Chlorine
Atomic Data
| Atomic Number | 17 |
| Symbol | Cl |
| Atomic Weight | 35.45 u |
| Density (STP) | 3.214 g/L |
| Melting Point | −101.5 °C (171.65 K) |
| Boiling Point | −34.04 °C (239.11 K) |
| Electronegativity | 3.16 (Pauling) |
| Electron Config. | 1s2 2s2 2p6 3s2 3p5 |
| Oxidation States | −1, +1, +3, +5, +7 |
| Phase at STP | Gas |
| Category | Halogen |
| Period / Group | 3 / 17 |
| CAS Number | 7782-50-5 |
Electron Configuration
[Ne] 3s2 3p5
| Shell | n | Subshell | Electrons | Cumulative |
|---|---|---|---|---|
| K | 1 | 1s | 2 | 2 |
| L | 2 | 2s | 2 | 4 |
| L | 2 | 2p | 6 | 10 |
| M | 3 | 3s | 2 | 12 |
| M | 3 | 3p | 5 | 17 |
| Total | 17 | 17 | ||
Isotopes of Chlorine
Chlorine has two naturally occurring stable isotopes. The most abundant is ³⁵Cl, comprising 75.76% of all naturally occurring Chlorine.
| Isotope | Symbol | Protons | Neutrons | Abundance | Stability |
|---|---|---|---|---|---|
| Chlorine-35 | ³⁵Cl | 17 | 18 | 75.76 | Stable |
| Chlorine-37 | ³⁷Cl | 17 | 20 | 24.24 | Stable |
Abundance & Occurrence
Chlorine is present in Earth's crust at approximately 145 ppm by mass and at approximately 210 ppm by mass throughout the universe.
Earth's Crust (ppm by mass)
Universe (ppm by mass)
Discovery & History
Read more about the discovery of the periodic table of elements →
Safety & Handling
- Inhalation toxicity: Chlorine gas is severely toxic by inhalation; concentrations above 1 ppm cause throat irritation, above 3 ppm cause coughing, and above 25 ppm are immediately dangerous to life — it was used as a chemical weapon in World War I.
- Pulmonary damage: Chlorine reacts with moisture in the lungs to form hydrochloric and hypochlorous acid, causing chemical pneumonia and pulmonary oedema that may be delayed; individuals exposed to chlorine gas must be monitored for 24–48 hours.
- Incompatibilities: Never mix chlorine bleach (sodium hypochlorite) with ammonia or acids — both reactions generate toxic chlorine gas or chloramine; a common cause of domestic poisoning accidents.
- Skin and eye contact: Chlorine gas or concentrated chlorine solutions cause chemical burns; eyes are particularly sensitive and must be flushed immediately with copious water for at least 15 minutes.
- Storage and transport: Chlorine cylinders must be stored in dedicated, ventilated locations away from flammables and reducing agents; chlorine-specific gas detectors and emergency chlorine self-contained breathing apparatus (SCBA) must be available on site.
Chlorine in the Real World
Real-World Uses
- Water disinfection — Chlorine gas and hypochlorite salts kill bacteria, viruses, and protozoa in municipal drinking water and swimming pools, representing one of the most impactful public health interventions in history.
- PVC plastics — Polyvinyl chloride (PVC) is one of the world's most produced plastics, used in pipes, window profiles, flooring, electrical cable insulation, medical tubing, and packaging films.
- Bleaching agents — Sodium hypochlorite (household bleach) and chlorine-based compounds whiten paper pulp and textiles, and disinfect surfaces in food processing facilities and healthcare settings.
- Pharmaceuticals — Chlorine atoms appear in many drugs including the antihistamine loratadine, the antibiotic amoxicillin, and the antidepressant sertraline; chlorination frequently improves a drug's lipophilicity and metabolic stability.
- Agrochemicals — Chlorine-containing pesticides (organochlorines such as lindane) and herbicides have been widely used, though many legacy compounds have been banned due to persistence in the environment.
- Solvents and refrigerants — Chlorinated solvents such as dichloromethane and trichloroethylene are used in metal degreasing, paint stripping, and pharmaceutical extraction; chlorofluorocarbons (now banned) were formerly the dominant refrigerants.
Downloadable Resources
Free periodic table reference sheets for classrooms, study sessions, and laboratory use.
Frequently Asked Questions
What is chlorine used for?
Chlorine is used to disinfect drinking water and swimming pools, to manufacture PVC plastic (polyvinyl chloride), as bleach (sodium hypochlorite) in household cleaning and paper production, in pharmaceutical synthesis, as a precursor to many agrochemicals and solvents, and in the production of hydrochloric acid for industrial metal cleaning and chemical processing.
Is chlorine dangerous?
Chlorine gas is highly toxic. At low concentrations it causes eye, nose, and throat irritation; at higher concentrations it causes pulmonary oedema (fluid in the lungs) and can be fatal. Chlorine gas was used as a chemical weapon in World War I. However, the trace amounts of chlorine used to disinfect drinking water (measured in parts per million) are safe for human consumption and prevent diseases such as cholera, typhoid fever, and dysentery.
How does chlorine kill bacteria in water?
Chlorine kills micro-organisms primarily through the action of hypochlorous acid (HOCl), formed when chlorine dissolves in water. HOCl is a small, uncharged molecule that penetrates bacterial cell membranes and reacts with enzymes, proteins, and DNA, disrupting metabolism and causing cell death. The effectiveness of disinfection depends on the pH (HOCl is more active at lower pH) and the contact time, which is why water treatment plants maintain a residual chlorine level throughout the distribution system.
What is PVC and why does it require chlorine?
PVC (polyvinyl chloride) is one of the world's most produced plastics, used in pipes, window profiles, electrical cable insulation, flooring, and medical tubing. It is produced by polymerising vinyl chloride monomer (CH₂=CHCl), which contains chlorine. The chlorine atoms in the PVC polymer chain are what give the material its inherent flame retardancy (chlorine suppresses combustion), chemical resistance, and electrical insulating properties that make PVC so versatile.