Fluorine, chlorine, bromine, iodine and astatine, known collectively as the halogens, are poisonous, non-metallic elements found in group 17 of the periodic table. Fluorine and chlorine are gases, bromine a liquid, and iodine and astatine are solids. The group derives its name from the Greek words {Ihal}, meaning 'salt', and {Igen}, meaning, 'forming', in allusion to the fact that the group members combine with metals to form salts. Sodium chloride, or common salt, is the most familiar of these. Rock salt is a form of sodium chloride.
The halogens are further characterized by their great reactivity. None of them are found uncombined in nature, and fluorine is the most reactive of all elements: it will combine with every element save for the noble gases, helium, argon and neon. When xenon, an unreactive noble gas, was first induced to form a compound, in the 1960s, it was with fluorine. All the halogens are reactive, but reactivity decreases as we descend the group.
Each of the elements contains 7 electrons in its outer shell, and forms monovalent, negatively-charged ions. For example: Cl-; Br-.
