Chlorine gas was first observed in 1774 by Swedish chemist Carl Wilhelm Scheele (1742–1786), who obtained it by reacting hydrochloric acid with manganese dioxide. Scheele noted that this yellow-green gas bleached plants and had a suffocating odor, but he believed it contained oxygen. In 1810, Humphry Davy (1778–1829) demonstrated that this substance was a distinct chemical element, not an oxygen compound as previously thought. He proposed the name chlorine (from Greek khlôros = pale green) in reference to its characteristic color. In 1811, Joseph Louis Gay-Lussac (1778–1850) and Louis-Jacques Thénard (1777–1857) confirmed the elemental nature of chlorine and proposed the French name chlore. The identification of chlorine as an element played a crucial role in abandoning the phlogiston theory.
Chlorine (symbol Cl, atomic number 17) is a halogen in group 17 (formerly group VIIA) of the periodic table. Its atom has 17 protons, 17 electrons, and usually 18 neutrons in its most abundant isotope (\(\,^{35}\mathrm{Cl}\)). Two stable isotopes exist: chlorine-35 (\(\,^{35}\mathrm{Cl}\)) and chlorine-37 (\(\,^{37}\mathrm{Cl}\)).
At room temperature, elemental chlorine is a diatomic gas (Cl₂), yellow-green in color, about 2.5 times denser than air (density ≈ 3.214 g/L at 0 °C). It has a sharp, suffocating odor, detectable at very low concentrations. Melting point of dichlorine: 171.6 K (−101.5 °C). Boiling point: 239.11 K (−34.04 °C). Chlorine gas is toxic and corrosive, severely irritating the respiratory tract and mucous membranes. Chlorine is one of the most electronegative and reactive elements in the periodic table.
| Isotope / Notation | Protons (Z) | Neutrons (N) | Atomic mass (u) | Natural abundance | Half-life / Stability | Decay / Remarks |
|---|---|---|---|---|---|---|
| Chlorine-35 — \(\,^{35}\mathrm{Cl}\,\) | 17 | 18 | 34.968853 u | ≈ 75.76 % | Stable | Major isotope of natural chlorine. |
| Chlorine-37 — \(\,^{37}\mathrm{Cl}\) | 17 | 20 | 36.965903 u | ≈ 24.24 % | Stable | Second stable isotope; significant natural proportion. |
| Chlorine-36 — \(\,^{36}\mathrm{Cl}\) | 17 | 19 | 35.968307 u | Cosmogenic trace | 301,000 years | Radioactive β\(^-\) and electron capture yielding \(\,^{36}\mathrm{Ar}\) and \(\,^{36}\mathrm{S}\). Used to date ancient groundwater. |
| Chlorine-38 — \(\,^{38}\mathrm{Cl}\) | 17 | 21 | 37.968010 u | Non-natural | 37.24 minutes | Radioactive β\(^-\) decaying into argon-38. Produced in laboratories. |
| Other isotopes — \(\,^{28}\mathrm{Cl}\) to \(\,^{51}\mathrm{Cl}\) | 17 | 11 — 34 | — (variable) | Non-natural | Milliseconds to seconds | Very unstable isotopes produced artificially; research in nuclear physics. |
N.B. :
Electron shells: How electrons organize around the nucleus.
Chlorine has 17 electrons distributed across three electron shells. Its full electronic configuration is: 1s² 2s² 2p⁶ 3s² 3p⁵, or simplified as: [Ne] 3s² 3p⁵. This configuration can also be written as: K(2) L(8) M(7).
K Shell (n=1): contains 2 electrons in the 1s subshell. This inner shell is complete and highly stable.
L Shell (n=2): contains 8 electrons distributed as 2s² 2p⁶. This shell is also complete, forming a noble gas configuration (neon).
M Shell (n=3): contains 7 electrons distributed as 3s² 3p⁵. The 3s orbitals are complete, while the 3p orbitals contain only 5 out of 6 possible electrons. Thus, 1 electron is missing to saturate this outer shell.
The 7 electrons in the outer shell (3s² 3p⁵) are the valence electrons of chlorine. This configuration explains its chemical properties:
By gaining 1 electron, chlorine forms the Cl⁻ ion (oxidation state -1), its most stable and common state, thus adopting the configuration of argon [Ar].
By losing or sharing electrons, chlorine can exhibit positive oxidation states: +1, +3, +5, and +7, particularly in its oxygenated compounds (acids and oxyacids).
