A halogen is a chemical element from Group 17 (in the IUPAC convention) (formerly VII, VIIA) of the periodic table, composed of fluorine, chlorine, bromine, iodine, and astatine. The man-made element 117 is predicted to be a halogen. The Swedish chemist Berzelius coined the term “halogen” from the Greek háls, “salt,” and gen, meaning “come to be” – for an element that produces a salt with a metal. The halogens are the only periodic table group that contains elements in all three familiar states of matter at standard temperature and pressure. At room temperature and pressure, fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids. The halogens show several trends as the atomic number increases, including decreasing electronegativity and reactivity and increasing melting and boiling points.
In their elemental form, the halogens exist as diatomic molecules, but these are relatively unstable. Due to their high reactivity and electron affinity,...