A polyatomic ion is an ion made of many atoms (poly = many, atomic = atoms); most of the time there is one main atom and several other oxygen atoms. The atoms in a polyatomic ion are nonmetals and are bonded together by covalent bonds. This means that within the polyatomic ion, electrons are shared. Even with this sharing, however, most of those atom clusters don't have enough electrons to be stable, meaning that don't fulfuill the octet rule. In order to solve this problem, the polyatomic molecule picks up extra electrons to become an ion. Most polyatomic ions are anions, that means they must gain an electron and are negatively charged. Only one or two of them lose an electron to become positively charged. Unlike the main group elements, you cannot look at the periodic table to figure out the charge of a polyatomic ion. They have been studied for a long time and are listed in a table by their charge.

Polyatomic ions have special nomenclature as well. Even though they are covalent compounds (and mostly anions), they do not follow the rules for covalent compounds or end in the -ide ending. With few exceptions, all polyatomic ions end in -ate or -ite. The difference has to do with the number of oxygens attached to the main element. Ions ending in -ate have one more atom than ions ending in -ite. So for example, sulfate has four oxygen atoms and sulfite has three. Despite this, both the -ate and -ite versions have the same charge: -2 in this case. Sometimes polyatomic ions can have one more oxygen than the -ate version or one less than the -ite version. In this case, the prefixes per- and hypo- are used. For example, chlorate has one chlorine and three oxygens. Chlorite has one chlorine and two oxygens. Perchlorate has one chlorine and four oxygens and hypochlorite has one chlorine and one oxygen. Regardless of the number of oxygens, the charge is the same for all of them. This is true for all polyatomic ion "families."