Chlorine

Chlorine was produced first in 1774 by Carl Wilhelm Scheele, who collected the gas released by the reaction of pyrolusite (manganese dioxide) with the substance we now call hydrochloric acid. It had, according to Scheele, "a very perceptible suffocating smell, which was most oppressive to the lungs... and gives the water a slightly acidic taste... the air in it acquires a yellow color..." Scheele also noted the reactivity and bleaching qualities of the new gas he had made: "all metals were attacked... fixed alkali was converted into common salt... all vegetable flowers - red, blue, and yellow - became white in a short time; the same thing also happened with green plants... insects immediately died. ⁽¹⁾

Despite the accuracy of his observations, Scheele mistakenly thought the resulting gas was a compound that contained oxygen.

Sir Humphry Davy in 1810, however, found he could not get the new gas to react with a charcoal electrode, which caused him to believe it may not contain oxygen. In reactions with phosphorus and ammonia, he demonstrated the new gas could not contain oxygen. He used a huge, 2000 plate voltaic pile (battery) to see whether he could get oxygen out of the gas's compounds with phosphorus and sulfur, but again found no oxygen. ^(1a)

In 1811, Davy concluded the new gas was in fact a new element. ^(1b) He named it chlorine, from the Greek word 'chloros', meaning pale green.

Chlorine is a toxic gas that irritates the skin, the eyes and the respiratory system.

Chlorine is a greenish-yellow, diatomic, dense gas with a sharp smell (the smell of bleach).

It is not found free in nature as it combines readily with nearly all other elements.

Chlorine occurs in nature mainly as common salt (NaCl), carnallite [ KMgCl₂.6(H₂O) ], and sylvite (KCl).

In its liquid and solid form it is a powerful bleaching, oxidizing and disinfecting agent.

Chlorine is used for producing safe drinking water.

Chlorinated compounds are used mostly for sanitation, pulp bleaching, disinfectants, and textile processing.

Chlorine is also used for the manufacture of chlorates and it is important in organic chemistry, forming compounds such as chloroform, carbon tetrachloride, polyvinyl chloride, and synthetic rubber.

Other uses of chlorine compounds include dyestuffs, petroleum products, medicines, antiseptics, insecticides, foodstuffs, solvents, paints and plastics. 

Chlorine gas is produced commercially by the electrolysis of sodium chloride (NaCl) from seawater or brine from salt mines.

Chlorine has 16 isotopes whose half-lives are known, with mass numbers 31 to 46. Of these, two are stable: ³⁵Cl and ³⁷Cl.

References
1. Henry M. Leicester, Herbert S. Klickstein, A Source Book in Chemistry, 1400-1900., (1969) p111. Harvard University Press.
1a. Henry M. Leicester, Herbert S. Klickstein, A Source Book in Chemistry, 1400-1900., (1969) p241. Harvard University Press.
1b. Henry M. Leicester, Herbert S. Klickstein, A Source Book in Chemistry, 1400-1900., (1969) p257. Harvard University Press.