Wikipedia Swimming Pool Chlorine

Chlorine (from the Greek language χλωρóς chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. As the chloride ion, which is part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including humans. In its elemental form, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable suffocating odor and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants.