Cesium
alkali metal · period 6 · group 1 (alkali metals (and hydrogen)) · s-block
Caesium (IUPAC spelling) (also spelled cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of 28.5 °C (83.3 °F), which makes it one of only five elemental metals that are liquid at or near room temperature.
What the numbers say
At standard temperature and pressure Cesium is a solid, sitting in period 6, group 1 of the s-block.
Its melting point of 302 K (29 °C) is the 95th highest of the 110 elements with a measured value — mid-range among the elements.
At 1.879 g/cm³ it is the 77th densest of 96 elements with a quoted density.
Pauling electronegativity is 0.79. Paired with oxygen (3.44) that is a difference of 2.65, which puts the bond predominantly ionic — expect a stable oxide ceramic.
In the solid state it takes a body-centered cubic structure — BCC packing is stronger but shows a ductile-to-brittle transition as temperature drops.
It is reported in 2 oxidation states (-1, 1), which sets how many distinct compounds and phases it can form.
First identified in 1860.
Crystal structure
Solid Cesium adopts a Body-centered Cubic arrangement, in the Cubic (isometric) system — axes a = b = c, angles α = β = γ = 90°. Symmetry controls the anisotropy of stiffness and thermal expansion, and the available slip systems set how far the metal deforms before it fractures.
Check it against computed structures
The structure search queries the Materials Project, OQMD and the Open Materials Database for computed crystal structures containing Cs — with DFT formation energy, hull stability and band gap where the provider publishes them.
K 2 · L 8 · M 18 · N 18 · O 8 · P 1
Properties
| Atomic number | 55 |
|---|---|
| Atomic mass | 132.905451966 u |
| Series | alkali metal |
| Group | 1 |
| Period | 6 |
| Block | s-block |
| Phase at STP | Solid |
| Density | 1.879 g/cm³ |
| Melting point | 301.7 K (29 °C) |
| Boiling point | 944 K (671 °C) |
| Electronegativity (Pauling) | 0.79 |
| Electron configuration | [Xe] 6s¹ |
| Crystal structure | Body-centered Cubic |
| Oxidation states | -1,1 |
| Discovered | 1860 |
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See Cs in context with all 118 elements.
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