Category: Halogen
Atomic Number: 17
Period: 3 | Group: 17
Block: p-block
A pale green gas that kills bacteria, bleaches fabric — and, bound to sodium, sits on your dinner table.
| Property | Value |
|---|---|
| Phase at STP | Gas |
| Density | 0.00321400 g/cm³ |
| Melting Point | 171.60 K (-101.55 °C) |
| Boiling Point | 239.11 K (-34.04 °C) |
| Heat Capacity | N/A |
| Property | Value |
|---|---|
| Atomic Mass | 35.4500 u |
| Electronegativity (Pauling) | 3.16 |
| Oxidation States | -1, +1, +3, +5, +7 |
| Electron Configuration | [Ne] 3s² 3p⁵ |
| Discovered By | Unknown |
|---|---|
| Discovery Year | 1774 |
| Named After | Greek chloros, "pale green" |
The Discovery and History of Chlorine
Early Chemical Investigations
The story of chlorine begins with Carl Wilhelm Scheele (1742-1786), a Swedish pharmacist working in Uppsala, Sweden. On a winter day in 1774, Scheele heated pyrolusite (manganese dioxide) with muriatic acid (hydrochloric acid) and observed a pungent, greenish-yellow gas that bleached flowers and burned his lungs. He called this mysterious substance "dephlogisticated muriatic acid," believing it was muriatic acid with its phlogiston removed.
Scheele recorded the gas's yellow-green colour, a smell like that of aqua regia, its bleaching of litmus and its deadly effect on insects. His reaction (MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂) had created the first recorded sample of pure chlorine gas.
The Elemental Controversy
Claude Louis Berthollet (1748-1822), the renowned French chemist, studied Scheele's gas extensively from 1785-1789. Initially supporting Scheele's compound theory, Berthollet first used the gas to bleach textiles (1785) and developed Javel water, a sodium hypochlorite bleaching liquid (1789). However, he remained convinced the gas contained oxygen and called it "oxygenated muriatic acid."
The controversy intensified when Antoine Lavoisier (1743-1794) insisted all acids must contain oxygen (from Greek "oxys genes" - acid producer). This created a 30-year scientific debate about chlorine's true nature.
Humphry Davy's Definitive Proof
The mystery was solved by Sir Humphry Davy (1778-1829) at the Royal Institution in London. Using his newly invented electrolysis apparatus, Davy attempted to decompose the green gas and obtain its supposed oxygen component. Despite months of experiments in 1807-1810, he failed to extract any oxygen.
On 15 November 1810, Davy presented his conclusion to the Royal Society: the gas was not a compound but an element. He named it "chlorine" from the Greek "chloros" (pale green), finally recognizing it as a fundamental element.
Davy's colleague Michael Faraday (1791-1867) provided crucial support by successfully liquefying chlorine in 1823, demonstrating its distinct physical properties and confirming its elemental nature.
Industrial Revolution and Bleaching Powder
Charles Tennant (1768-1838), a Scottish chemist and businessman, transformed the textile industry by developing bleaching powder in 1799. His factory in St. Rollox, Glasgow, became the world's largest chemical works, producing calcium hypochlorite [Ca(ClO)₂] by passing chlorine gas over slaked lime.
Tennant's innovation transformed textile manufacturing - traditional bleaching with buttermilk and sunlight required 6-8 months, while chlorine bleaching took just hours. By 1815, his company produced 9,000 tons annually, employing over 1,000 workers and generating enormous profits.
The Chloralkali Industry Birth
The modern chlorine industry began with Hamilton Young Castner (1858-1899) and Karl Kellner (1851-1905), who independently developed electrolytic processes for producing chlorine and sodium hydroxide from salt water. Castner and Kellner developed similar mercury cells independently in the early 1890s and joined forces in 1895.
Herbert Henry Dow (1866-1930) transformed American chlorine production by founding Dow Chemical Company in 1897. His innovative electrolytic cells used Midland, Michigan's underground brine deposits, making Dow the world's largest chlorine producer by 1920.
World War I and Chemical Warfare
Chlorine's dark chapter began on April 22, 1915, at the Second Battle of Ypres when German forces released 168 tons of chlorine gas, creating the first large-scale chemical weapon attack. Fritz Haber (1868-1934), later Nobel laureate for ammonia synthesis, directed this program despite his wife Clara's opposition and eventual suicide in protest.
