Pale green-yellow gas which smells like bleach m.p. -100.98 ºC; -149.76 ºF b.p. -34.6 ºC; -30.3 ºF density 0.003214 g/cc; 0.200643 pounds/cubic foot	memory peg
	1774 Carl Wilhelm Scheele, Sweden
	χλωρος (chlōros) = light green, green-yellow (Greek) named by Sir Humphry Davy in 1810

Chlorine was discovered in 1774 by the Swedish chemist Carl Wilhelm Scheele (1742-1786). He produced this element by heating pyrolusite (MnO₂) with hydrogen chloride, HCl, an acid first made by alchemists in the fifteenth century and named "muriatic acid" by the French chemist Antoine Lavoisier. Scheele thought that the yellow-green product gas was a compound of oxygen and, believing that the pyrolusite had removed phlogiston from "muriatic acid", Scheele named this air dephlogisticated muriatic acid (also oxygenated muriatic acid, oxymuriatic acid).

Meanwhile Antoine Lavoisier proposed to replace both the four element theory and phlogiston with numerous new elements and a concept of combustion involving a new gaseous element. Lavoisier named the gas responsible for combustion oxygen, meaning acid forming, because all compounds containing oxygen seemed to be acidic. Lavoisier's colleague Claude Berthollet therefore proposed that Scheele's dephlogisticated muratic acid gas must be a combination of oxygen and an undiscovered element, muriaticum.

Louis-Joseph Gay-Lussac (1778-1850) and Louis-Jacques Thénard (1777-1857) tried to decompose dephlogisticated muratic acid gas and did not succeed; they published a report "On the nature and the properties of muriatic acid and of oxygenated muriatic acid" (Mémoires de Physique et de Chimie de la Société d'Arcueil 2, 339-358 (1809) [translated and excerpted in Alembic Club Reprints #13 The Early History of Chlorine], on-line). The authors consider the possibility that it is an element, but are not convinced and conclude:

"In fact, oxygenated muriatic acid is not decomposed by charcoal, and it might be supposed, from this fact and those which are communicated in this Memoir, that this gas is a simple body. The phenomena which it presents can be explained well enough on this hypothesis; we shall not seek to defend it however, as it appears to us that they are still better explained by regarding oxygenated muriatic acid as a compound body."

"As the new compound in its purest form is possessed of a bright yellow green colour, it may be expedient to designate it by a name expressive of this circumstance, and its relation to oxymuriatic gas. As I have named that elastic fluid Chlorine, so I venture to propose for this substance the name Euchlorine, or Euchloric gas from ευ and χλωρος. The point of Nomenclature I am not, however, inclined to dwell upon. I shall be content to adopt any name that may be considered as most appropriate by the able chemical philosophers attached to this Society."

• Japanese: 塩 = Chinese character for salt, and 素 "so" (normally, plain).

• Mary Elvira Weeks, Discovery of the Elements, comp. rev. by Heny M. Leicester (Easton, Pa.: Journal of Chemical Education, 1968), pp. 701-708.
• Chlorine Chemistry Council, Chlorine: What is it?