Chlorine: the essentials

Chlorine is a greenish yellow gas which combines directly with nearly all elements. Chlorine is a respiratory irritant. The gas irritates the mucous membranes and the liquid burns the skin. As little as 3.5 ppm can be detected as an odour, and 1000 ppm is likely to be fatal after a few deep breaths. It was used as a war gas in 1915. It is not found in a free state in nature, but is found commonly as NaCl (solid or seawater).

Table: basic information about and classifications of chlorine.

Chlorine: historical information

Chlorine was discovered by Carl William Scheele at 1774 in Sweden. Origin of name: from the Greek word "chloros" meaning "pale green".

Chlorine was discovered in 1774 by Carl Wilhelm Scheele. He obtained it through the reaction of the mineral pyrolusite (manganese dioxide, MnO2) with hydrochloric acid (HCl, then known as muriatic acid). Scheele thought the resulting gas contained oxygen. Sir Humphry Davy proposed and confirmed chlorine to be an element in 1810, and he also named the element.

Chlorine: physical properties

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Chlorine: orbital properties

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Isolation: it is rarely necessary to make chlorine in the laboratory as it is readily available commercially in cylindes. Chlorine is found largely in seawater where it exists as sodium chloride. It is recovered as a reactive, corrosive, pale green chlorine gas from brine (a solution of sodium chloride in water) by electrolyis. Electrolysis of molten salt, NaCl, also succeeds, in which case the other product is sodium metal rather than sodium hydroxide.

Na+ + Cl- + H2O → Na+ + 1/2Cl2 + 1/2H2 + OH-

In the laboratory under carefully controlled conditions, chlorine can be made by the action of an oxidizing agent such as manganese dioxide, MnO2, upon concentrated hydrochloric acid - the same reaction used by Scheele in 1774 when discovering chlorine.

MnO2 + 4HCl → MnCl2 + Cl2 + 2H2O

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chlorine atomic number