A polyatomic ion, 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.
Ions in chemistry can be a single charged atom, or they can be a group of atoms that act as an ion. These groups of atoms are called polyatomic ions. Polyatomic ions each carry a specific charge, which is determined by their numbers of valence electrons. Many chemistry classes require students to know at least some of the basic polyatomic ions. While there are some ways of figuring out the charges on each ion, as well as tricks to remembering others, there are no solid rules on how they are named and what charges they carry. The only way to be sure of the charges and names of these ions is to memorize them.
Calculate from Oxidation Number
Determine the oxidation number of each atom in the ion. For example, consider the hydroxide ion, which has an oxygen atom and a hydrogen atom. The oxidation number of oxygen is -2, and the oxidation number of hydrogen is +1.
Add together the oxidation numbers of all the atoms in the polyatomic ion. In the example, -2 +1 = -1. This is the charge on the polyatomic ion.
Write this charge as a superscript to the right of the ion's formula. For a single charge, write - or + instead of 1- or 1+. In the example, the hydroxide ion is expressed as OH^-.