Chlorine(Cl)
Chlorine is a reactive halogen gas. Its chemistry is dominated by the -1 oxidation state.
KEY PROPERTIES
Electron configuration & shells
[Ne] 3s² 3p⁵
Shell model is schematic — see lesson on atomic structure for orbitals. Oxidation states: -1, +1, +3, +5, +7. Common ions: Cl⁻.
Measured properties
Values at stated conditions; missing data shown as “—”. Radii distinguished by type; densities at stated phase/temperature.
- Electronegativity (Pauling)
- 3.16
- First ionization energy
- 1251 kJ·mol⁻¹ (first ionization, gas phase)
- Atomic radius
- 79 pm (calculated)
- Covalent radius
- 102 pm (covalent)
- Van der Waals radius
- 175 pm (van der Waals)
- Density
- 0.003214 g·cm⁻³ (gas at STP)
- Melting point
- 171.6 K (1 atm)
- Boiling point
- 239.11 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 |
|---|---|---|---|
| 35 | 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.
- Water treatment
- PVC
- Hydrochloric acid
BIOLOGICAL ROLE
Biologically relevant — see uses/occurrence for context.
Discovery
Unknown — —
Safety
Compounds featuring Cl
Reactions involving Cl
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