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

1 Answer
Oscar L. · mrpauller.weebly.com
Aug 9, 2016

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.

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