Caesium
[Xe] 6s¹ · 2 · 8 · 18 · 18 · 8 · 1
The most electropositive stable element and the softest metal: caesium melts in your hand at 28 °C. The SI second is defined by a transition frequency in the caesium-133 atom.
3D model
Bohr model: nucleus and electron shells from the real configuration. Valence electrons are highlighted.
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Atomic properties
- Atomic number
- 55
- Atomic mass
- 132.905 а.е.м.
- Electron configuration
- [Xe] 6s¹
- Electrons per shell
- 2 · 8 · 18 · 18 · 8 · 1
- Block
- s-block
- Group
- Alkali metals
- Period
- 6
- Electronegativity (Pauling)
- 0.79
- Electronegativity (Allen)
- 0.66
- Atomic radius
- 298 pm
- Covalent radius
- 244 pm
- Van der Waals radius
- 343 pm
- Ionisation energy 1
- 375.7 kJ/mol
- Ionisation energy 2
- 2 234.3 kJ/mol
- Ionisation energy 3
- 3 400 kJ/mol
- Electron affinity
- 45.5 kJ/mol
- Common oxidation states
- +1
- Oxidation states
- -1, +1
Physical properties
- State at 25 °C
- solid
- Density
- 1.93 g/cm³
- Melting point
- 301.7 K · 28.6 °C
- Boiling point
- 944 K · 670.9 °C
- Speed of sound
- no data
Thermal properties
- Heat of fusion
- 2.09 kJ/mol
- Heat of vaporisation
- 63.9 kJ/mol
- Specific heat capacity
- 0.242 J/(g·K)
- Thermal conductivity
- 35.9 W/(m·K)
Mechanical properties
- Young's modulus
- 1.7 GPa
- Shear modulus
- no data
- Bulk modulus
- 1.6 GPa
- Poisson ratio
- no data
- Mohs hardness
- 0.2
- Brinell hardness
- 0.14 MPa
Electrical and magnetic properties
- Electrical resistivity
- 205 nΩ·m
- Magnetic ordering
- paramagnetic
- Curie point
- no data
- Néel point
- no data
- Superconducting point
- no data
Crystal structure
- Crystal structure
- body-centred cubic (bcc)
- Lattice constants
- a = 614.1 pm
Abundance
- In the crust
- 3 mg/kg
- In the ocean
- 0.0003 mg/L
- In the universe
- 0.008 mg/kg
- In the human body
- 0.02 mg/kg
Isotopes
| Isotope | Abundance | Half-life | Decay mode |
|---|---|---|---|
| 133Cs | 100 % | stable | — |
| 137Cs | — | 30.07 yr | β− |
Caesium-133 is the only stable isotope and the basis of the second: 9 192 631 770 periods of the radiation from its hyperfine transition.
Discovery
- Year of discovery
- 1860
- Discovered by
- Robert Bunsen and Gustav Kirchhoff
- Where
- Germany
- Origin of the name
- from Latin caesius (sky blue), after two blue spectral lines
History
Bunsen and Kirchhoff discovered caesium in 1860 while studying the spectrum of Dürkheim mineral water — the first element ever found by spectral analysis. Two bright blue lines gave the name, from the Latin for sky blue.
Where it occurs
3 mg/kg of the crust. The only commercial mineral is pollucite, and virtually all world production comes from the single Tanco mine in Canada.
How it is produced
From pollucite via the chloride, then calcium reduction in vacuum. World output is measured in tonnes.
Role in living things
No biological role. The body treats caesium as potassium and distributes it through muscle.
Safety
The metal explodes on contact with water — even with ice at −116 °C. Caesium-137 from Chernobyl and Fukushima fallout is the principal long-lived soil contaminant.
Uses
- Atomic clocks: the caesium standard defines the second and disciplines GPS
- Caesium formate heavy drilling fluids — a brine denser than most metals
- Photocells and low-work-function cathodes
- Caesium-137 in industrial radiography and radiotherapy
Curiosities
- The SI second is defined by caesium: since 1967 time has been measured by an atom, not by the Earth's rotation.
- Caesium is the most electropositive element you can hold; francium is formally stronger but cannot be made in weighable amounts.
- Caesium-137, half-life 30 years, remains the central problem of the Chernobyl exclusion zone.
- A caesium atomic clock drifts by a second in roughly 30 million years.