Question

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

Answer 1

They indicate the oxidation state of the element in the ion, usually a metal. If the compiund is ionic and the ion has only one atom, this is also the charge of the ion.

Explanation:

For example, iron `color(blue)(("III"))` sulfate is the sulfate of iron where the iron has an oxidation state of `color(blue)(+3)`, thus `"Fe"_2"(SO"_4")"_3`. The two iron atoms in the formula each belong to separate monatomic ions, so each ion is `"Fe"^{3+}`.

Mercury `color(blue)(("I"))` chloride is likewise the chloride of mercury where mercury has the oxidation state `color(blue)(+1)`. In the mercury `(I)` ion, however, two atoms are joined together in one ion so the chloride is `"Hg"_2"Cl"_2` and the ion is `"Hg"_2^{2+}`. The two mercury atoms in the `+1` oxidation state combine to make an ion with two positive charges.

This video provides some additional examples of how to use Roman numerals when naming compounds.