Chlorine

With the electron configuration [Ne] 3s2 3p5, chlorine sits one electron short of a complete outer shell, and that single vacancy defines its entire chemical personality. As a Group 17 halogen, it is among the most electronegative elements on the periodic table at 3.16, which drives its relentless appetite for electrons and its tendency to form the stable chloride ion (Cl⁻) in virtually every aqueous environment it encounters. At room temperature it exists as a diatomic gas, Cl2, with a sharp, acrid odor detectable by the human nose at concentrations as low as 0.5 parts per million. Its high electron affinity of 3.617 eV — the second highest of any element after fluorine — makes it an exceptionally powerful oxidizing agent, a property that underlies both its industrial value and its toxicity.

Municipal water treatment is chlorine's most consequential application: even small doses of dissolved chlorine or hypochlorite kill bacteria, viruses, and protozoa that once made waterborne disease a leading cause of death worldwide. Bleach, typically a 3–8% sodium hypochlorite solution, disinfects household surfaces and sanitizes food-processing equipment. The polymer polyvinyl chloride (PVC) — produced by polymerizing vinyl chloride monomer derived from chlorine and ethylene — is the third most widely produced synthetic plastic, used in pipes, window frames, flooring, and medical tubing. Chlorine also anchors the synthesis of chlorinated solvents such as trichloroethylene and methylene chloride, which serve as industrial degreasers and paint strippers. In the pharmaceutical industry, roughly 85% of drug molecules incorporate chlorine at some stage of synthesis, either as a functional group in the final compound or as part of a reactive intermediate.

Swedish-German chemist Carl Wilhelm Scheele first isolated chlorine gas in 1774 by reacting manganese dioxide with hydrochloric acid, though he mistakenly believed the greenish-yellow product was an oxygen compound and called it 'dephlogisticated marine acid.' Humphry Davy established in 1810 that it contained no oxygen at all and was in fact a new element, proposing the name chlorine from the Greek chloros, meaning pale green — a direct reference to the gas's distinctive color. Chlorine's most infamous chapter opened on April 22, 1915, at Ypres, Belgium, when German forces released roughly 150 tons of chlorine gas from pressurized cylinders, creating the first large-scale chemical weapons attack in modern warfare. Thousands of Allied soldiers were killed or permanently injured as the heavier-than-air cloud settled into trenches. That event accelerated international efforts that eventually produced the Chemical Weapons Convention of 1993, which bans the use of chlorine as a weapon.

Chlorine is too reactive to occur as a free element in nature; instead it is found almost exclusively as chloride ions (Cl⁻) bonded to cations in solution or mineral form. Seawater holds approximately 1.9% chloride by weight, making it the most abundant dissolved anion in the ocean and a major driver of seawater's ionic strength. On land, it accumulates as halite (NaCl, rock salt) in evaporite deposits formed when ancient seas evaporated, with major formations found in the United States, Germany, Poland, and China. The human body contains about 115 grams of chloride, distributed throughout blood plasma, extracellular fluid, and gastric acid (hydrochloric acid, HCl). In terms of cosmic abundance, chlorine ranks roughly 18th among the elements by number of atoms, synthesized predominantly through neutron capture reactions in massive stars and dispersed into interstellar space via supernovae.

Sodium chloride (NaCl), common table salt, is the most familiar chlorine compound and an essential electrolyte in mammalian physiology that also underpins the chlor-alkali industry — the electrolysis of brine that produces chlorine gas, sodium hydroxide, and hydrogen at industrial scale. Hydrogen chloride (HCl) dissolves in water to form hydrochloric acid, a strong acid critical to metal pickling, leather tanning, and gastric digestion. Sodium hypochlorite (NaOCl) is the active ingredient in bleach and disinfecting cleaners. Polyvinyl chloride (PVC), derived from vinyl chloride monomer (CH2=CHCl), is one of the most-produced plastics globally. Chloroform (CHCl3) was among the first general anesthetics used in surgery in the 1840s and remains a solvent and chemical intermediate today. Carbon tetrachloride (CCl4), once a common dry-cleaning agent and fire suppressant, was phased out after its role in stratospheric ozone depletion became clear.

• Chlorine's atomic mass of 35.45 is not a whole number because natural chlorine is a near-equal mixture of two stable isotopes — chlorine-35 (about 75.8%) and chlorine-37 (about 24.2%) — and 35.45 is the weighted average of those two masses.
• The characteristic smell of a swimming pool does not come from chlorine gas itself but from chloramines — compounds formed when chlorine reacts with nitrogen in sweat, urine, and skin oils; a strong pool odor actually signals that the disinfection chemistry is being overwhelmed, not that the water is clean.
• Chlorine was the first element used as a chemical weapon in industrial-scale warfare, yet the same reactivity that makes it toxic to lungs at parts-per-million concentrations is what allows trace doses to reliably kill cholera-causing bacteria in drinking water — a public health intervention credited with saving hundreds of millions of lives.
• Despite chlorine's fearsome reputation as a gas, more than 90% of the chlorine produced industrially is immediately incorporated into solid or liquid compounds rather than used as a gas, and most of those products — PVC plumbing, pharmaceuticals, disinfectants — carry no special hazard in normal use.
• Chloride channels in cell membranes regulate the electrical behavior of neurons: the drug class of benzodiazepines works by enhancing the opening of chloride channels, allowing Cl⁻ ions to flow in and hyperpolarize the neuron, making it harder to fire.