How are oxidation numbers used when naming ionic compounds?
You write the oxidation numbers as Roman numerals in the name.
Most transition metals and a few other metals form cations with more than one oxidation number. To distinguish these cations, we add the oxidation number as a capital Roman numeral in parentheses.
We write the name of the ionic compound as
name of cation(oxidation number in Roman numerals) + name of anion
Note that there is no space between the name of the cation and the parenthesis.
For example, the oxidation number of Fe in FeCl₂ is +2. Fe²⁺ is iron(II), and the name of the compound is iron(II) chloride.
The oxidation number of Fe in FeCl₃ is +3. Fe³⁺ is iron(III), and the name of the compound is iron(III) chloride.
Other examples are
PbO = lead(II) oxide; PbO₂ = lead(IV) oxide
Hg₂Cl₂ = mercury(I) chloride; HgCl₂ = mercury(II) chloride