Atomic number 55

Caesium

Cs
+ phase
− phase

Shells Orbitals 6s

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55 electrons in 6 shells: 2, 8, 18, 18, 8, 1.

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

Category: Alkali metal
Period: 6
Also spelled: Cesium (US)
Group: 1
Block: s
Standard atomic weight: 132.91(1) Da
Electron configuration: [Xe] 6s¹
Phase at 25 °C: solid
Melting point: 28.5 °C
Boiling point: 671 °C
Density: 1.93 g/cm³
Electronegativity (Pauling): 0.79
Discovered: 1860, Robert Wilhelm Bunsen and Gustav Robert Kirchhoff

Neighbours in the table

Xe Ba

In 1860 Robert Bunsen and Gustav Kirchhoff found a new element in mineral water from Durkheim by its light alone. Los Alamos describes bright lines in the blue, with others in the red, yellow and green. They named it caesium (cesium in US spelling) after the Latin caesius, sky blue, the colour of those blue lines. The metal itself waited until 1882, when Carl Setterberg first isolated it.

Only one 6s electron lies outside the xenon core in caesium, whose 55 electrons form shells of 2, 8, 18, 18, 8 and 1. Atoms grow down group 1, and in a larger atom the outermost electron sits farther from the nucleus, where it is easier to remove. Caesium’s first ionisation energy, the energy needed to pull one electron off the neutral atom, is 3.894 eV on Jefferson Lab’s figure. Los Alamos calls it the most electropositive (readiest to shed electrons) and most alkaline element.

That electron goes to almost any partner. Caesium reacts directly with every nonmetal except the noble gases, giving ionic compounds of Cs⁺, and with excess oxygen it forms the superoxide CsO₂. Los Alamos also lists −1 as an oxidation state, a bookkeeping number assuming ionic bonds. The metal melts at 28.5 °C, only a few degrees above 25 °C.

Where it is found

Jefferson Lab’s figures in PubChem’s record give 3 milligrams per kilogram of crust and 3 × 10⁻⁴ milligrams per litre of seawater. None of it is free metal, because the alkali metals are too reactive to stay uncombined. The ore that counts is pollucite, CsAlSi₂O₆, a silicate of aluminium and caesium. The US Geological Survey finds it chiefly in lithium-rich granite pegmatites, very coarse-grained granite bodies, alongside lepidolite or petalite.

Los Alamos counts Bernic Lake in Manitoba among the richest caesium sources in the world, with an estimated 300,000 tons of pollucite averaging 20 per cent caesium. The Geological Survey states grade on another basis, as caesium oxide: 5 to 32 per cent in most pollucite. It reports that during 2024 one Canadian company produced caesium intermittently at the Tanco Mine, from ore and stockpiles, and that some ore was mined in China.

Freeing the metal has more than one route. OpenStax describes sodium reducing molten caesium chloride: caesium is the more volatile metal, and drawing off its vapour shifts the equilibrium towards more caesium. Los Alamos names electrolysis of the fused cyanide among several methods.

Why it matters

Caesium keeps time. The US Geological Survey describes caesium isotopes as the resonance frequency standard inside atomic clocks, on which aircraft guidance, satellite positioning and internet and mobile telephone networks depend.

Measured by weight, caesium’s biggest job is underground. The Geological Survey names caesium formate brines, used in drilling at high pressure and temperature, as the primary application by gross weight. About 11,000 tons of formate were in use, with 5 per cent lost and replaced each year and nearly 85 per cent of the brine recovered. Most other uses take only a few grams.

The metal combines readily with oxygen, so it serves as a getter, binding and removing trace gases left inside electron tubes. On ion engines, Los Alamos says the metal has recently found use in ion propulsion, while Jefferson Lab says caesium ions may one day be used there.

Radioactive caesium-137 is made, not mined. The Survey lists it for industrial gauges and sterilising food, sewage and surgical equipment, though, IUPAC says, not in large-scale commercial food irradiation. Weapons tests from the late 1940s and the Chernobyl accident of April 1986 spread it widely. IUPAC notes that this makes it useful for dating lake sediments up to 60 years old.

Properties

Properties of caesium 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	55
Standard atomic weight	132.91(1) Da	Da	IUPAC/CIAAW 2024
Melting point	28.5 °C	°C	Los Alamos: 28.5 °C
Boiling point	671 °C	°C	Los Alamos: 671 °C
Density	1.93 g/cm³	g/cm³
Electronegativity (Pauling)	0.79
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s¹
Phase at 25 °C	solid

Safety

Caesium metal carries the signal word Danger in PubChem’s classification. With water it releases a flammable gas that may ignite by itself, it burns skin and eyes severely, and PubChem’s hazard data add that it catches fire in air unprompted. The stable caesium in rocks is another matter: the US Geological Survey links it to no known human health issues, and the radioactive isotopes are a separate, man-made hazard. For the metal, the PubChem Laboratory Chemical Safety Summary gives the full classification.

Questions

Is caesium a liquid at room temperature?

Only in a warm room. PubChem's data give caesium as a solid at 25 °C, and Los Alamos puts its melting point at 28.5 °C, so a few degrees of extra warmth turn it liquid. Los Alamos groups caesium with gallium and mercury among the metals liquid at room temperature, true of caesium only in a room warmer than 28.5 °C. Jefferson Lab ranks its melting point second lowest among the metallic elements.

What happens when caesium touches water?

Water takes caesium's single outer electron. In group 1 that electron leaves more easily as atomic number rises, because the ionisation energy falls. The hazard data in PubChem's record say the reaction gives off enough heat to ignite the hydrogen it frees, and it leaves caustic caesium hydroxide behind. Los Alamos says the metal reacts explosively even with cold water.

Is caesium radioactive?

Natural caesium is not; it is all caesium-133, the one stable isotope. Caesium-134 and caesium-137 come from nuclear explosions or from uranium breaking down in reactor fuel. The US toxic-substances agency ATSDR says both give off beta particles as they decay. PubChem's isotope data add, for caesium-134, a tiny electron-capture branch of 0.00030 per cent, the nucleus capturing an electron. IUPAC adds that caesium-137 gives off high-energy gamma rays. Their half-lives are 2.0650 and 30.04 years.

Why is it hard to separate caesium from rubidium?

The two are chemically so alike that Jefferson Lab, quoted in PubChem's record, calls caesium difficult to purify: its ores are often contaminated with rubidium. The US Geological Survey says the pair are interchangeable in many applications because their physical properties and atomic radii are similar. Swapping gains little, though, since rubidium is mined as a byproduct of caesium production and of lithium mining.

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