Rubidium (Rb)

Rubidium (symbol Rb) is the chemical element with atomic number 37: an alkali metal, solid at 0 °C, with an atomic mass of 85.468 u. Discovered in 1861 (Robert Bunsen and Gustav Kirchhoff).

Rubidium is an alkali metal so reactive that it bursts into flame in air and explodes on contact with water. More abundant than copper, it nonetheless forms no ore of its own. It keeps the beat in atomic clocks and, in 1995, made it possible to create a new state of matter.

Key facts

Atomic number: 37
Symbol: Rb
French name: Rubidium
Atomic mass: 85.468 u
Molar mass: 85.468 g/mol
Family: Alkali metals
Position: period 5, group 1, s-block
State at 0 °C: Solid
Melting point: 39.3 °C
Boiling point: 687.9 °C
Density: 1.53 g/cm³
Electronegativity (Pauling): 0.82
First ionization energy: 403 kJ/mol
Atomic radius: 235 pm
Electron configuration: [Kr] 5s¹
Abundance in Earth's crust: 90 ppm (22nd most abundant element)
Indicative price: 16,000 $/kg (order of magnitude)
Radioactive: no
Discovery: 1861, Robert Bunsen and Gustav Kirchhoff

Origin of the name

From the Latin rubidus, deep red, the color of its spectral lines.

What does rubidium look like?

A very soft, silvery-white metal that melts at 39 °C, barely above body temperature. It is stored under oil or in sealed vacuum ampoules.

Where is rubidium found?

At about 90 ppm of Earth's crust, it is more abundant than copper or zinc. But it is scattered through potassium and lithium minerals and never forms a deposit of its own.

Natural sources

Lepidolite
Pollucite
Carnallite
Rubicline (rare)

How is rubidium produced?

It is extracted as a by-product of processing lepidolite (a lithium ore) or pollucite (a caesium ore). World output is tiny, on the order of a few tonnes a year, and its price is very high.

Top producers

China
Canada
Zimbabwe

What is rubidium used for?

Compact atomic clocks (GPS satellites, telecom networks).
Quantum physics research (ultracold atoms).
PET heart imaging (rubidium-82).
Dating ancient rocks (rubidium-strontium method).
The purple color of some fireworks.

In everyday life

Your phone's GPS signal (satellite clocks)
Cell towers (time reference)

Role in living things

With no known role, it is absorbed by the body as if it were potassium. An adult contains a few hundred milligrams, supplied by fruit, vegetables and coffee.

Hazards of rubidium

The pure metal ignites in air and reacts violently with water, producing hydrogen and corrosive caustic hydroxide. Its salts have low toxicity.

Cosmic origin

Supernovae: Forged in the cores of massive stars, then scattered when they explode.
Neutron stars: Created when two neutron stars collide — the “forge” of gold.

History of rubidium

In 1861, Robert Bunsen and Gustav Kirchhoff, inventors of spectroscopy, analyzed the light given off by a mineral, lepidolite, and found two unknown deep-red lines. They named the element after the Latin rubidus, “deep red.” It was the second element discovered with this technique, after caesium.

Did you know?

The first Bose-Einstein condensate, a state of matter in which thousands of atoms behave as one, was made in 1995 with rubidium-87 (Nobel Prize 2001).
Rubidium-87 is slightly radioactive, with a half-life of 49 billion years, more than three times the age of the universe.
Despite its name, the metal isn't red at all: the color comes from its light signature.

Isotopes of rubidium

A single stable isotope, rubidium-85; nearly one atom in four is weakly radioactive rubidium-87. Its slow transformation into strontium dates the oldest rocks.

rubidium-82: radioactive, half-life 1.26 min. PET imaging of blood flow in the heart muscle.
rubidium-85: 72.17% of natural rubidium, stable
rubidium-87: 27.83% of natural rubidium, radioactive, half-life 4.97 × 10¹⁰ years. Rubidium-strontium dating of rocks and meteorites.

Related elements

Same family (alkali metals): Lithium (Li), Sodium (Na), Potassium (K), Caesium (Cs), Francium (Fr).

Same column (group 1): Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Caesium (Cs), Francium (Fr).