On the periodic table, group 17 is traditionally called the halogens. The word halogen comes from Greek roots meaning “salt-former,” reflecting the tendency of these elements to react with metals and produce salts. Group 17 is also called group VIIA in older numbering systems.
The halogens are fluorine, chlorine, bromine, iodine, astatine, and tennessine. They occupy the column immediately to the left of the noble gases. Their atoms generally have seven valence electrons, so they commonly gain one electron in chemical reactions and form ions with a −1 charge.
The physical states change down the group. Fluorine and chlorine are gases at room temperature, bromine is a liquid, and iodine is a solid. Astatine is radioactive and extremely rare, while tennessine is a synthetic superheavy element. Halogens are not all equally reactive: fluorine is generally the most reactive, and reactivity decreases down the group. The term should not be confused with chalcogens, the name for group 16.