# Question #4b705

Apr 30, 2014

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 ($\text{II}$) means the iron has a $2 +$ charge
Iron ($\text{III}$) means that the iron has a $3 +$ charge

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

Iron ($\text{III}$) oxide would have a chemical formula of ${\text{Fe"_2"O}}_{3}$
(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.