Chlorine
Atomic Data
| Atomic Number | 17 |
| Symbol | Cl |
| Atomic Weight | 35.45 u |
| Density (STP) | 3.214 g/L |
| Melting Point | −101.50 °C (171.65 K) |
| Boiling Point | −34.04 °C (239.11 K) |
| Electronegativity | 3.16 (Pauling) |
| Electron Config. | 1s² 2s² 2p&sup6; 3s² 3p&sup5; |
| Oxidation States | −1, +1, +3, +5, +7 |
| Phase at STP | Gas |
| Category | Halogen |
| Period / Group | 3 / 17 |
| CAS Number | 7782-50-5 |
Electron Configuration
1s2 2s2 2p6 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 stable naturally occurring isotopes. Neither is radioactive, and their unequal proportions explain why chlorine’s standard atomic weight (35.45 u) falls between two whole numbers.
| 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 not found in its elemental form in nature because it reacts readily with most substances. It occurs almost entirely as chloride ions, most conspicuously as sodium chloride dissolved in seawater, and in evaporite mineral deposits such as halite and sylvite. Chlorine constitutes about 145 ppm of Earth’s crust and roughly 1.9% of seawater by mass, making seawater its most accessible natural reservoir.
Seawater Dissolved Ions (By Mass)
Earth’s Crust Composition (ppm by mass, selected)
Discovery & History
Safety & Handling
- Toxic gas: chlorine is a respiratory irritant at concentrations as low as 1 ppm. Exposure above 10 ppm causes severe pulmonary damage, and concentrations above 25 ppm are immediately dangerous to life and health (IDLH).
- Chemical weapon history: chlorine was used as a choking agent in World War I and remains prohibited as a weapon under the Chemical Weapons Convention.
- Storage: store in cool, well-ventilated areas in leak-proof steel cylinders, kept upright and secured, away from flammable materials, reducing agents, and ammonia compounds.
- PPE requirements: handle only with a supplied-air respirator or SCBA, chemical-resistant gloves, and full face shield. Never work alone when handling chlorine gas cylinders.
- Hydrogen chloride (HCl): a common chlorine compound that forms a corrosive, fuming acid in moist air. Causes severe burns to skin and mucous membranes; treat exposure by prolonged flushing with water.
- Bleach mixing hazard: mixing household bleach (sodium hypochlorite) with acidic cleaners or ammonia-based products releases chlorine gas and chloramine vapours — a common cause of domestic chemical accidents requiring immediate ventilation.
Chlorine in the Real World
Real-World Uses
- Water disinfection — Chlorine kills bacteria, viruses, and other pathogens in municipal drinking water and swimming pools. It remains the most widely used and cost-effective water treatment method, protecting billions of people from waterborne diseases including cholera and typhoid.
- PVC plastic production — Approximately 35% of all chlorine produced globally goes into manufacturing polyvinyl chloride (PVC), one of the world’s most widely used plastics. PVC is found in pipes, window profiles, flooring, and electrical cable insulation.
- Bleaching agents — Chlorine-based compounds such as sodium hypochlorite and chlorine dioxide are used to bleach paper pulp and textiles, and to sanitise food-processing equipment and surfaces in hospitals.
- Pharmaceutical synthesis — Around 85% of pharmaceutical drugs involve chlorine chemistry at some stage in their manufacture, including the synthesis of antiretrovirals, antibiotics, and anaesthetics.
- Solvents — Chlorinated solvents such as dichloromethane and trichloroethylene serve as industrial degreasers, paint strippers, and extraction solvents in chemical and pharmaceutical manufacturing.
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 primarily to disinfect drinking water and swimming pools, to manufacture PVC plastics, as a bleaching agent in paper and textile production, and in pharmaceutical synthesis. It is one of the most commercially important chemicals in the world.
Is chlorine dangerous to humans?
Chlorine gas is toxic and acts as a respiratory irritant. Concentrations above 1 ppm cause throat and eye irritation; above 10 ppm it poses serious pulmonary risk. It was deployed as a chemical weapon in World War I. At the trace levels used in water treatment, however, it is safe and essential for public health.
Who discovered chlorine?
Chlorine was first isolated in 1774 by Swedish chemist Carl Wilhelm Scheele, who produced it by reacting manganese dioxide with hydrochloric acid. He initially believed it to be an oxygen compound. Sir Humphry Davy established in 1810 that it was a distinct element and gave it the name chlorine, from the Greek chloros meaning pale green.
What is the difference between chlorine and chloride?
Chlorine (Cl₂) is the diatomic elemental form — a toxic yellow-green gas. Chloride (Cl−) is the negatively charged ion formed when chlorine gains one electron. Chloride ions are found in common table salt (sodium chloride, NaCl), seawater, and biological fluids, where they are essential and non-toxic.
How many isotopes does chlorine have?
Chlorine has two stable naturally occurring isotopes: chlorine-35, which makes up about 75.76% of all natural chlorine, and chlorine-37, which accounts for the remaining 24.24%. This two-isotope mix is why the standard atomic weight of chlorine (35.45 u) is not a whole number.