# What do the Roman numerals in a cation's name indicate?

For example, iron $\textcolor{b l u e}{\left(\text{III}\right)}$ sulfate is the sulfate of iron where the iron has an oxidation state of $\textcolor{b l u e}{+ 3}$, thus ${\text{Fe"_2"(SO"_4")}}_{3}$. The two iron atoms in the formula each belong to separate monatomic ions, so each ion is ${\text{Fe}}^{3 +}$.
Mercury $\textcolor{b l u e}{\left(\text{I}\right)}$ chloride is likewise the chloride of mercury where mercury has the oxidation state $\textcolor{b l u e}{+ 1}$. In the mercury $\left(I\right)$ ion, however, two atoms are joined together in one ion so the chloride is ${\text{Hg"_2"Cl}}_{2}$ and the ion is ${\text{Hg}}_{2}^{2 +}$. The two mercury atoms in the $+ 1$ oxidation state combine to make an ion with two positive charges.