Rubidium · element 37
Named for a deep red spectral line, it now keeps time inside GPS satellites and made the first Bose-Einstein condensate
Key facts
Rubidium is a soft, silvery alkali metal that would melt on a hot summer's day, catches fire in air and explodes on contact with water. It is more common in the Earth's crust than copper, yet you have almost certainly never seen any, because it never gathers into an ore of its own. Its name comes from the Latin rubidus, deep red, after the two red lines it produces in a spectroscope.
It sits in group 1 under potassium and above caesium. Its chemistry is simple and violent: it wants to lose one electron and does so at the slightest opportunity. That very simplicity is what makes rubidium atoms so useful in atomic clocks, quantum experiments and medical imaging.
Where it is found
Rubidium ions are almost the same size as potassium ions, so wherever potassium minerals form, rubidium tags along as a trace substitute. It is found in the lithium mica lepidolite, in the caesium mineral pollucite and in potash deposits. The crust holds roughly 90 parts per million, which makes rubidium about the twenty-third most abundant element.
Nobody mines rubidium deliberately. It is recovered as a by-product of processing lithium and caesium ores, notably from pegmatite deposits in Canada and Zimbabwe. Seawater contains a small amount. Natural rubidium is about 28 per cent rubidium-87, an isotope that decays so slowly that its half-life is about three times the age of the universe.
How it was discovered
In 1859 Robert Bunsen and Gustav Kirchhoff in Heidelberg invented the spectroscope, an instrument that splits the light from a flame into lines that act like a fingerprint for each element. Within a year they had found caesium in mineral water by its blue lines. In 1861 they processed a large quantity of lepidolite from Saxony and noticed two dark red lines that matched nothing known.
They named the new element rubidium. Bunsen went on to isolate the metal itself by reducing its compounds, and studied its reactions with water and air. Rubidium and caesium were the first elements ever discovered by spectroscopy, and the method soon uncovered thallium and indium too.
What we use it for
The single most valuable use is timekeeping. Rubidium atoms absorb microwaves at an extremely precise frequency, and a rubidium oscillator locked to that frequency makes a compact, affordable atomic clock. Every GPS satellite carries rubidium clocks, and they synchronise mobile phone networks, broadcasting and financial systems on the ground.
In physics, rubidium-87 was the atom used to create the first Bose-Einstein condensate in 1995, a new state of matter in which thousands of atoms behave as a single quantum object, work that won the 2001 Nobel Prize. Rubidium atoms trapped by lasers are now a leading platform for quantum computers and ultra-sensitive magnetometers. In medicine, short-lived rubidium-82 is injected to image blood flow in the heart with PET scanners. Geologists use the slow decay of rubidium-87 into strontium-87 to date rocks that are billions of years old.
Reading the numbers
Rubidium melts at 39 °C, so it would liquefy in a car parked in the sun, and boils at 688 °C. Its density of 1.53 grams per cubic centimetre is about one and a half times that of water. Lithium, sodium and potassium all float on water; rubidium sinks, then reacts so violently that the question is academic.
The configuration [Kr] 5s1 says it all: a single electron in a fifth shell, far from the nucleus and shielded by 36 inner electrons. That electron is very easy to remove, and rubidium's first ionisation energy of 403 kJ/mol is among the lowest of any element. The second ionisation energy jumps to 2633 kJ/mol, more than six times higher, confirming that only one electron is ever given up. The electronegativity of 0.82 is near the bottom of the scale. Its place at the far left of period 5 makes it the most reactive metal that most school laboratories are ever likely to see, and even then only in a sealed ampoule.
Safety and the environment
Rubidium metal ignites spontaneously in air and reacts explosively with water, producing hydrogen and rubidium hydroxide, a strong alkali. It is stored under dry mineral oil or sealed in glass under argon. Rubidium compounds are only mildly toxic. The body handles rubidium ions much like potassium ions, and a typical adult carries a fraction of a gram of rubidium without any known effect.
The natural radioactivity of rubidium-87 is far too weak to be a health concern. Environmental impact is minimal because production volumes are small and the element is recovered from mining that would happen anyway for lithium and caesium.
Did you know?
- Rubidium-87 has a half-life of about 49 billion years, more than three times the current age of the universe.
- The first Bose-Einstein condensate, a cloud of atoms cooled to within a whisker of absolute zero, was made from rubidium-87 in 1995.
- Every GPS satellite in orbit carries rubidium atomic clocks to keep its timing signals accurate.
- Rubidium melts at 39 °C, only a few degrees above the temperature of the human body.
- The human body naturally contains more rubidium than most trace elements, because it is absorbed alongside potassium.
- Rubidium was named after its red spectral lines, while its heavier cousin caesium was named for its blue ones.
More in the alkali metals
These share a column with rubidium, so their outer electrons are arranged in a similar way and they tend to behave alike.
See all 6 alkali metals on one page, or browse all 118 elements.
Frequently asked questions
Does rubidium explode in water?
Yes. Rubidium reacts with water instantly, releasing hydrogen gas and enough heat to ignite it, which produces a violent explosion and a strongly alkaline solution. It is more reactive than potassium and only slightly less so than caesium. Even moisture in the air is enough to start a reaction.
What is rubidium used for?
Its most important use is in rubidium atomic clocks, which provide precise timing for GPS satellites, mobile networks and broadcasting. It is also central to quantum physics experiments, including quantum computers based on trapped atoms. Rubidium-82 is used in heart scans and rubidium-87 decay dates ancient rocks.
Is rubidium radioactive?
Slightly. About 28 per cent of natural rubidium is rubidium-87, which decays to strontium-87 with a half-life of roughly 49 billion years. The activity is so low that it poses no health risk, but it is useful to geologists for dating very old rocks.
Why is rubidium so reactive?
Rubidium has one electron in its outermost shell, sitting far from the nucleus and shielded by four full shells beneath. It takes very little energy to remove that electron, so rubidium hands it over to almost anything nearby, including oxygen and water. Reactivity increases down group 1, so rubidium is fiercer than sodium or potassium.
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Written by Anthony. Updated September 2026. Numerical data from Periodic-Table-JSON (CC BY-SA), cross-checked against the Rubidium reference article. Spotted an error? Tell us.