Element: Chlorine
General data
| Element name | Chlorine |
|---|---|
| Chemical symbol | Cl |
| Atomic number Z | 17 |
| Year discovered | 1774 |
| Standard state (25°C) | Gas |
| Category | Halogen |
| Block | p |
| Atomic mass | 35.45 |
| CPK color | #1FF01F |
Physical properties
| Melting point | 171.65 K (-101.50 °C) |
|---|---|
| Boiling point | 239.11 K (-34.04 °C) |
| Density | 0.003214 g/cm³ |
Energy properties
| First ionization energy | 12.968 eV |
|---|---|
| Electron affinity | 3.617 eV |
Atomic structure
| Protons | 17 (Atomic number) |
|---|---|
| Neutrons (Cl-35, most abundant isotope) | 18 |
| Electrons | 17 |
| Electron configuration | [Ne] 3s² 3p⁵ |
| Oxidation states | +7, +5, +1, -1 |
| Electronegativity (Pauling) | 3.16 |
| Atomic radius (van der Waals) | 175 pm |
Ionization energies - NIST ASD (17 states)
| State | Charge | Ioniz. energy (eV) | Type |
|---|---|---|---|
| Cl I | 0+ | 12.967633 ± 0.000016 | measured |
| Cl II | 1+ | 23.81364 ± 0.00012 | measured |
| Cl III | 2+ | 39.8 ± 0.11 | semi-empirical |
| Cl IV | 3+ | 53.24 ± 0.12 | semi-empirical |
| Cl V | 4+ | 67.68 ± 0.1 | semi-empirical |
| Cl VI | 5+ | 96.94 ± 0.04 | semi-empirical |
| Cl VII | 6+ | 114.2013 ± 0.0006 | measured |
| Cl VIII | 7+ | 348.306 ± 0.007 | measured |
| Cl IX | 8+ | 400.851 ± 0.012 | measured |
| Cl X | 9+ | 456.7 ± 0.4 | semi-empirical |
| Cl XI | 10+ | 530 ± 0.4 | semi-empirical |
| Cl XII | 11+ | 591.58 ± 0.19 | semi-empirical |
| Cl XIII | 12+ | 656.3 ± 0.12 | semi-empirical |
| Cl XIV | 13+ | 750.23 ± 0.19 | semi-empirical |
| Cl XV | 14+ | 809.198 ± 0.007 | calculated |
| Cl XVI | 15+ | 3658.34366 ± 0.00022 | calculated |
| Cl XVII | 16+ | 3946.29179 ± 0.00007 | calculated |
Value type as given by NIST ASD: measured - from evaluated experimental data; calculated - from ab initio calculations (mostly highly ionised atoms that have not been measured; for the simplest ones, e.g. H I, the calculation is more accurate than measurement); semi-empirical - from interpolation, extrapolation or another semi-empirical method.
Photos
Isotopes (25)
| A | N | Half-life | Decay |
|---|---|---|---|
| Cl-28 | 11 | unknown | p |
| Cl-29 | 12 | < 20 ns | p |
| Cl-30 | 13 | < 30 ns | p |
| Cl-31 | 14 | 190 ms | EC/β⁺ |
| Cl-32 | 15 | 299.1 ms | EC/β⁺ |
| Cl-33 | 16 | 2.5060 s | EC/β⁺ |
| Cl-34 | 17 | 1.5265 s | EC/β⁺ |
| Cl-35 | 18 | stable | - |
| Cl-36 | 19 | 3.013×10⁵ a | β⁻ |
| Cl-37 | 20 | stable | - |
| Cl-38 | 21 | 37.236 min | β⁻ |
| Cl-39 | 22 | 56.2 min | β⁻ |
| Cl-40 | 23 | 1.33 min | β⁻ |
| Cl-41 | 24 | 38.4 s | β⁻ |
| Cl-42 | 25 | 6.8 s | β⁻ |
| Cl-43 | 26 | 3.13 s | β⁻ |
| Cl-44 | 27 | 540 ms | β⁻ |
| Cl-45 | 28 | 410 ms | β⁻ |
| Cl-46 | 29 | 232 ms | β⁻ |
| Cl-47 | 30 | 101 ms | β⁻ |
| Cl-48 | 31 | unknown | β⁻ |
| Cl-49 | 32 | unknown | β⁻ |
| Cl-50 | 33 | unknown | β⁻ |
| Cl-51 | 34 | unknown | β⁻ |
| Cl-52 | 35 | unknown | β⁻ |
Data sources
- PubChem Periodic Table API - Retrieved on 29 June 2026 - licence: CC0
- IUPAC CIAAW - Abridged Standard Atomic Weights 2024 - Table version: 2024 - licence: Public domain
- NNDC Wallet Cards 2023 - Data as of 15 October 2024 - licence: Public domain
- NIST Atomic Spectra Database (ASD) - Retrieved on 29 June 2026 - licence: © U.S. Secretary of Commerce
- Images of Elements - Retrieved on 29 June 2026 - licence: CC BY 3.0