Originally Answered: How do I use roman numerals when naming transition metals in compounds?
These roman numerals are used to specify the oxidation state, or charge of that atom. This means that Fe(II) is the same as Fe2+, which means that the Fe atom has lost 2 electrons, leaving it with a positive charge of 2+. This is important to know in order to balance compounds and do reactions. For example, if someone says "what is the formula for iron (II) nitrate?", I can see that the charge on iron, Fe, is 2+ because of the II in parenthesis. I know that nitrate, NO3- has a charge of -1, so I need 2 nitrates for every iron in order to cancel the charge. Therefor, iron (II) nitrate is Fe(NO3)2. Alternatively, if I see the compound Fe(NO3)2, I can figure out the name by knowing that since N03- has a negative charge, and there are 2 of them, together they have a charge of -2, so I need a charge of +2 to cancel it out. Therefore, iron, Fe, has a charge (oxidation state) of 2+, and so it is written as Iron (II) Nitrate. The same goes if it has a charge/oxidation state of 3, it is written and Iron (III). Hope this helps!