Chlorine
Chlorine is a reactive yellow-green halogen gas; chloride ions in salt are chemically different from the gas.
Chlorine helps make clean water possible when used in controlled treatment systems.
Explore Chlorine's atom- Atomic number
- 17
- Standard atomic weight
- [35.446, 35.457]
- Family
- Halogen
- Period
- 3
- Group
- 17
- Standard state
- Gas
In everyday life
Chlorine compounds are used in water treatment and PVC plastics.
Something to remember
Chlorine is a halogen, but chloride ions in salt behave very differently from chlorine gas.
Meet its electrons
[Ne] 3s2 3p5
Published neutral-atom ground-state configuration; excited atoms and ions can differ.
Electrons per shell: 2 · 8 · 7.
The playground shows an illustrative shell model. Real electrons are described by quantum orbitals, rather than tiny planets traveling along fixed paths.
A connection to explore
Chlorine and sodium form table salt, whose ions behave differently from the pure elements.
Meet Sodium (Na)Properties and conditions
- Standard atomic weight
- [35.446, 35.457]CIAAW 2024 standard atomic weight (dimensionless). The interval reflects variation in normal terrestrial isotope compositions, not measurement uncertainty. Source flags: m.CIAAW standard atomic weights ↗ · Retrieved 2026-10-15
- Standard state
- GasPubChem classification, including explicit predictions where stated. Reference temperature and pressure are not supplied in the table.PubChem periodic table ↗ · Retrieved 2026-10-15
- First ionisation energy
- 12.968 eVEnergy to remove the first electron from an isolated neutral gas-phase atom. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electronegativity
- 3.16 Pauling scaleA dimensionless relative measure; values depend on the chosen scale. Uncertainty is not supplied.PubChem periodic table ↗ · Retrieved 2026-10-15
- Melting point
- 171.65 KPubChem reported value. Pressure, allotrope and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Boiling point
- 239.11 KPubChem reported value. Pressure and measurement uncertainty are not specified in the table; consult the cited source.PubChem periodic table ↗ · Retrieved 2026-10-15
- Density
- 0.003214 g/cm³PubChem reported value. Temperature, pressure, allotrope and measurement uncertainty are not specified in the table; gas densities are especially condition-dependent.PubChem periodic table ↗ · Retrieved 2026-10-15
- Oxidation states
- +7, +5, +1, -1Selected PubChem oxidation states; formal electron bookkeeping, not a list of stable free-ion charges or every possible compound.PubChem periodic table ↗ · Retrieved 2026-10-15
- Discovery
- 1774PubChem discovery attribution. Ancient knowledge and competing priority claims cannot always be represented by a single year.PubChem periodic table ↗ · Retrieved 2026-10-15
- Atomic radius
- 175 pmPubChem tabulated radius. Radius definitions depend on bonding and convention; measurement definition and uncertainty are not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
- Electron affinity
- 3.617 eVPubChem reported gas-phase electron affinity. Sign conventions and electronic states matter; uncertainty is not supplied in this table.PubChem periodic table ↗ · Retrieved 2026-10-15
Isotopes, abundance and decay
Isotopes share a proton count but have different neutron counts. This selection includes naturally tabulated compositions and a reference isotope for other elements. It is not a list of every known isotope.
Explore 2 isotopes of Cl
| Isotope | Relative atomic mass | Natural fraction | Half-life | Decay |
|---|---|---|---|---|
| Cl-35 | 34.968852682(37) | 0.7576(10) | No decay observed | No decay tabulated |
| Cl-37 | 36.965902602(55) | 0.2424(10) | No decay observed | No decay tabulated |
Fractions use NIST’s terrestrial isotopic compositions, which can vary by sample. Parentheses denote uncertainty in the last digits. Half-lives and decay branches use NUBASE2020; # marks estimates, >/< mark limits. “No decay observed” follows that evaluation and does not prove permanent stability. Decay branch intensities are percentages; SF means spontaneous fission, EC electron capture, IT an isomeric transition.
Follow the science
Use these original references to explore the data and context. Atomic weights can vary with isotope composition; values for elements without stable isotopes may describe a representative isotope rather than a standard atomic weight.