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Caesium (Cs)
Caesium (symbol Cs) is the chemical element with atomic number 55: an alkali metal, solid at 0 °C, with an atomic mass of 132.91 u. Discovered in 1860 (Robert Bunsen and Gustav Kirchhoff).
Caesium is an extremely reactive alkali metal, one of the few gold-colored metals. Since 1967, the second has been defined by the oscillations of the caesium-133 atom in atomic clocks. Its radioactive isotope, caesium-137, is the main long-lasting contaminant after Chernobyl and Fukushima.
Key facts
- Atomic number
- 55
- Symbol
- Cs
- French name
- Césium
- Atomic mass
- 132.91 u
- Molar mass
- 132.91 g/mol
- Family
- Alkali metals
- Position
- period 6, group 1, s-block
- State at 0 °C
- Solid
- Melting point
- 28.6 °C
- Boiling point
- 670.9 °C
- Density
- 1.93 g/cm³
- Electronegativity (Pauling)
- 0.79
- First ionization energy
- 376 kJ/mol
- Atomic radius
- 260 pm
- Electron configuration
- [Xe] 6s¹
- Abundance in Earth's crust
- 3 ppm (46th most abundant element)
- Indicative price
- 62,000 $/kg (order of magnitude)
- Radioactive
- no
- Discovery
- 1860, Robert Bunsen and Gustav Kirchhoff
Origin of the name
From the Latin caesius, sky blue, the color of its spectral lines.
What does caesium look like?
A soft, pale golden-yellow metal that melts at 28.5 °C: it turns liquid on a summer day. It can only be handled in sealed ampoules under inert gas.
Where is caesium found?
About 3 ppm of Earth's crust. It is mined almost exclusively from pollucite, a mineral found in a few pegmatites, including the Tanco mine in Canada.
Natural sources
- Pollucite
- Lepidolite (traces)
- Carnallite (traces)
How is caesium produced?
Pollucite is attacked with acid to produce caesium salts, notably the formate. The world market is no more than a few hundred tonnes of compounds a year, and only a handful of mines work this mineral.
Top producers
- Canada
- Zimbabwe
- Australia
What is caesium used for?
- Dense caesium formate oil-drilling fluids.
- Reference atomic clocks.
- Catalysts in organic chemistry.
- Caesium-137 for radiotherapy and industrial gauges.
- Photoelectric cells.
In everyday life
- Your phone's official time (reference atomic clocks)
- GPS signal (synchronized to atomic clocks)
Role in living things
No biological role; the body mistakes it for potassium. That is why radioactive caesium-137 spreads through the muscles after contamination, before being eliminated within a few months.
Hazards of caesium
The metal ignites spontaneously in air and explodes on contact with water, even more violently than potassium. Caesium-137, with a half-life of 30 years, contaminates soil and food for a long time after a nuclear accident.
Cosmic origin
- Neutron stars: Created when two neutron stars collide — the “forge” of gold.
- Dying stars: Made slowly in red giants (stars like the Sun at the end of their lives).
History of caesium
In 1860, Robert Bunsen and Gustav Kirchhoff analyzed mineral water from Bad Dürkheim with a spectroscope and found two unknown blue lines: it was the first element discovered by spectroscopy. They named it after the Latin caesius, “sky blue.” They had to evaporate tens of thousands of liters of water to obtain a few grams of salt.
Did you know?
- One second is exactly 9,192,631,770 oscillations of radiation emitted by the caesium-133 atom.
- In 1987, in Goiânia, Brazil, scrap dealers opened an abandoned radiotherapy source containing caesium-137; fascinated by the glowing blue powder, they shared it around, causing four deaths.
- Caesium, gold and copper are the only pure metals with a distinct color.
Isotopes of caesium
A single stable isotope, caesium-133, whose vibrations define the second in atomic clocks. Caesium-137 is the main long-lasting contamination after Chernobyl and Fukushima.
- caesium-131: radioactive, half-life 9.69 days. Brachytherapy seeds for certain cancers.
- caesium-133: 100% of natural caesium, stable. Its atoms beat time in atomic clocks and define the second.
- caesium-134: radioactive, half-life 2.07 years. Fallout from nuclear accidents, shorter-lived than caesium-137.
- caesium-135: radioactive, half-life 1.33 × 10⁶ years. Long-lived fission product in nuclear waste.
- caesium-137: radioactive, half-life 30.1 years. A major soil contaminant after Chernobyl; also used for sterilization and industrial gauges.
Related elements
Same family (alkali metals): Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Francium (Fr).
Same column (group 1): Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Francium (Fr).