Rubidium Facts – Rb or Element 37

Rubidium Facts
Rubidium is an alkali metal with atomic number 37 and symbol Rb.
Rubidium Element Metal
Rubidium is a soft silver metal that melts at a temperature a bit higher than body temperature. (Dnn87, Creative Commons Attribution 3.0 Unported)

Rubidium is a soft, silvery-white metallic element of the alkali metal group, known for its strikingly colorful flame test. People encounter rubidium in specialized applications such as atomic clocks, medical research, and occasionally in fireworks for its characteristic red-purple flame coloration. Here is a closer look at the discovery, properties, uses, and sources of this element.

Symbol: Rb
Atomic Number: 37


Rubidium was discovered in 1861 by the German chemists Robert Bunsen and Gustav Kirchhoff. They made the discovery while they were analyzing the spectrum of the mineral lepidolite. They noticed a pair of prominent red lines that did not match any previously known elements, which led to the identification of rubidium.


The name “rubidium” originates from the Latin word “rubidus,” meaning deep red. This refers to the bright red spectral lines of this element.


Rubidium is a soft, silvery-white metal. It is similar in appearance to other alkali metals like potassium and sodium.

Rubidium Properties

Rubidium Flame Test Color
Rubidium produces a red to violet flame in the flame test. (Didaktische.Medien, Creative Commons Attribution-Share Alike 3.0 Unported)
  • Rubidium is highly reactive, especially with water and air. In water, it reacts violently, producing hydrogen gas and rubidium hydroxide.
  • It has a very low melting point for a metal.
  • Rubidium conducts electricity well.


There are two naturally occurring isotopes of rubidium: 85Rb and 87Rb. Of these, 85Rb is more abundant. Rubidium-87 is a beta emitter that has a half-life of 4.9 x 1010 year. So, natural sources of rubidium are slightly radioactive. Many synthetic radioisotopes of the element are also known.


Rubidium is relatively rare, but it can be found in certain minerals such as lepidolite and pollucite. Although it’s not abundant, large deposits occur in Canada, Russia, and Namibia.

Rubidium Uses

  1. Atomic Clocks: The most accurate timekeeping devices, atomic clocks, use the frequency of electronic transitions in the 87Rb isotope.
  2. Medical Imaging: Rubidium chloride find use in medical imaging.
  3. Fireworks: Rubidium produces a red-purple flame when burned, making it useful in fireworks.
  4. Research: It’s used in various scientific research settings, particularly in physics and chemistry labs.
  5. Radioisotope Dating: 87Rb is an isotope of importance in geological dating methods, particularly for dating old rocks and minerals.
  6. Thermoelectric Generators and Ion Engines: While rubidium finds use in powering spacecraft, there are also alternatives.

Additionally, rubidium removes oxygen from vacuum tubes, is a component of certain types of glass, and may be beneficial to plants and animals as a supplement in certain situations.

Oxidation States

Rubidium typically has an oxidation state of +1 in its compounds, similar to other alkali metals. However, it sometimes has the -1 oxidation state.

Biological Role and Toxicity

Rubidium does not play a essential role in the biology of living organisms, although it occurs in small amounts in nearly all animal tissues. In the body, it participates in many of the same reactions as potassium. Dialysis patients who suffer from depression tend to have lower than average rubidium levels. In large amounts, rubidium compounds can be toxic and even lethal. For example, when rubidium replaces half of the potassium in rat heart muscles, the animals die. Because it reacts with water, the pure metal produces burns on contact with skin or mucous membranes.

Important Rubidium Facts for Chemists

Atomic Number37
Group1 (Alkali metals)
Electron Configuration[Kr] 5s¹
Electrons Per Shell2, 8, 18, 8, 1
Melting Point312.46 K (39.31 °C, 102.76 °F)
Boiling Point961 K (688 °C, 1270 °F)
Density1.532 g/cm³
Heat of Vaporization69 kJ/mol
Heat of Fusion2.19 kJ/mol
Oxidation States-1, +1
Pauling Electronegativity0.82
Ionization Energies403 kJ/mol (1st)
Atomic Radius248 pm
Crystal StructureBody-centered cubic (bcc)
Magnetic OrderingParamagnetic

Interesting Rubidium Facts

  • Eric Cornell, Wolfgang Ketterle, and Carl Wiemen used rubidium-87 to produce Bose-Einstein condensate, earning them the 2001 Nobel Prize in Physics.
  • While not particularly common, rubidium is the 23rd most abundant element in the Earth’s crust. This is about the same abundance as zinc and higher than the abundance of copper.
  • Rubidium is the second most electropositive element, after cesium.
  • Rubidium melts a bit over body temperature, so it is a liquid element on hot days.
  • Like other alkali metals, rubidium reacts violently with water. It is stored in an inert atmosphere or under dry oil to prevent ignition.
  • Rubidium forms alloys with gold, cesium, potassium, and sodium. It forms an amalgam with mercury.
  • Because rubidium is so similar to potassium, it is a good biomarker for tracing potassium uptake by living organisms.


  • Campbell, N. R.; Wood, A. (1908). “The Radioactivity of Rubidium”. Proceedings of the Cambridge Philosophical Society. 14: 15.
  • Fieve, Ronald R.; Meltzer, Herbert L.; Taylor, Reginald M. (1971). “Rubidium chloride ingestion by volunteer subjects: Initial experience”. Psychopharmacologia. 20 (4): 307–14. doi:10.1007/BF00403562
  • Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). Boca Raton, FL: CRC Press. ISBN 1439855110.
  • Meites, Louis (1963). Handbook of Analytical Chemistry (New York: McGraw-Hill Book Company.
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.