Rubidium

Rubidium is a soft alkali metal so reactive that it catches fire spontaneously when exposed to air and explodes in water. It was discovered using spectroscopy: the technique of identifying elements by the unique colour of light they emit, and gave its name directly from those red spectral lines. Today it has a niche but vital role in atomic clocks, the GPS system and quantum computing research.

  • Atomic Number3737 protons, 37 electrons
  • Atomic Mass85.468 uAbout 85× heavier than hydrogen
  • State at Room TempSolidsoft, silvery metal
  • Density1.53 g/cm³Slightly denser than water
  • Melting / Boiling39.3°C / 687.9°CMelts at just 39°C, barely above room temperature
  • Discovered1861Bunsen & Kirchhoff, 1861

Rubidium is the fourth alkali metal in Group 1.

It continues the steady increase in mass and reactivity as you go down Group 1.

Atomic Mass Comparison
Sodium23 u
Potassium39.1 u
Rubidium85.5 u
Iron55.8 u
Caesium133 u

Rubidium at 85.5 u is about twice as heavy as potassium (39 u) and very close to caesium in reactivity. The steady increase of mass down Group 1 reflects the filling of increasingly large electron shells, with one electron always in the outermost shell.

What is rubidium?

Rubidium is an alkali metal in Group 1 of the periodic table, sitting below potassium. It has 37 protons and a single outer electron, like all alkali metals, that it gives away so readily the metal ignites spontaneously in air and reacts explosively with water. Pure rubidium is a soft, silvery metal that must be sealed in an ampoule of argon or kept under mineral oil. It melts at just 39.3°C, slightly above room temperature, so it can melt on a warm day.

Rubidium gets its name from the Latin rubidus, meaning deep red, chosen because rubidium produces two distinctive deep-red lines in its emission spectrum. The element was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff using the technique of spectroscopy: the same team that also discovered caesium. The symbol Rb comes from the name.

Fact Rubidium was discovered using spectroscopy, a technique so new that Bunsen and Kirchhoff had invented it themselves just the year before. By analysing the light emitted by burning substances through a prism, they could identify elements from their unique spectral "fingerprints". Using this method, they discovered both rubidium and caesium in 1861, finding two new elements in a single year.

Where you find rubidium

On Earth

Rubidium is moderately abundant in the Earth's crust at approx. 90 parts per million but is always found dispersed in small amounts in other minerals, never in economically workable deposits of its own.

  • Lepidolite (lithium mica). Rubidium substitutes for potassium or lithium in certain micas. Much commercial rubidium is recovered from lepidolite.
  • Pollucite. This rare mineral also contains both rubidium and caesium. Pollucite deposits in Canada, Zimbabwe and Namibia are the main sources of commercial rubidium and caesium.
  • By-product of lithium mining. Some rubidium is recovered as a by-product of processing lithium minerals from pegmatite deposits.

How we use rubidium

  • Atomic clocks. Rubidium atomic clocks use the precise oscillation frequency of rubidium-87 atoms to measure time to extraordinary accuracy. They are used as secondary frequency standards in GPS satellites and telecommunication networks.
  • Research. Bose-Einstein condensates, a state of matter where atoms become quantum-mechanically identical at near absolute zero, were first created in 1995 using rubidium atoms. This Nobel Prize-winning achievement opened up quantum physics research.
  • Vapour cells. Rubidium vapour cells are used in magnetometers (devices that measure magnetic fields) sensitive enough to detect the magnetic fields produced by the human heart and brain.
  • Specialty glass. Rubidium compounds are used in specialty optical glasses.
Did you know? In 1995, physicists Eric Cornell and Carl Wieman cooled a gas of rubidium atoms to within a hundred-billionth of a degree of absolute zero and created a Bose-Einstein condensate, a bizarre fifth state of matter where thousands of atoms all enter the same quantum state and behave as a single quantum entity. They shared the 2001 Nobel Prize in Physics for the achievement, which has opened up entirely new areas of quantum physics research.

How it was discovered

Rubidium was discovered in 1861 by Robert Bunsen and Gustav Kirchhoff in Heidelberg, Germany. They were applying the new technique of flame spectroscopy to samples of lepidolite and pollucite minerals. After removing the known elements, they saw unfamiliar red spectral lines that matched no known element. They named the new element rubidium from the Latin for red and isolated small amounts of the metal. The discovery of rubidium and caesium in the same year established spectroscopy as a powerful tool for discovering new elements.

Deeper dive: Bose-Einstein condensates and rubidium atomic clocks

Rubidium-87 is slightly radioactive, decaying slowly to strontium-87 with a half-life of approx. 49 billion years, much longer than the age of the universe. This decay is so predictable that geologists use rubidium-strontium ratios to date ancient rocks, similar to potassium-argon dating.

The rubidium atomic clock works by exciting rubidium-87 atoms with microwave radiation at exactly 6.834 GHz: the frequency at which rubidium atoms transition between two ground states. When the microwave frequency exactly matches this transition, maximum absorption occurs. By locking the microwave frequency to this absorption peak, the clock produces an extremely stable frequency reference. Rubidium atomic clocks are less accurate than caesium clocks (which define the second internationally) but are far cheaper and more compact, making them practical for GPS satellites, mobile phone base stations and scientific instruments.

The Bose-Einstein condensate (BEC) created with rubidium atoms represents a fundamentally new state of matter. At temperatures billionths of a degree above absolute zero, quantum mechanical effects dominate and thousands of rubidium atoms all collapse into the lowest possible quantum state. The condensate behaves as a single quantum entity, individual atoms lose their separate identities. BECs are used to study quantum mechanics in new ways and are being explored for use in ultra-precise sensors, quantum computers and atom lasers.

Rubidium is a reactive alkali metal with a remarkable life in quantum physics and atomic timekeeping. Moving to 38 protons brings us to strontium, the alkaline earth metal whose compounds produce red in fireworks.