Rubidium
Rubidium is a chemical element with symbol Rb and atomic number 37. Rubidium is a soft, silvery-white metallic element of the alkali metal group, with an atomic mass of 85.4678. Elemental rubidium is highly reactive, with properties similar to those of other alkali metals, such as very rapid oxidation in air.
Identity
- Atomic mass
- 85.468 u
- Category
- Alkali metals
- Group
- 1
- Period
- 5
- Block
- s
- Oxidation States
- +1
- Phase (25 °C)
- Solid
- Appearance
- grey white
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s1
- Short configuration
- [Kr] 5s1
- Electrons per shell
- 2, 8, 18, 8, 1
- Electronegativity (Pauling)
- 0.820
- Electron affinity
- 46.9 kJ/mol
- 1st ionization energy
- 403 kJ/mol
- Ionization energies
- 403 · 2633 · 3860 · 5080 · 6850… kJ/mol
Physical properties
- Melting point
- 39.3 °C (312 K)
- Boiling point
- 687.9 °C (961 K)
- Density
- 1.532 g/cm³
- Molar heat capacity
- 31.1 J/(mol·K)
Chemical behavior of rubidium
Rubidium has a single electron in the 5s orbital ([Kr] 5s¹), located at a great distance from the nucleus and shielded by inner electron shells. Consequently, it presents an extremely low ionization energy and cedes this valence electron spontaneously to constitute the Rb⁺ cation with the electron configuration of the noble gas krypton. It forms exclusively ionic bonds and lacks other oxidation states in solution. Following the periodic pattern of group 1, it is much more reactive, electropositive and softer than lithium, sodium and potassium, igniting spontaneously on contact with air.
Reactivity
2 Rb + 2 H₂O → 2 RbOH + H₂
Reacts violently and explosively with liquid water and even with ice at temperatures up to −100 °C, releasing hydrogen that ignites instantaneously.
Rb + O₂ → RbO₂
It ignites spontaneously in air forming rubidium superoxide, an orange crystalline compound with strongly ionic bonds.
2 Rb + Cl₂ → 2 RbCl
It burns with liveliness in the presence of gaseous chlorine forming rubidium chloride, with emission of bright violet light.
2 Rb + 2 C₂H₅OH → 2 C₂H₅ORb + H₂
It reacts intensely with alcohols producing rubidium ethoxide and hydrogen gas with high thermal release.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Rubidium chloride | RbCl | Reagent in cellular biochemistry for introducing DNA into competent bacteria and physiological tracer |
| Rubidium carbonate | Rb₂CO₃ | Special optical glass for telecommunications, high-tech ceramics and catalysts |
| Rubidium hydroxide | RbOH | Hyperstrong inorganic base, specialized chemical stripper and reagent in industrial synthesis |
| Rubidium formate | HCOORb | Dense saline solutions for gravimetric separation and petroleum completion fluids |
Electron shell distribution
View in the atom builderIsotopes
Natural rubidium is made up of two isotopes: rubidium-85 (⁸⁵Rb), stable with 48 neutrons (72.17 %), and rubidium-87 (⁸⁷Rb), a primordial beta-emitting radionuclide with 50 neutrons (27.83 %) and a gigantic half-life of 49.2 billion years (more than three times the age of the universe). Due to this extremely slow decay into strontium-87, the isotope pair ⁸⁷Rb/⁸⁷Sr constitutes one of the most reliable geochronological and cosmochemical dating methods for dating lunar rocks, meteorites and the terrestrial mantle.
Occurrence and production
It is the twenty-second most abundant element in the Earth's crust (about 90 ppm), being more copious than common metals such as copper or zinc, but it is found extremely dispersed without forming its own economically exploitable minerals. It is found as a substitutional impurity of potassium in minerals such as , carnallite and pollucite. It is obtained commercially as a byproduct of lithium and cesium ore processing, separating by fractional crystallization of alum or by liquid-liquid extraction with organic solvents.
History and origin of the name
It was discovered in 1861 by German scientists and in samples of mineral water from Bad Dürkheim and using the spectroscope method they had invented themselves. By analyzing the light emitted by the sample in the flame, they observed two spectral lines of bright dark red never before recorded. They gave it the name "rubidium" from the Latin word "rubidus", meaning dark red or intense crimson, in allusion to those characteristic optical lines.
