On the periodic table, chlorine has the symbol Cl, and its name comes from the Greek word khlōros, meaning greenish-yellow.
Chlorine is atomic number 17 and belongs to group 17, the halogens. At room temperature and pressure, elemental chlorine is a diatomic gas, Cl₂, with a distinctive greenish-yellow color and a sharp odor. Its name describes that visible color rather than a property of chlorine compounds in general.
Swedish chemist Carl Wilhelm Scheele produced chlorine-containing gas in 1774, but he did not recognize it as a new element. Humphry Davy later demonstrated in 1810 that it was elemental chlorine and gave it the name used today.
Chlorine is widely used to disinfect drinking water and swimming pools because it forms reactive compounds that damage microorganisms. It is also part of hydrochloric acid and many industrial chemicals. Chloride ions, however, are common and essential in biology, including nerve function and fluid balance.