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Why do some ionic compound names include Roman numerals?

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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, Roman numerals are used in the names of ionic compounds to specify the oxidation state or charge of a metal ion when that metal is capable of forming more than one type of cation.123

Many metal atoms, such as iron, lead, tin, and copper, can exist in multiple ionic forms with different charges. For example, iron can form both ${Fe}^{2 +}$ and ${Fe}^{3 +}$ ions. To distinguish between the resulting compounds, the Roman numeral is placed in parentheses after the name of the metal to indicate its specific charge.12

Consequently, ${FeCl}_{3}$ is named iron(III) chloride because it contains the ${Fe}^{3 +}$ ion, while ${FeCl}_{2}$ is named iron(II) chloride because it contains the ${Fe}^{2 +}$ ion. This systematic nomenclature ensures clarity and precision, as the chemical properties of these compounds differ based on the oxidation state of the metal.123