<span>Answer:
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
â’
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.)</span>
