Periodic table › Chlorine
1. Identity and classification
Colored, pungent, reactive diatomic nonmetal (gas, volatile liquid, or crystalline solid)
- Name and symbol
- Chlorine (Chlorine) · Cl
- Atomic number
- 17
- Relative atomic weight / mass number
- 35.45 ± 0.01
- Group · period · block
- 17 · 3 · p
- Category
- Halogen
- State near room temperature
- Gas
- Stable isotopes
- At least one stable isotope
- CAS number
- 7782-50-5
Square brackets [A] indicate a representative isotope mass number, not an average atomic weight. Neutron counts differ between isotopes.
2. Atomic structure and electron configuration
- Protons · electrons in a neutral atom
- 17 · 17
- Neutrons in reference isotope
- 18 (35Cl)
- Full electron configuration
1s² 2s² 2p⁶ 3s² 3p⁵- Abbreviated configuration
[Ne]3s² 3p⁵- Electrons per shell
- 2, 8, 7
- Listed oxidation states
- +7, +5, +1, -1
- Electronegativity
- 3.16 (Pauling)
- First ionization energy
- 1251.222 kJ/mol · 12.968 eV
- Electron affinity
- 348.987 kJ/mol
- Covalent radius
- 102 pm
- van der Waals radius
- 175 pm
Configurations of superheavy elements may include theoretical predictions. Different definitions of atomic radius are not interchangeable.
3. Physical and thermal properties
- Density
- 0.003214 g/cm³
- Melting point
- 171.65 K · -101.5 °C · -150.7 °F
- Boiling point
- 239.11 K · -34.04 °C · -29.272 °F
- Crystal structure
- Orthorhombic
- Specific heat
- 0.479 J/(g·K)
- Thermal conductivity
- 0.0089 W/(m·K)
- Enthalpy of fusion
- 6.406 kJ/mol
- Enthalpy of vaporization
- 20.41 kJ/mol
- Magnetic ordering
- Diamagnetic
- Mohs hardness
- Not established
- Speed of sound
- 206 m/s
Density and structure depend on temperature, pressure and allotrope. 25°C is a room-temperature reference, not STP. Unestablished values are not zero.
4. Chemical behavior and compounds
The properties of Cl₂ gas differ greatly from those of chloride salts.
Selected compounds: ClO / Cl₂O₃ (Oxide) · ClClₙ (Chloride) · ClFₙ (Fluoride)
Reaction reference
Chlorine is an oxidizing halogen gas.
Chlorine reacts reversibly with water to give hydrochloric and hypochlorous acids.
5. Isotopes
| Isotope | Status | Natural abundance | Half-life |
|---|---|---|---|
| 35Cl | Stable | 75.80% | Stable |
| 36Cl | Radioactive | — | 3.013E+5 Y |
| 37Cl | Stable | 24.20% | Stable |
Half-life units: s seconds, m minutes, h hours, d days, Y years. A dash means no natural-abundance value is given, not zero. Listed records are selected ground states; metastable states are distinct.
6. Discovery and occurrence
- Discovery
- 1774 CE
- Scientists associated with discovery
- Carl Wilhelm Scheele
- Location
- Sweden (Uppsala)
- Origin
- Natural
- Listed crustal abundance
- Not established
Minerals and naming reference
Halite (NaCl), Sylvite (KCl), Carnallite, Seawater brines
Named from scientific/historical roots associated with Chlorine (Cl), discovered or characterized by Carl Wilhelm Scheele.
7. Applications
Municipal drinking water disinfection, PVC (polyvinyl chloride) pipes, hydrochloric acid, stomach acid (HCl) digestion
Detailed applications by field
- everyday
- Municipal drinking water disinfection, PVC (polyvinyl chloride) pipes, hydrochloric acid, stomach acid (HCl) digestion
- industry
- Used in halogen processing, specialized alloys, and chemical manufacturing (Municipal drinking water disinfection, PVC (polyvinyl chloride) pipes, hydrochloric acid, stomach acid (HCl) digestion).
- electronics
- Applied in electronic components, semiconductors, or functional coatings where Chlorine's electronic structure ([Ne] 3s² 3p⁵) provides target conductivity.
- medicine
- Diagnostic imaging, biomedical instrumentation, or pharmaceutical synthesis research.
- energy
- Utilized in power generation, battery materials, catalysts, or nuclear technology involving Chlorine.
- aerospace
- High-performance materials, structural alloys, or space instrumentation utilizing Chlorine.
- agriculture
- Soil geochemistry analysis, trace micronutrient research, or analytical environmental monitoring.
- science
- Spectroscopic standard, crystallography, and quantum chemistry studies of Period 3 p-block elements.
8. Biological role and safety
Hazard depends on the isotope, chemical form, dose and exposure route.
Elemental substances, ions, compounds and radioisotopes can have different health effects. This is an educational reference; laboratory work requires the material safety data sheet and trained supervision.
Biological role reference
Classified as essential in human and biological systems.