Chlorine
![Chlorine, liquefied under a pressure of 7.4 bar at room temperature, displayed in a quartz ampule embedded in acrylic glass.](/Images/godic/202501/04/Chlorine_liquid_in_an_ampoule4707.jpg")
![Carl Wilhelm Scheele](/Images/godic/202501/04/PSM_V31_D740_Carl_Wilhelm_Scheele4707.jpg")
![Liquid chlorine analysis](/Images/godic/202501/04/Liquid_chlorine_in_flask4707.jpg")
![Membrane cell process for chloralkali production](/Images/godic/202501/04/Chloralkali_membrane.svg4707.png")
Chlorine is a chemical element with symbol Cl and atomic number 17. It has a relative atomic mass of about 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen, following fluorine. The element is a yellow-green diatomic gas under standard conditions. Chlorine has the highest electron affinity and the third highest electronegativity of all the reactive 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.