1.Definition of halogen
The halogen elements include: five kinds of fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At). Among them, astatine is a radioactive element. The halogens that people usually refer to are four elements: fluorine, chlorine, bromine, and iodine. The first four elements exist in the form of organic compounds in products, especially in polymer materials.
