II.

Properties and Occurrence

At ordinary temperatures, chlorine is a greenish-yellow gas that can readily be liquefied under a pressure of 6.8 atmospheres at 20° C (68° F). The gas has an irritating odour and in large concentrations is dangerous; it was the first substance used as a poison gas in World War I (see Chemical and Biological Warfare).

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 relative density of 1.41 at -35° C (-31° F); the atomic weight of the element is 35.453.