CHLORINE

Chlorine is a chemical element with symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine is a yellow-green gas at room temperature. It is an extremely reactive element and a strong oxidising agent: among the elements, it has the highest electron affinity and the third-highest electronegativity, behind only oxygen and fluorine.

Uses

Chlorine kills bacteria – it is a disinfectant. It is used to treat drinking water and swimming pool water. It is also used to make hundreds of consumer products from paper to paints, and from textiles to insecticides.

About 20% of chlorine produced is used to make PVC. This is a very versatile plastic used in window frames, car interiors, electrical wiring insulation, water pipes, blood bags and vinyl flooring.

Another major use for chlorine is in organic chemistry. It is used as an oxidising agent and in substitution reactions. 85% of pharmaceuticals use chlorine or its compounds at some stage in their manufacture.

In the past chlorine was commonly used to make chloroform (an anaesthetic) and carbon tetrachloride (a dry-cleaning solvent). However, both of these chemicals are now strictly controlled as they can cause liver damage.

Chlorine gas is itself very poisonous, and was used as a chemical weapon during the First World War.

Natural abundance

Chlorine is not found uncombined in nature. Halite (sodium chloride or ‘common salt’) is the main mineral that is mined for chlorine. Sodium chloride is a very soluble salt that has been leached into the oceans over the lifetime of the Earth. Several salt beds, or ‘lakes’ are found where ancient seas have evaporated, and these can be mined for chloride.

Chlorine is also found in the minerals carnallite (magnesium potassium chloride) and sylvite (potassium chloride).

40 million tonnes of chlorine gas are made each year from the electrolysis of brine (sodium chloride solution). This process 

also produces useful sodium hydroxide.

Common oxidation states	7, 5, 3, 1, -1
Isotopes	Isotope	Atomic mass	Natural abundance (%)	Half life	Mode of decay
35Cl	34.969	75.76	-	-
37Cl	36.966	24.24	-	-