Caesium(Cs)
Caesium (Cs) is a alkali metal in period 6 of the periodic table. Its properties reflect its electron configuration and position among neighbouring elements.
KEY PROPERTIES
Electron configuration & shells
[Xe] 6s¹
Shell model is schematic — see lesson on atomic structure for orbitals. Oxidation states: +1. Common ions: Cs⁺.
Measured properties
Values at stated conditions; missing data shown as “—”. Radii distinguished by type; densities at stated phase/temperature.
- Electronegativity (Pauling)
- 0.79
- First ionization energy
- 376 kJ·mol⁻¹ (first ionization, gas phase)
- Atomic radius
- 298 pm (calculated)
- Covalent radius
- 244 pm (covalent)
- Van der Waals radius
- 343 pm (van der Waals)
- Density
- 1.873 g·cm⁻³ (solid at 20 °C, 1 atm)
- Melting point
- 301.59 K (1 atm)
- Boiling point
- 944 K (1 atm)
Standard conditions: 1 atm unless noted. Values with null are not established or not comparable across allotropes. Radii are distinguished by type. Densities at stated phase/temperature.
| Mass number | Abundance | Stability | Half-life |
|---|---|---|---|
| 133 | trace/synthetic | stable | — |
Isotopic abundances are natural terrestrial values where applicable; synthetic isotopes shown as trace.
Occurrence & uses
Occurs naturally; abundance and extraction depend on geochemistry. For heavy synthetics (Z > 92), no natural occurrence.
- Industrial use
- Laboratory reagent
- Research
Discovery
Unknown — —
Safety
RELATED CONTENT
SOURCES
- [nist-atomic-weights]nist-atomic-weights (2022)/#nist-atomic-weights
- [nist-2022]nist-2022 (2022)/#nist-2022
- [iupac-gold]iupac-gold (2022)/#iupac-gold
Last reviewed: 2026-08-22 • Reviewer: dr-example