Cl

Chlorine

Chlorine is a yellow-green gas with atomic number 17 and symbol Cl. It is a highly reactive halogen that’s widely used as a disinfectant in water treatment and as a key ingredient in many industrial and household products. Chlorine is the second-lightest halogen. Chlorine in swimming pools is a powerful disinfectant that kills harmful bacteria and prevents algae growth, but it’s not the chlorine that causes the distinctive pool smell. That odor is actually caused by chloramines, which form when chlorine reacts with contaminants like sweat, urine, and body oils introduced by swimmers.