Halogen compounds have been known since ancient times. Rock salt - or sodium chloride - for example, was used at least five thousand years ago. But the elements themselves, because of their great reactivity, were isolated relatively recently.
Being the most reactive of all the elements, fluorine was especially difficult to produce in elemental form. Discovered by Carl Wilhelm Scheele in 1771, over a hundred years elapsed before the French chemist, Henri Moissan, succeeded in isolating the gas.
With chlorine, however, Scheele was more successful, isolating the element in 1774. This was the first of the halogens to be seen uncombined. Even so, Scheele believed that what he had prepared was a compound, and only in 1810 did Davy demonstrate the true nature of his discovery.
Bromine, which resembles chlorine in many ways, was not recognised as a separate element until 1826, when the French chemist, Antoine-Jerome Balard, succeeded in isolating the liquid from a sample of Mediterranean sea water. The light-sensitive silver bromide was first used in photography in about 1840.
Iodine was discovered by Bernard Courtois in 1811. The French manufacturer of saltpetre extracted the element from seaweed ash. It was named by Joseph Louis Gay-Lussac in 1813, in allusion to its violet colour, {Iiodes} being Greek for 'violet'.
The radioactive astatine was synthesized at the University of California in 1940 by a team of American scientists which included Emilio SegrĪ.
