Chlorine is a chemical element with symbol Cl and atomic number 17. Chlorine is in the halogen group and is the second lightest halogen after fluorine. The element is a yellow-green gas under standard conditions, where it forms diatomic molecules. It has the highest electron affinity and the third highest electronegativity of all the elements; for this reason, chlorine is a strong oxidizing agent. Free chlorine is rare on Earth, and is usually a result of direct or indirect oxidation by oxygen.Principal applications of chlorine are in the production of a wide range of industrial and consumer products.By 1918, the US Department of Treasury called for all drinking water to be disinfected with chlorine. Chlorine is presently an important chemical for water purification (such as water treatment plants), in disinfectants, and in bleach.