Halogen-free (halides) and chemical elements of the main sub-group VII group of the periodic system D. I. Mendeleev: fluorine (see), chlorine (see), bromine (see), iodine (see), and astatine (astatin). Typical non-metals,- have high chemical activity, directly connected with the majority of chemical elements, forming halides. The Halogens and their compounds are widely used in industry and medicine.