When is it appropriate to use Roman Numerals in naming compounds?
It is appropriate to name Compounds with roman numerals when a transition metal say iron has more than one ion ie; FeO is written Iron(II)Oxide because oxygen has a 2- charge and iron therefore has a 2+ charge making it Iron(II) ion. A subscript in any formula stands for the number of atoms/ions the preceding symbol has in the formula so H2O has 2 hydrogen atoms therefore the 2 subscript is after H.