Chlorine

One electron short

A chlorine atom has seventeen electrons, seven of them in its outer shell — one short of the full set that makes argon, its neighbour, so inert. Nearly everything chlorine does follows from its hunger for that eighth electron. It takes one from almost any metal to become the chloride ion, Cl⁻; it shares one with carbon, hydrogen or another chlorine atom. Its electronegativity, 3.16, is beaten only by fluorine's and oxygen's.

The free element is a diatomic gas, Cl₂, yellow-green, two and a half times denser than air, with a choking smell the nose detects at a few parts per million. It attacks the eyes and lungs, and in quantity it kills. Mixed with hydrogen it explodes when a light is shone on it; sodium burns in it to table salt; with water it forms hypochlorous acid, the compound that does the real work of bleaching and disinfection.

Salt, sea and brine

Chlorine is too reactive to occur free. As chloride it is everywhere: about 145 grams in every tonne of the crust, much of it in rock salt, and 19 grams in every kilogram of seawater, where it is the most abundant of the dissolved ions. Ancient seas that dried up left beds of salt — under Cheshire, Poland, Kansas, the Alps — that are mined, or dissolved and pumped out as brine.

Nearly all the chlorine industry uses comes from brine, by the chlor-alkali process: an electric current passed through a solution of common salt gives chlorine at one electrode and hydrogen and caustic soda at the other. The process dates from the 1890s and makes tens of millions of tonnes a year. The gas is hazardous to store and ship in bulk, so most of it is turned into something else within reach of the plant that made it.

Bleach, clean water and gas

Chlorine's first trade was bleaching. Claude Louis Berthollet found in 1785 that the gas whitened cloth, and Charles Tennant of Glasgow patented bleaching powder, chlorine taken up by lime, in 1799. The second was killing germs. From 1908, when Jersey City chlorinated its water supply for good, the practice spread round the world, and it has probably saved more lives than any drug. Today the largest single use is PVC; chlorine also goes into solvents, hydrochloric acid, titanium white and a large share of all medicines and pesticides.

The body uses it too: chloride is the main negative ion of blood and of the fluid around the cells, the stomach secretes hydrochloric acid to digest food, and white blood cells make hypochlorous acid — bleach in miniature — to destroy bacteria.

The dark side is as real. At Ypres on 22 April 1915 the German army released some 170 tonnes of chlorine from cylinders, the first large-scale use of a chemical weapon. And in 1974 Mario Molina and Sherwood Rowland showed that chlorofluorocarbons, the refrigerant gases, release chlorine atoms in the stratosphere, where each destroys thousands of ozone molecules; the ozone hole over Antarctica followed, and in 1987 the Montreal Protocol.

Discovery and name

An apothecary's gas, and the man who called it an element

Carl Wilhelm Scheele, a Swedish apothecary, made the gas in 1774 by warming pyrolusite, an ore of manganese, with hydrochloric acid. He noted its colour, its suffocating smell, its attack on metals and the way it bleached flowers and leaves, but took it for a compound, “dephlogisticated muriatic acid”, and chemists after him, following Berthollet, assumed it contained oxygen. Humphry Davy, at the Royal Institution, could get no oxygen out of it by any means, and in 1810 declared it an element. He named it chlorine, from the Greek chloros, yellow-green, after the one property nobody disputed.