we've learned how to name ionic compounds with monoatomic and polyatomic ions and we've learned that these ions come together to form neutral compounds in particular ratios but this has depended on knowing the common charge of these ions and it's not always the case that there is only one formal charge that an ion of a particular element can have many of the transition metals can have different charges or oxidation States for this reason when we list the names of ionic compounds containing these particular elements we have to have a way to list their charge in the name the way we do this is with Roman numerals let's say we are combining iron and chlorine the chloride ion is always 1 minus but iron can be either 2+ or 3+ this means that iron and chlorine when combined can either make fecl2 or fe3 these would both be called iron chloride but we must include information in the name that reports the charge or oxidation state of the iron atom fe2 will be Iron 2 chloride with a Roman numeral 2 in parentheses after the word iron fecl3 will be called iron 3 chloride with the Roman numeral three this is important to list because if we simply say iron chloride no one can be certain precisely what molecule we are describing there is also an old convention to give different names to these differently charged metal ions for example we can call fe2+ the feris ion and fe3+ the feric ion which can result in the compounds Ferris chloride and feric chloride but this convention has been largely abandoned though still used by some areas of industry in general we simply have to know that if you are naming an ionic comp compound and the cation involved is a transition metal we will need to include a Roman numeral in parentheses after the name of the cation to specify the charge or oxidation state that the cation is exhibiting in the compound