the chemical elements fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and astatine (At), which form the main subgroup of Group VII of the Mendeleev periodic system. They are called halogens (from the Greek hals, salt) because they form salts (for example, common salt, NaCl) upon combination with metals. The name “halides” is sometimes used. There are seven electrons (configuration s²p⁵) in the outer electron shell of the halogen atoms, that is, they are one electron short of the stable eight-electron inert-gas configuration (s²p⁶). Upon reacting with a metal, each halogen atom removes an electron from it, thus showing oxidizing properties. All halogens are very reactive, combining directly with most chemical elements. The chemical reactivity of halogens decreases from fluorine to iodine as the atomic radius increases. Under ordinary conditions fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. Astatine is a radioactive element. The molecules of the halogens are diatomic.