The oxidation state 0 corresponds to dichlorine Cl₂, its natural molecular form, where two chlorine atoms share a pair of electrons.
The electronic configuration of chlorine, with 7 electrons in its valence shell, classifies it among the halogens. This structure gives it characteristic properties: very high chemical reactivity (it only needs one electron to achieve the stability of a noble gas), strong electronegativity (ability to attract electrons), and powerful oxidizing power. Chlorine easily forms ionic bonds by capturing an electron to give the chloride ion Cl⁻, present in table salt (NaCl). It can also form covalent bonds by sharing electrons with other atoms. Its high electron affinity and reactivity make chlorine an essential element in chemistry, used notably for water disinfection, the production of many organic and inorganic compounds, and as a bleaching agent.
Chlorine is extremely reactive and never found in its elemental state in nature. It reacts directly with almost all elements, except noble gases, nitrogen, and oxygen (under normal conditions). Chlorine vigorously combines with metals to form chlorides and with hydrogen to form hydrogen chloride (HCl), which dissolves in water to form hydrochloric acid. It exists in several oxidation states: -I (chlorides, the most common state), +I (hypochlorites), +III (chlorites), +V (chlorates), and +VII (perchlorates). Chlorine is a powerful oxidant, capable of removing electrons from many substances. This oxidizing property is exploited for disinfection and bleaching. Dichlorine reacts with water to form a mixture of hydrochloric acid and hypochlorous acid (HOCl), the latter being responsible for chlorine's disinfectant power.
The chlorination of drinking water, introduced in the early 20th century, represents one of the most important advances in public health. It has eradicated or significantly reduced waterborne diseases such as cholera, typhoid, and dysentery in developed countries, likely saving more lives than any other public health intervention. Chlorine destroys pathogenic bacteria, viruses, and protozoa by oxidizing their cell membranes and disrupting their metabolic processes. However, chlorine can react with organic matter in water to form disinfection byproducts (trihalomethanes, haloacetic acids), some of which are potentially carcinogenic at high doses. Water treatment standards seek to optimize the balance between effective disinfection and minimizing these byproducts.
Chlorine is the 19th most abundant element in the Earth's crust (about 0.017% by mass) and is primarily found as sodium chloride (NaCl) in the oceans (about 1.9% by mass of seawater) and rock salt deposits (halite). Other chlorinated minerals include sylvite (KCl) and carnallite (KMgCl₃·6H₂O). Industrial chlorine is mainly produced by electrolysis of sodium chloride solutions (brine) using three processes: diaphragm cells, mercury cells (being phased out for environmental reasons), and membrane cells. This electrolysis simultaneously produces chlorine gas, hydrogen, and sodium hydroxide (caustic soda), forming the basis of the chlor-alkali industry. Global chlorine production exceeds 75 million tons per year.
While chlorine has essential beneficial applications, some of its organic compounds have caused serious environmental problems. Chlorofluorocarbons (CFCs), used as refrigerants and aerosol propellants, were identified as destroyers of the stratospheric ozone layer, leading to the Montreal Protocol (1987), which gradually banned their production. Persistent organochlorine pesticides like DDT, although effective, accumulate in food chains and have been widely banned. Chlorinated dioxins and furans, byproducts of certain industrial processes and incineration, are among the most toxic substances known. The chemical industry has developed less persistent alternatives and cleaner processes to minimize these environmental impacts.
Chlorine is produced in massive stars during nucleosynthesis, primarily through neutron capture and fusion reactions. Although relatively abundant on Earth, chlorine is a minor element on a cosmic scale. It has been detected in some evolved stars, meteorites, and the interstellar medium. Cosmogenic chlorine-36 is produced by the interaction of cosmic rays with atmospheric argon. Chloride salts have been detected on Mars by rovers, suggesting the past presence of liquid saltwater. Chlorine compounds are also present in the atmospheres of some exoplanets, although their significance for habitability is complex.
N.B.:
Chlorine gas was used as the first modern chemical weapon during World War I. On April 22, 1915, the German army released 168 tons of chlorine gas near Ypres, Belgium, creating a deadly yellow-green cloud that drifted toward Allied trenches. The gas, denser than air, accumulated in the trenches and caused thousands of deaths by asphyxiation and pulmonary edema. This attack paved the way for chemical warfare, which later used even more toxic agents such as phosgene and mustard gas. Today, the use of chemical weapons is prohibited by the Chemical Weapons Convention (1997), but this dark chapter reminds us that chemical elements can be used for both good and evil, depending on humanity's choices.
