Question

Question #4b705

Answer 1

The Roman numerals are used to show the oxidation number of transition metals because some elements have more than one possible oxidation state.

Explanation:

Some metals have the ability to form differently charged ions. For example, iron can form `2+` or `3+` ions. If you simply gave the name iron chloride, you would not know which charge the iron ion possessed.

A Roman numeral is to indicate the charge of the iron.

Iron (`"II"`) means the iron has a `2+` charge
Iron (`"III"`) means that the iron has a `3+` charge

So, iron (`"II"`) oxide would have a chemical formula of `"FeO"`.
(The oxide ion has a `2-` charge to balance the `2+` of the iron to form a neutral compound.)

Iron (`"III"`) oxide would have a chemical formula of `"Fe"_2"O"_3`
(Here you need to find the common multiple of 6, so two iron ions with a `3+` charge will balance the charge of three oxide ions with a `2+` charge.)

Here is a video with added information about how to use Roman numerals in names of compounds.