Nomenclature Based on Oxidation States

Many metals exhibit two or more oxidation states. This is particularly true of the transition metals, where cations with ONs of +2 and +3 are common, but also of the heavier main-group metals such as thallium (ONs +1 and +3), and tin and lead (ONs of +2 and +4).

The modern system of nomenclature uses ONs directly in naming compounds. Thus in FeO and Fe₂O₃, the ONs of Fe are +2 and +3, respectively, and we name these compounds iron(II) oxide and iron(III) oxide. Similarly, phosphorus(III) cholride is PCl₃ and phosphorus(V) chloride is PCl₅.

The traditional nomenclature for salts of such metals is to use the suffix -ic for the higher ON and –ous for the lower, often using the Latin name for the element, e.g., ferrous (Fe²⁺) and ferric (Fe³⁺), stannous (Sn²⁺) and stannic (Sn⁴⁺), chromous (Cr²⁺) and chromic (Cr³⁺).

Monatomic and polyatomic anions containing a single element are named with the -ide suffix, regardless of the formal oxidation state, e.g., chloride (Cl^-), sulfide (S^2-), and triiodide (I₃^-).

Both metals and nonmetals form oxoanions with the central atom in a variety of oxidation states, e.g., NO₂^- and NO₃^-, MnO₄^2- and MnO₄^-. The traditional names reflect these differences by using the suffix - ate for the higher oxidation number and -ite for the lower. When more than two oxidation states are found, the highest uses the -ate suffix with the prefix per-, and the lowest uses the -ite suffix with the prefix hypo-. Some oxoanions with two central atoms linked by an oxygen use the di- prefix (indicating 2), others use the pyro- prefix, (indicating their origins as products obtained by heating—pyrolysis).

Some of the more common polyatomic anions are listed in Table 1.1.

Table 1.1. Common Anions Exhibiting Variable ON's