Rubidium
Atomic Data
| Atomic Number | 37 |
| Symbol | Rb |
| Atomic Weight | 85.468 u |
| Density (STP) | 1.532 g/cm³ |
| Melting Point | 39.31 °C (312.46 K) |
| Boiling Point | 687.85 °C (961 K) |
| Electronegativity | 0.82 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 5s1 |
| Oxidation States | +1 |
| Phase at STP | Solid |
| Category | Alkali Metal |
| Period / Group | 5 / 1 |
| CAS Number | 7440-17-7 |
Electron Configuration
[Kr] 5s1
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 6 | 18 |
| M | 3 | 3d | 10 | 28 |
| N | 4 | 4s | 2 | 30 |
| N | 4 | 4p | 6 | 36 |
| O | 5 | 5s | 1 | 37 |
| Total | 37 | 37 | ||
Isotopes of Rubidium
Rubidium has two naturally occurring stable isotopes. The most abundant is ⁸⁵Rb, comprising 72.17% of all naturally occurring Rubidium.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Rubidium-85 | ⁸⁵Rb | 37 | 48 | 72.17 | Stable |
| Rubidium-87 | ⁸⁷Rb | 37 | 50 | 27.83 | Stable |
Abundance & Occurrence
Rubidium is present in Earth's crust at approximately 90 ppm by mass and at approximately 1 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Extreme water reactivity: Rubidium reacts more violently with water than sodium or potassium, igniting instantly and potentially detonating; even traces of moisture cause fire: handle only in an inert atmosphere.
- Spontaneous ignition in air: Rubidium ignites spontaneously in air; it must be stored under dry mineral oil or inert gas and never exposed to atmosphere outside a glove box.
- Severe burns: Contact with rubidium metal or rubidium hydroxide causes severe alkaline chemical burns to skin and eyes; the hydroxide is extremely caustic.
- Rubidium-87 radioactivity: Natural rubidium contains 27.8% Rb-87, a weak beta emitter (t½ = 4.9 × 1010 years); the activity is very low and poses no measurable radiation hazard in normal use.
Rubidium in the Real World
Real-World Uses
- Atomic clocks and GPS: Rubidium-87 frequency standards exploit a hyperfine transition at 6.8 GHz to provide highly stable oscillator references used in telecommunications base stations, GPS satellite secondary clocks, and network synchronisation equipment.
- Photomultiplier tubes: Rubidium (and caesium) coatings on photocathodes in photomultiplier tubes convert single photons into detectable electron avalanches, used in radiation detectors, night-vision instruments, and scientific spectrometers.
- Bose-Einstein condensate research: Rubidium-87 was used in the first experimental creation of a Bose-Einstein condensate in 1995 (by Cornell and Wieman), enabling study of quantum macroscopic phenomena; it remains a preferred atom for cold-atom physics experiments.
- Magnetoencephalography (MEG): Optically pumped rubidium vapour magnetometers detect the tiny magnetic fields produced by neural activity in the brain, offering a wearable alternative to cryogen-based SQUID-based MEG scanners.
- Specialty glasses: Rubidium compounds are used in specialty optical glasses with specific refractive index or dispersion properties, and as a flux in the production of high-purity crystals for laser applications.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is rubidium used for?
Rubidium's most important application is in atomic clocks: rubidium frequency standards are compact, inexpensive, and accurate to about one part in 10^11, making them widely used in GPS satellites, telecommunications networks, and portable timing instruments. Rubidium is also used in magnetometers, in photomultiplier tubes (rubidium iodide as a photocathode), and in research on Bose-Einstein condensates.
Is rubidium dangerous?
Rubidium is a highly reactive alkali metal that ignites spontaneously in air and reacts explosively with water, releasing hydrogen gas. It must be stored under an inert atmosphere or in mineral oil. The metal itself is not a common toxic hazard since handling it requires specialist conditions, but its compounds can cause irritation. Rubidium's chemistry is very similar to potassium, so it partially mimics potassium in biological systems.
How was rubidium discovered?
Rubidium was discovered in 1861 by German chemists Robert Bunsen and Gustav Kirchhoff using the newly developed technique of flame spectroscopy. They detected two characteristic deep-red spectral lines in samples of the mineral lepidolite that did not match any known element. The name comes from the Latin 'rubidus', meaning deep red, describing those spectral lines.
How accurate is a rubidium atomic clock?
A rubidium atomic clock is accurate to roughly one second in 300 years under typical operating conditions. While this is less precise than caesium primary standards (which are accurate to one second in 300 million years), rubidium clocks are far smaller, cheaper, and lower-power, making them practical for GPS satellites, mobile base stations, and portable military systems where a primary caesium clock would be impractical.