In a Stock system name such as iron (III) sulfate, what do the Roman numeral tells us?
The Roman numerals indicate the ionic charge of the cation. In this case, it is an ionic charge of
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:
Another composition of iron is iron (II). If reacting with sulfate, it will yield
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 :)