The attack killed 5,000 Allied soldiers and wounded 10,000 more, opening a new era of chemical warfare. Both sides subsequently developed more sophisticated chemical weapons, though chlorine's visibility and crude delivery methods limited its effectiveness.
Public Health
Chlorine's greatest legacy emerged through water disinfection. John L. Leal (1858-1914) conducted the first continuous municipal water chlorination in Jersey City, New Jersey, in 1908. Despite legal challenges from residents fearing "chemical poisoning," Leal's system eliminated typhoid fever deaths within two years.
Abel Wolman (1892-1989) and Linn Enslow refined chlorination science, establishing dosage calculations and residual monitoring that became global standards. Their work enabled safe drinking water for billions and ranks among the greatest public health achievements of the 20th century.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Cl-35 | 18 | 75.80% | Stable | - |
| Cl-36 | 19 | - | 3.013 × 10⁵ yrs | β⁻ (98.1%), ε/β⁺ (1.9%) |
| Cl-37 | 20 | 24.20% | Stable | - |
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Industrial Applications of Chlorine
Water Treatment and Public Health
Chlorine transformed public health as the most widely used water disinfectant worldwide. Municipal water treatment employs three primary chlorination methods:
Free chlorination: Chlorine gas (Cl₂) or sodium hypochlorite (NaClO) creates hypochlorous acid (HOCl), the active disinfectant
Breakpoint chlorination: Eliminates ammonia and organic nitrogen compounds by adding chlorine until residual forms
Combined chlorination: Forms chloramines for longer-lasting disinfection in distribution systems
The World Health Organization recommends 0.2-0.5 mg/L residual chlorine for safe drinking water. Major suppliers include Olin Corporation , Occidental Chemical , and ERCO Worldwide , while world chlorine production is about 97 million tons a year (2022).
Chemical Manufacturing Hub
Chlorine serves as a building block for countless chemicals through chloralkali electrolysis :
Polyvinyl chloride (PVC): 40% of chlorine production creates PVC for pipes, siding, flooring
Methylene chloride (CH₂Cl₂): Paint strippers and pharmaceutical intermediates
Chloroform (CHCl₃): Refrigerant production and chemical synthesis
Carbon tetrachloride (CCl₄): Specialty cleaning applications (heavily regulated)
Phosgene (COCl₂): Polyurethane and polycarbonate plastics manufacturing
Pulp and Paper Industry
Chlorine dioxide (ClO₂) transformed paper bleaching, replacing elemental chlorine to reduce environmental impact. Kraft pulp mills use ECF (Elemental Chlorine Free) bleaching sequences like D₀-E₁-D₁-E₂, where D represents ClO₂ stages and E represents alkaline extraction. Companies like International Paper and Georgia-Pacific consume roughly 20-45 kg ClO₂ per ton of bleached pulp.
Swimming Pool and Spa Sanitization
Pool chlorination maintains 1-3 ppm free chlorine levels through various delivery systems:
Sodium hypochlorite (liquid bleach): 12.5% solutions for commercial pools
Calcium hypochlorite: 65-70% available chlorine in powder/tablet form
Trichlor tablets: Slow-dissolving 90% available chlorine with cyanuric acid stabilizer
Salt chlorine generators: Electrolytic cells convert salt (NaCl) to chlorine on-site
Pharmaceutical and Healthcare
Chlorine compounds are essential in pharmaceutical manufacturing:
Antibiotics: Chloramphenicol contains chlorine in its active structure
Anesthetics: Historical use of chloroform, modern halothane derivatives
Antiseptics: Sodium hypochlorite solutions (Dakin's solution) for wound care
Sterilization: Chlorine dioxide gas sterilizes medical equipment and pharmaceutical facilities
Agrochemicals and Pest Control
Organochlorine compounds historically dominated pesticide markets, though many are now restricted:
Currently used: 2,4-D herbicide, chlorpyrifos insecticide (restricted use)
Soil fumigation: Methyl bromide alternatives using chlorine-based compounds
Wood preservation: Copper-chrome-arsenic (CCA) alternatives with chlorinated compounds
Metal Processing and Ore Beneficiation
Chlorine metallurgy extracts titanium, zirconium, and rare earth metals through chlorination processes. The Kroll process uses chlorine to convert rutile (TiO₂) to titanium tetrachloride (TiCl₄), then reduces with magnesium to produce titanium metal for aerospace applications.
Rock salt, seawater
| Earth's Crust | 0.0001450000 ppm |
|---|---|
| Universe | 0.0000001000 % |