Applications and relevance of rubidium
Uses of the element
Elemental rubidium has cutting-edge technological applications in quantum physics and telecommunications. It is the central component of rubidium atomic clocks, which provide time references of enormous stability in cellular telephone base stations, telecommunications networks and the internet. Likewise, its atomic vapors are used as laser cooling targets to create Bose-Einstein condensates and in precision atomic magnetometers.
Uses of its compounds
Its salts are incorporated in the manufacture of special optical fiber glasses to increase the refractive index and reduce light signal dispersion. In molecular biology, rubidium chloride is used to induce permeability in bacterial membranes during plasmid transformation, and in fireworks it produces purple sparks.
Health, safety and environment
Biological role
It does not play any known biological role in living beings. Due to its similarity in ionic radius with potassium, the Rb⁺ ion is rapidly absorbed by animal and plant cells through potassium channels, distributing through muscle tissues without acute toxic accumulation in normal trace amounts.
Safety
Metallic rubidium is extremely reactive and pyrophoric: it burns spontaneously on contact with environmental oxygen and reacts with explosive violence against any trace of moisture or water. It must be handled under a rigorously anhydrous argon atmosphere in vacuum-sealed glass ampoules. Its fires require specific Class D metal powder extinguishing agents.
Environment
Due to its very low free concentration and because its ions are strongly fixed in soil clays like potassium, rubidium does not represent an ecological threat nor does it present toxic bioaccumulation potential in waterways or food chains.
Frequently asked questions about Rubidium
Why are rubidium atomic clocks so popular in telecommunications and satellites?
Rubidium clocks take advantage of the extremely stable hyperfine transition frequency of the rubidium-87 atom (6.834 gigahertz) to govern a reference quartz oscillator. Although cesium clocks are the absolute primary standard, rubidium devices are much more compact, economical, robust and low power. For this reason they are massively installed in GPS and Galileo navigation satellites, 5G cellular networks and telephone exchanges to synchronize data transfers without delays.
How do rubidium and strontium isotopes help calculate the age of meteorites?
Rubidium-87 slowly transforms into strontium-87 radioactively, over billions of years. By measuring the amounts of these isotopes in minerals of a meteorite, scientists can estimate when those materials formed or were modified. This radiometric dating method has contributed to determining the age of the solar system, which formed about 4.567 billion years ago.
How is DNA introduced into bacteria to study it in the laboratory?
In some experiments, scientists prepare bacteria so they can incorporate small circular DNA molecules called plasmids. Rubidium salts can be part of that treatment, which temporarily modifies the cells' capacity to accept genetic material. Thus, researchers can study specific genes or make bacteria produce a certain protein. It is a laboratory technique, not a process that normally occurs in nature.
How can rubidium vapor help measure very weak magnetic fields?
Rubidium atoms inside a small vapor cell respond to the magnetic fields around them. If they are illuminated with a laser, that response can be measured with great sensitivity. Instruments based on this principle allow detecting very weak magnetic fields, including those generated by brain activity. This technology is investigated to study brain activity and other phenomena without resorting to bulky equipment or sensors that require extremely low temperatures.
What did scientists discover by cooling rubidium atoms almost to absolute zero?
In 1995, several research teams managed to cool rubidium atoms to extraordinarily low temperatures, close to absolute zero. Under those conditions, many atoms began to behave collectively as if they formed a single quantum entity. This state of matter is called Bose-Einstein condensate and allowed studying quantum physics phenomena at an observable scale. The discovery opened new research avenues and was recognized with the 2001 Nobel Prize in Physics.
What happens if rubidium is thrown into water or ice?
Rubidium is one of the most reactive metals in nature. On contact with liquid water or even with a block of ice at forty degrees below zero, it rips hydrogen atoms with immediate violence. The reaction releases so much heat that it instantly ignites the released hydrogen gas, causing an instantaneous explosion and a striking violet flare.
Why does rubidium burn spontaneously in air while sodium requires a heat source?
Rubidium is a pyrophoric metal due to its extraordinary electropositivity and low atomization enthalpy. Its surface metal atoms oxidize at supersonic speed on contact with atmospheric oxygen molecules at ordinary temperature, forming very stable crystalline superoxides. The heat generated by this initial reaction does not dissipate in time and raises the local temperature above its very low ignition point, triggering spontaneous combustion without the need for a previous spark.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.