most compounds in which it participates. Rubidium forms a great number of halides, salts, and oxides, which include rubidium iodide, bromide, chloride, fluoride, and monoxide.

The most commonly prepared rubidium compounds are listed below:

  • Rubicline
  • Rubidium acetate
  • Rubidium azide
  • Rubidium bromide
  • Rubidium carbonate
  • Rubidium chloride
  • Rubidium cyanide
  • Rubidium fluoride
  • Rubidium hydride
  • Rubidium hydrogen sulfate
  • Rubidium hydroxide
  • Rubidium iodide
  • Rubidium nitrate
  • Rubidium oxide
  • Rubidium perchlorate
  • Rubidium silver iodide
  • Rubidium sulfate
  • Rubidium sulfide
  • Rubidium telluride
  • Rb6O
  • Rb9O2
  • Rubidium monoxide (superoxide Rb2O)
  • Rubidium-82 chloride

5 Interesting Facts and Explanations

  1. The radioactive rubidium-87 isotope can easily substitute potassium in minerals, as well as in some medical conditions.
  2. Lepidolite is a type of aluminum, potassium, and lithium silicate.
  3. The American scientists Eric A. Cornell, Wolfgang Ketterle, and Carl E. Wieman shared the 2001 Nobel prize in Physics for succeeding to produce the Bose-Einstein condensate from the rubidium-87 isotope.
  4. Rubidium nitrate gives a purple color to fireworks.
  5. Some foods and beverages that we consume on an everyday basis, like asparagus, coffee, freshwater fish, and black tea, are relatively rich in rubidium.