# In a Stock system name such as iron (III) sulfate, what do the Roman numeral tells us?

Mar 17, 2017

The Roman numerals indicate the ionic charge of the cation. In this case, it is an ionic charge of ${3}^{+}$.

#### Explanation:

Transition metals are notorious for having multiple ionic charges. Different from being an isotope - an atom that has a different number of neutrons - elements that have Roman numerals right after their chemical name indicate the specific charge of that element.

For example, iron (III) sulfate: $F {e}_{\text{2"(SO_"4")_"3}}$ means that the iron component of the compound is a transition metal that has an ionic charge of ${3}^{+}$ - negative because it acts as the cation.

Another composition of iron is iron (II). If reacting with sulfate, it will yield $F e S {O}_{\text{4}}$. Unlike the previous compound, this one has only 1 iron atom (with an ionic charge of ${2}^{+}$), and 1 sulfate atom, instead of 2 iron atoms, and 3 sulfate atoms. This results in a new compound being dealt with.

There are many other atoms that behave this way - having multiple ionic charges.

This video discusses additional examples of how to use the stock system for naming compounds.

Hope this helps :)