Answer:
Oxidation State
In chemistry, when naming ionic compounds there will sometimes be a roman numeral after the first element. This number represents the oxidation state of the element. An oxidation state is defined as the hypothetical charge of an atom, assuming that all of its bonds are fully ionic. In other words, the oxidation state shows how many electrons an atom gives or receives after ionization.
In ionic compounds, there will only ever be a roman numeral after the first element. This element will always be the cation (positively charged ion). So, the roman numeral will show how many electrons each cation gave.
When Roman Numerals are Necessary
This is necessary when you deal with transition metals or any other element that has multiple oxidation states. For example, iron can give 2 or 3 electrons. So, when naming a compound with iron you need to use a (II) or (III) to show how many electrons each iron atom gave.
When Roman Numerals are Unnecessary
On the other hand, roman numerals are unnecessary when using an element that only has one possible oxidation state. For example, Na can only give 1 electron ever, so it does not need a roman numeral. Additionally, elements in the second group like Mg can only ever give 2 electrons, so they also don't need a roman numeral to follow their name.