Chlorine
. Chlorine, symbol Cl, greenish-yellow gaseous element. In group 17 (or VIIa) of the periodic table , chlorine is one of the halogens . The atomic number of chlorine is 17. Elementary chlorine was first isolated in 1774 by the Swedish chemist Carl Wilhelm Scheele , who thought that the gas was a compound; it was not until 1810 that the British chemist Sir Humphry Davy proved that chlorine was an element and gave it its present name. At ordinary temperatures, chlorine is a greenish-yellow gas that can readily be liquefied under pressure of 5170 torr, or 6.8 atmospheres, at 20° C (68° F). The gas has an irritating odor and in large concentration is dangerous; it was the first substance used as a poison gas in World War I (1914-1919). Free chlorine does not occur in nature, but its compounds are common minerals, and it is the 20th most abundant element in the earth's crust. Chlorine melts at -101° C (-149.8° F), boils at -34.05° C (-29.29° F) at one atmosphere pressure, and has a spe...