A polyatomic ion, also known as a molecular ion, is a charged chemical species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered to be acting as a single unit. The prefix poly- means "many," in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group. In contemporary usage, the term radical refers to free radicals that are (not necessarily charged) species with an unpaired electron.An example of a polyatomic ion is the hydroxide ion; consisting of one oxygen atom and one hydrogen atom, hydroxide has a charge of −1. Its chemical formula is OH−. An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: it has a charge of +1, and its chemical formula is NH4+.Polyatomic ions are often useful in the context of acid-base chemistry or in the formation of salts. A polyatomic ion can often be considered as the conjugate acid/base of a neutral molecule. For example, the conjugate base of sulfuric acid (H2SO4) is the polyatomic hydrogen sulfate anion (HSO4−). The removal of another hydrogen ion yields the sulfate anion (SO42−).