Why do certain compounds require a roman numeral?
Some metals can have more than one charge and they are used to show the charge.
For example, in PbBr
From the periodic table (or google) we can find out that Br has an ionic charge of -1 ( This means that Br would need 1 more electron to become a noble gas (Krypton), and since it's bonded it did get that electron. Furthermore, since it has one more electron, it now has a charge of -1)
Since there are 4 atoms of Br, the total charge of all the Br is -4 (charge times amount = -1*4)
For the compound to be balanced, it needs to have a charge of 0. What plus -4 is equal to 0? 4. This means that Pb needs to have a charge of 4.
So, since Pb has a charge of 4, we would write it like this: Lead (IV) Bromide. (The subscript 4 at the end of Bromide is not included since it can be figured out from Pb's charge).
This video provides some additional examples of how to use Roman numerals when naming compounds.
So there you go. Ask if you have any more questions.