Chlorine
Chlorine is a chemical element with symbol Cl and atomic number 17. It also has a relative atomic mass of 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen following fluorine.
Identity
- Atomic mass
- [35.446, 35.457] u
- Category
- Halogens
- Group
- 17
- Period
- 3
- Block
- p
- Oxidation States
- −1, +1, +3, +5, +7
- Phase (25 °C)
- Gas
- Appearance
- pale yellow-green gas
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p5
- Short configuration
- [Ne] 3s2 3p5
- Electrons per shell
- 2, 8, 7
- Electronegativity (Pauling)
- 3.16
- Electron affinity
- 349 kJ/mol
- 1st ionization energy
- 1.25e+3 kJ/mol
- Ionization energies
- 1251 · 2298 · 3822 · 5159 · 6542… kJ/mol
Physical properties
- Melting point
- -101.5 °C (172 K)
- Boiling point
- -34.0 °C (239 K)
- Density
- 0.003214 g/cm³
- Molar heat capacity
- —
Chemical behavior of chlorine
Chlorine has seven valence electrons ([Ne] 3s² 3p⁵) and is the second halogen: a very powerful oxidant that seeks the missing electron up to -1, almost always in ionic or polar bonds (chlorides). But its larger size than fluorine allows it to give electrons to oxygen and fluorine, showing positive states +1, +3, +5 and +7 in oxoanions and oxides. That duality — reducing metals, oxidising non-metals — makes it the most versatile halogen and the universal disinfectant.
Reactivity
H₂ + Cl₂ → 2 HCl
With hydrogen it reacts explosively in light (chain radical reaction) giving hydrogen chloride. It is the direct synthesis of industrial hydrochloric acid.
2 Fe + 3 Cl₂ → 2 FeCl₃
Metals burn in chlorine giving chlorides in their high state: iron goes to +3. The reaction releases so much heat that steel can ignite in a chlorine atmosphere.
Cl₂ + H₂O ⇌ HCl + HClO
In water it disproportionates into hydrochloric and hypochlorous acid: the latter is the one that disinfects pools and bleaches. It is the reaction that purifies the planet.
Cl₂ + 2 NaOH → NaCl + NaClO + H₂O
With cold lye it gives chloride and hypochlorite: household bleach. With hot one forms chlorate, base of matches and herbicides. All bleaching chemistry originates here.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Sodium chloride (common salt) | NaCl | Food, de-icing and chemical raw material |
| Hydrochloric acid | HCl | Gastric juices, cleaning and synthesis |
| Sodium hypochlorite (bleach) | NaClO | Universal disinfection and bleaching |
| Vinyl chloride (PVC) | C₂H₃Cl | Pipes, windows and cables |
| Potassium chlorate | KClO₃ | Matches, pyrotechnics and chemical oxygen |
Electron shell distribution
View in the atom builderIsotopes
Natural chlorine mixes two stable isotopes: chlorine-35 (³⁵Cl, ~76%) and chlorine-37 (~24%), both with seventeen protons. This mixture explains its fractional atomic mass (35.45). Cosmogenic chlorine-36, with a 300,000-year half-life, dates millennia-old groundwater.
Occurrence and production
It is the twentieth element in the crust but the most abundant dissolved in the sea (19,000 ppm as Cl⁻). It appears in halite, sylvite and carnallite. It is obtained by electrolysis of brine (chlorine-alkali process), which also produces soda and hydrogen: chlorine is the byproduct that disinfects the world.
History and origin of the name
Carl Wilhelm Scheele obtained it in 1774 from muriatic acid and took it for an oxide; Humphry Davy showed in 1810 that it was an element and baptized chlorine for its yellowish-green colour. Its bleach changed the textile revolution; its gas asphyxiated in Ypres (1915); and chlorinated water has saved more lives than any drug.
Applications and relevance of chlorine
Uses of the element
Gaseous Cl₂ purifies the water of billions of people and disinfects pools; it bleaches paper and textiles. Its transport as liquefied gas requires the strictest protocols of the chemical industry.
Uses of its compounds
Table salt is food and de-ices roads; HCl digests in your stomach and cleans metals; bleach disinfects homes and hospitals; PVC sheathes the planet. Chlorine is in the kitchen, the bathroom and the pipes.
Health, safety and environment
Biological role
The chloride ion is essential: it regulates hydration, forms stomach HCl and transmits nerve impulses (GABA). Its deficiency causes alkalosis; excess salt raises blood pressure. Without chloride there is no digestion or calm neurons.
Safety
Gaseous Cl₂ was the first modern chemical weapon: it irritates eyes and lungs and at 1,000 ppm kills in minutes. Mixing bleach with ammonia or acids releases chloramines or deadly chlorine: it is the most serious domestic accident. Never mix bleach.
Environment
CFCs with chlorine destroyed the ozone layer (banned in Montreal 1987, recovering). Persistent organochlorines (DDT, dioxins) bioaccumulate; DDT was restricted in Stockholm 2001. Common salt chloride, however, is harmless to the environment.
Frequently asked questions about Chlorine
Is pool chlorine the gas of World War I?
The same element, opposite doses: at Ypres (1915) tons of Cl₂ were released that asphyxiated the trenches; in your pool there are 1-3 parts per million that kill microbes without harming you. Like fluorine in toothpaste: dose makes the poison. Smelling like a pool is smelling safety: without that chlorine residual, water would spread cholera and typhoid. The weapon became guardian.
Is it dangerous to drink chlorinated water?
No at the doses for purification (0.2-0.5 mg per litre): a century of evidence shows that it prevents cholera, typhoid and dysentery without damage, and by-products are controlled by regulation. The chlorine residual that you smell guarantees that water arrives alive with microbes to the tap. Letting water rest evaporates the smell if it bothers. The real risk is water without chlorine.
Is common salt a chlorine compound?
Yes: each grain is NaCl, chlorine trapped as a stable chloride along with sodium. The deadly chlorine gas and explosive sodium combined give the vital condiment: chemistry transforms poisons into everyday food. Your blood, your tears and the sea carry the same dissolved chloride. Eating a little salt is the most peaceful and tasty way to ingest chlorine daily.
Is PVC toxic because it contains chlorine?
The installed tube, no: rigid PVC is inert and that is why the world's drinking water is piped without risk. The problem is in its manufacture (mutagen carcinogen, under strict control) and in uncontrolled burning (dioxins), not in everyday use. Recycling it is difficult due to its various additives. Like almost everything with chlorine: useful on site, dangerous outside it.
Why should we never mix bleach with other products?
Because with acids it releases chlorine gas and with ammonia it forms toxic chloramines: both attack eyes and lungs and can kill in an unventilated bathroom. It is the most serious and also the most common cleaning accident. Bleach alone, diluted and well rinsed, disinfects more than enough: it doesn't need 'reinforcement'. If you mix by mistake, ventilate and run: your lungs have no reserve.
What destroyed the ozone layer?
CFCs, gases in refrigerators and sprays: they rose intact to the stratosphere, where the sun released their chlorine and each atom destroyed thousands of ozone molecules. The Antarctic hole alarmed the world and the Montreal Protocol (1987) banned them: it is the greatest environmental success, with the layer recovering towards 2060. The stratospheric chlorine drops each year.
How did chlorine win the life of disinfecting?
By oxidising all microscopic life: hypochlorous acid crosses the membranes of bacteria and viruses and destroys their proteins and DNA in seconds. It does not discriminate: that is why it kills pathogens but irritates eyes if in excess. It has been doing so for more than a century in waters, hospitals and kitchens. It is the cheapest and most effective killer of microbes ever employed by humanity.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.