Chlorine is the chemical element with symbol Cl and atomic number 17. At room temperature, elemental chlorine exists as Cl₂, a dense yellow-green gas with a sharp, choking odour. Chlorine is a highly reactive halogen in group 17 of the periodic table.
Elemental chlorine is toxic, but chlorine chemistry is indispensable to modern society. Chloride ions are essential in biology, while chlorine compounds are used to disinfect drinking water, manufacture plastics such as PVC, produce medicines and chemicals, and bleach or sanitise materials.
Chlorine quick facts
| Property | Value |
| Symbol | Cl |
| Atomic number | 17 |
| Relative atomic mass | 35.45 |
| Group | 17 |
| Period | 3 |
| Block | p-block |
| Classification | Halogen / nonmetal |
| State at 20°C | Gas |
| Appearance | Yellow-green gas |
| Density | 0.002898 g/cm³ |
| Melting point | −101.5°C |
| Boiling point | −34.04°C |
| Electron configuration | [Ne] 3s² 3p⁵ |
| CAS number | 7782-50-5 |
| Stable natural isotopes | ³⁵Cl, ³⁷Cl |
Where is chlorine on the periodic table?
Chlorine is in group 17, period 3 and the p-block. Its electron configuration is [Ne] 3s² 3p⁵. With seven valence electrons, chlorine readily gains or shares one electron, which explains the prevalence of chloride compounds and chlorine’s strong oxidising chemistry.
See the complete CleverlySmart periodic table. Chlorine lies below fluorine in the halogen group.
Physical properties of chlorine
Chlorine gas consists of diatomic Cl₂ molecules. It is heavier than air and can be condensed to a yellowish liquid at low temperature or elevated pressure. Its melting point is about −101.5°C and its boiling point about −34.04°C.
The characteristic yellow-green colour gives chlorine its name: it derives from the Greek chloros, meaning greenish-yellow.
Chemical properties
Chlorine is a strong oxidising agent and reacts with many elements. The most common oxidation state in simple chloride salts is −1, although chlorine also forms compounds in positive oxidation states, especially with oxygen and fluorine.
Chlorine reacts with hydrogen to form hydrogen chloride:
H₂ + Cl₂ → 2HCl
Hydrogen chloride dissolves readily in water to form hydrochloric acid.
Important chlorine compounds
- Sodium chloride (NaCl): common salt and one of the most abundant chlorine compounds.
- Hydrochloric acid (HCl in water): an important industrial acid and a component of gastric acid.
- Hypochlorites: compounds such as sodium hypochlorite are widely used for bleaching and disinfection.
- Chlorates and perchlorates: powerful oxidising salts with specialised industrial, laboratory and energetic-material uses.
- Polyvinyl chloride (PVC): a major chlorine-containing plastic used in pipes, cable insulation, flooring and many building products.
How chlorine is produced
Most industrial chlorine is produced by the chlor-alkali process, in which an aqueous sodium chloride solution (brine) is electrolysed. The process produces chlorine gas together with hydrogen and sodium hydroxide, making it one of the fundamental processes of the chemical industry.
Uses of chlorine
- Drinking-water treatment: chlorine and hypochlorite compounds kill many disease-causing microorganisms.
- Swimming-pool sanitation: chlorine chemistry helps control bacteria and algae.
- PVC manufacture: chlorine is a major component of vinyl chloride, the monomer used to make PVC.
- Chemical synthesis: chlorine-containing intermediates are used in pharmaceuticals, solvents, agrochemicals, dyes and many other products.
- Bleaching and cleaning: hypochlorite solutions are widely used as household and industrial disinfectants.
Chlorine in biology
The biology of chlorine is mainly the biology of the chloride ion (Cl⁻), not elemental chlorine gas. Chloride is an essential electrolyte involved in fluid balance, electrical neutrality, nerve and muscle function, and gastric hydrochloric acid production.
Chlorine isotopes
Natural chlorine consists mainly of two stable isotopes, ³⁵Cl and ³⁷Cl. Their natural mixture gives chlorine its standard atomic weight of about 35.45. Chlorine isotope ratios can also be useful in geochemistry and environmental studies.
Radioactive chlorine isotopes also exist. For a wider reference, see the CleverlySmart complete isotope table and guide to nuclides.
Safety
Chlorine gas is highly toxic and corrosive. Inhalation can damage the eyes and respiratory tract and can cause severe lung injury at sufficiently high exposure. Chlorine must be handled only with suitable engineering controls, ventilation and protective equipment.
Household cleaning products containing hypochlorite should never be mixed with acids or ammonia-containing cleaners, because dangerous gases can be released.
History
Swedish chemist Carl Wilhelm Scheele produced chlorine gas in 1774, although he did not recognise it as a new element. In 1810, Humphry Davy demonstrated that chlorine was an element and gave it its modern name.
Frequently asked questions
Is chlorine the same as chloride?
No. Chlorine commonly refers to elemental Cl₂ gas, while chloride is the negatively charged Cl⁻ ion found in salts such as sodium chloride.
Why is chlorine used in drinking water?
At carefully controlled concentrations, chlorine-based disinfectants can destroy many pathogens and help maintain a disinfectant residual through parts of a water-distribution system.
What is chlorine’s electron configuration?
[Ne] 3s² 3p⁵.
Chlor-alkali chemistry and industrial production
Most chlorine is made by electrolysis of concentrated sodium chloride solution in the chlor-alkali process. The same process also produces hydrogen and sodium hydroxide. Chlorine is then used to manufacture PVC, solvents, disinfectants, inorganic chlorides and a wide range of intermediates for pharmaceuticals and industrial chemicals.
Disinfection chemistry and public health
When chlorine dissolves in water, it forms species including hypochlorous acid (HOCl), a strong disinfectant that damages microbial proteins and cell components. Chlorination has played a major role in making drinking water safer, although treatment must be carefully controlled to limit unwanted disinfection by-products.
Chloride ions are also essential in biology. They help regulate fluid balance, nerve function and stomach acidity. The hazards of elemental chlorine should therefore not be confused with the normal biological role of chloride salts.
Frequently asked questions
Why is chlorine gas dangerous?
Chlorine is a powerful oxidising, corrosive gas that reacts with moist tissues in the eyes and respiratory tract. Significant exposure can cause severe lung injury.
Can household bleach be mixed with other cleaners?
No. Mixing hypochlorite bleach with acids can release chlorine gas, while mixing it with ammonia can form hazardous chloramine gases.
