Chlorine is anelement denoted by the atomic symbol, Cl. It has an atomic number of 17 and the average atomic weight of its isotopes is 35.453. It is an extremely poisonous, yellowish-green gas with a sharp, suffocating odor. It has a melting point of -149.8°F (- 101°C) and boils at -29.3°F (-34.05°C). It is more than twice as heavy as air, and when inhaled, it strongly irritates the respiratory tract.

Chlorine is one of five closely related chemical elements called halogens, or "salt-formers, " because they readily combine with metals to make salts. In fact, chlorine is one of the most reactive elements known. When exposed to sunlight, a mixture of chlorine and hydrogen will violently explode. In nature, chlorine is always found in compounds, never as a free element. Its most common compound is ordinary table salt, sodium chloride.

Although chlorine compounds have...