Chlorine

Chlorine is the yellow-green gas that gives swimming pools their distinctive smell, kills bacteria in drinking water and forms one half of table salt. It is one of the most useful and also most dangerous elements, used as a poison gas in World War I, yet now saving millions of lives every year by disinfecting water supplies.

  • Atomic Number1717 protons, 17 electrons
  • Atomic Mass35.45 uAbout 35× heavier than hydrogen
  • State at Room TempGasyellow-green gas
  • Density0.003214 g/cm³About 2.5× heavier than air
  • Melting / Boiling-101.5°C / -34°CLiquefies at −34°C
  • Discovered1774Carl Wilhelm Scheele, 1774

How does chlorine compare to the other halogens?

Chlorine is the second lightest halogen, between fluorine above and bromine below in Group 17.

Atomic Mass Comparison
Fluorine19 u
Chlorine35.5 u
Bromine79.9 u
Iron55.8 u
Iodine127 u

Chlorine at 35.5 u is nearly twice as heavy as fluorine (19 u), but less than half the mass of iodine (127 u). Despite these differences, all halogens are reactive non-metals with seven outer electrons, eager to gain one more.

What is chlorine?

Chlorine is a halogen in Group 17 of the periodic table, sitting directly below fluorine. At room temperature it forms a diatomic molecule (Cl₂), a dense, yellow-green gas with a sharp, choking smell. Like all halogens, chlorine has seven electrons in its outer shell and is highly reactive, it combines with metals, organic compounds and water to form a wide range of important compounds.

Chlorine gets its name from the Greek word chloros, meaning yellow-green or pale green, which perfectly describes the colour of the gas. The name was given by the English chemist Humphry Davy in 1810, when he proved that chlorine was a true element and not a compound containing oxygen as had previously been believed.

Fact Chlorine was the first poison gas used on a large scale in warfare. On 22 April 1915, German forces released 168 tonnes of chlorine gas at Ypres, Belgium, creating a yellow-green cloud that drifted into Allied trenches. The attack shocked the world and led to rapid development of the gas mask.

Where you find chlorine

On Earth

Chlorine is one of the most abundant elements in the Earth's crust and oceans, but it is never found as free gas in nature, it is always bonded to other elements.

  • Table salt. Sodium chloride (NaCl) is by far the most common chlorine compound. It is found in vast underground deposits and dissolved in the oceans. Seawater contains approx. 1.9% chloride ions by mass.
  • Potassium chloride minerals. Sylvite and carnallite are mined as a source of potassium fertiliser, with chloride as a by-product.
  • Underground brines. Salt wells, deep underground pools of salty water, are pumped to the surface in many parts of the world to recover chlorine compounds for industry.

How we use chlorine

  • Water purification. Small amounts of chlorine or chlorine compounds are added to drinking water and swimming pools to kill bacteria and viruses. This simple step has saved more human lives than almost any other public health measure in history.
  • PVC plastic. Polyvinyl chloride (PVC) is one of the world's most widely produced plastics, used in pipes, window frames, cables and flooring. Chlorine makes up 57% of its weight.
  • Bleach and disinfectants. Household bleach is a solution of sodium hypochlorite, a chlorine compound. It disinfects surfaces, whitens laundry and kills pathogens in hospitals.
  • Pharmaceuticals. About 85% of all pharmaceutical drugs are made using chlorine chemistry at some stage. Aspirin, antibiotics and antihistamines all involve chlorine in their manufacture.
Did you know? The "swimming pool smell" is not actually chlorine itself. Pure chlorine smells more acrid. The familiar pool odour comes from chloramines, compounds formed when chlorine reacts with organic matter such as sweat and urine in the water. A strong pool smell actually means the pool water is not clean enough, not that it has too much chlorine.

How it was discovered

Chlorine was first produced in 1774 by the Swedish chemist Carl Wilhelm Scheele, who heated manganese dioxide with hydrochloric acid and noticed a dense, yellow-green gas with a choking smell. He called it "dephlogisticated marine acid air". Humphry Davy proved in 1810 that it contained no oxygen and was a true element, naming it chlorine from the Greek for its colour. Scheele had actually produced and described chlorine 36 years earlier, but Davy established its elemental nature.

Deeper dive: the chlor-alkali process and the ozone layer

The chlor-alkali process is the main industrial source of chlorine. Electrolysis of brine (sodium chloride solution) produces chlorine gas at the anode and sodium hydroxide (NaOH) at the cathode, with hydrogen gas also produced. This single process provides the feedstock for an enormous range of products, from PVC to bleach to solvents and pharmaceuticals.

Chlorofluorocarbons (CFCs) were once widely used as refrigerants and aerosol propellants. In the 1970s, scientists Mario Molina and Sherwood Rowland discovered that CFCs drifted into the stratosphere where ultraviolet light broke them apart, releasing chlorine atoms that destroyed ozone molecules in a chain reaction. Each chlorine atom could destroy 100,000 ozone molecules. The resulting ozone hole over Antarctica threatened all life by allowing more UV radiation through. The Montreal Protocol of 1987, phasing out CFCs globally, is considered one of the most successful international environmental agreements ever signed.

In water, chlorine dissolves to form hypochlorous acid (HOCl), which is the active disinfecting agent. HOCl penetrates bacterial cell walls and destroys their enzymes and DNA. At the pH of most drinking water, HOCl is far more effective than the hypochlorite ion (OCl⁻), which is why maintaining pool pH in the correct range matters so much for effective disinfection.

Chlorine is a powerful element, both a lifesaver in water treatment and a deadly gas in warfare. Moving to 18 protons brings us to argon, a noble gas so inert it was almost impossible to detect.