Atomic number 37

Rubidium

Rb
+ phase
− phase

Shells Orbitals 5s

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37 electrons in 5 shells: 2, 8, 18, 8, 1.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkali metal
Period: 5
Group: 1
Block: s
Standard atomic weight: 85.468(1) Da
Electron configuration: [Kr] 5s¹
Phase at 25 °C: solid
Melting point: 39.3 °C
Boiling point: 688 °C
Density: 1.53 g/cm³
Electronegativity (Pauling): 0.82
Discovered: 1861, Robert Bunsen and Gustav Kirchhoff

Neighbours in the table

Kr Sr

In 1861 Robert Bunsen and Gustav Kirchhoff examined the mineral lepidolite with a spectroscope and saw deep red lines new to both men. CIAAW, IUPAC’s atomic-weights commission, derives the name from the Latin rubidus, deepest red, after two of those lines. Bunsen isolated the metal itself in 1863. A flame coloured by rubidium looks violet, as Los Alamos describes it.

Los Alamos ranks rubidium second among all elements for electropositivity, the readiness to give up electrons. Behind that lies the arrangement [Kr] 5s¹, 37 electrons in shells of 2, 8, 18, 8 and 1, the last a single 5s electron beyond a krypton core. Pulling that electron off the neutral atom takes 4.177 eV, Jefferson Lab reports. OpenStax gives the trend behind it: down group 1 the atoms swell, and the loosest electron of a bigger atom lies farther from the nucleus and comes away more easily.

That easy loss is why the group 1 metals form stable ions of charge 1+, OpenStax says, and for rubidium the ion is Rb⁺. Given excess oxygen, the metal forms the superoxide RbO₂, a compound of Rb⁺ with the paired-oxygen ion O₂⁻, and Los Alamos counts four oxides in all. The soft, silvery-white metal is kept under dry mineral oil, in a vacuum or under an inert gas.

Where it is found

Jefferson Lab’s figures in PubChem’s record give 90 milligrams per kilogram of crust and 0.12 milligrams per litre of seawater. Los Alamos ranks it 16th in the crust by abundance. Yet the US Geological Survey says no mineral holds it as a major ingredient. No rubidium occurs as free metal, since OpenStax notes that the alkali metals are too reactive to stay uncombined.

On the Survey’s figures, lepidolite can hold up to 3.5 per cent rubidium oxide and pollucite up to 1.5 per cent. Los Alamos, by contrast, puts rubidium in lepidolite at about 1.5 per cent. The Survey says rubidium concentrate comes as a byproduct of mining lepidolite for lithium and pollucite for caesium.

No rubidium production was reported anywhere in 2024, the Survey says, although China may have produced some. Rubidium mining in Namibia stopped in the early 2000s, and the Bikita Mine in Zimbabwe ran out of pollucite ore in 2018. To free the metal, OpenStax describes sodium reducing molten rubidium chloride, and Los Alamos names calcium. OpenStax adds that such reduction costs much less than electrolysis, the route it gives for group 1 in general.

Why it matters

Rubidium helps keep networks in step. The US Geological Survey describes it as an atomic frequency reference that synchronises telecommunications networks and supports global positioning systems. The timescale of the US Naval Observatory rests on a weighted network of atomic clocks, six of them rubidium fountain clocks. Research on quantum computing also works with ultracold rubidium atoms.

Rubidium-87 makes the element a geological clock too. It forms 27.83 per cent of natural rubidium on CIAAW’s figure and 27.85 per cent on Los Alamos’s. It decays by one route only, beta emission (the nucleus giving out an electron), to strontium-87. Los Alamos gives the half-life as 4.9 × 10¹⁰ years. The strontium-87 in a rock, with its rubidium-to-strontium ratio, gives the age at which the rock crystallised.

Demand is small by weight: industry information, the Survey says, suggests US consumption under 2,000 kilograms a year, all of it imported. Specialty glass is the Survey’s leading market, where rubidium carbonate lowers electrical conductivity in fibre-optic networks. Rubidium-82, a radioactive isotope, behaves like potassium in the body, which is why it serves in positron emission tomography, a scanning method, to image blood flow through the heart. Los Alamos adds that RbAg₄I₅ conducts better at room temperature than any other known ionic crystal.

Properties

Properties of rubidium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	37
Standard atomic weight	85.468(1) Da	Da	IUPAC/CIAAW 2024
Melting point	39.3 °C	°C	Los Alamos: 39.3 °C
Boiling point	688 °C	°C	Los Alamos: 688 °C
Density	1.53 g/cm³	g/cm³
Electronegativity (Pauling)	0.82
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s¹
Phase at 25 °C	solid

Safety

Rubidium metal is corrosive and pyrophoric, able to catch fire in air unaided, and PubChem’s classification marks it Danger. With water it forms rubidium hydroxide and hydrogen, and the heat usually ignites the gas; contact burns skin and eyes severely. The trace rubidium in rocks and natural water is another matter, with no known human health issues according to the US Geological Survey. The PubChem Laboratory Chemical Safety Summary carries the full classification.

Questions

Can rubidium be a liquid at room temperature?

On a hot day, yes. PubChem's data list rubidium as a solid at 25 °C, and Los Alamos gives its melting point as 39.3 °C, so an ordinary room keeps it solid and a very warm one does not. OpenStax says every alkali metal melts below 200 °C, and rubidium clears that bar by a wide margin.

What happens when rubidium touches water?

Rubidium reacts violently with water, and PubChem's hazard data name the products: corrosive rubidium hydroxide and hydrogen gas. The same data say the heat released usually ignites that hydrogen. In group 1 the ionisation energy falls as atomic number rises, and OpenStax ties rising reactivity to that fall, so rubidium reacts more violently than potassium or sodium.

Is rubidium radioactive?

Weakly. Natural rubidium is a mix of rubidium-85 and rubidium-87, and the heavier isotope gives off beta particles; its half-life is 4.97 × 10¹⁰ years in the Atomic Mass Data Center's table on PubChem's record. Los Alamos says ordinary rubidium is radioactive enough to fog a photographic film within 30 to 60 days. The US Geological Survey reports no known human health issues from the rubidium that occurs naturally.

Can rubidium be used instead of caesium?

In many applications, according to the US Geological Survey, because the two have similar physical properties and atomic radii. Caesium is the more electropositive metal, though, and the Survey says that makes it the preferred choice for some uses. Los Alamos gives an example: rubidium ionises easily and was considered for ion engines in spacecraft, but caesium does that job somewhat more efficiently.