Britannica Sources used in this answer

How do the older names ferric and ferrous relate to iron(III) and iron(II)?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Two diagrams show the three-dimensional structure of a methane molecule: panel A uses a ball-and-stick model with a central carbon atom connected to four hydrogen atoms in a tetrahedral arrangement, while panel B illustrates the bond angle of 109.5 degrees and bond length of 1.09 angstroms between carbon and hydrogen atoms. — chemical compound Methane, in which four hydrogen atoms are bound to a single carbon atom, is an example of a basic chemical compound. The structures of chemical compounds are influenced by complex factors, such as bond angles and bond length.

In chemistry, the terms "ferrous" and "ferric" are traditional naming conventions used to distinguish between the two primary oxidation states of iron.1

Ferrous refers to iron in the +2 oxidation state, containing the ${Fe}^{2 +}$ ion.1
Ferric refers to iron in the +3 oxidation state, containing the ${Fe}^{3 +}$ ion.1

Modern chemical nomenclature typically uses the IUPAC system, which identifies these states more precisely as iron(II) and iron(III), respectively. These older terms remain common in industrial and historical contexts, such as in the naming of ferrous oxide ( $FeO$ ) and ferric oxide ( ${Fe}_{2} O_{3}$ ). Some compounds, such as ferrosoferric oxide ( ${Fe}_{3} O_{4}$ ), contain iron in both the +2 and +3 oxidation states.12