When are you supposed to use roman numerals when naming chemicals?

Roman numerals are usually used to indicate the oxidation state of metals, they are not really required though so I would just stay away from using Roman numerals in naming chemicals.





Just to add a comment to another explanation. the compounds FeCl3 and FeCl2 do not have to be both called Iron Chloride. The first one can be called ferric chloride and the second one ferrous chloride. You could also say Iron trichloride and iron dichloride. Just to emphasize the fact that this part of naming chemicals is not a requirement and I think that Roman numerals should not be used.

When are you supposed to use roman numerals when naming chemicals?
To Christophe G, you only use the prefixes (di,tir,tetra, etc.) if the compound is covalent, and seeing that iron is a metal and chloride is a non-metal, they are ionic. If the cation is an transition element, you HAVE to use roman numeral to explain the charge. Report Abuse

Reply:Roman numerals are used to describe the amount of charge on a transition metal in a compound. For group 1 and 2 metals, no roman numeral is needed because they will always have a +1 or +2 charge respectively. Transition metals, though, have more than one possible charge that they can carry (oxidation states). So when naming a compound, you need a way to state the charge.





For ex.





FeCl2 and FeCl3





Without any roman numerals, both would be named iron chloride. But clearly they are different compounds so their names need to be different.





The first is iron(II) chloride


the second is iron(III) chloride.





That's pretty much all it is.