You will have come across names like iron(II) sulphate and iron(III) chloride. The (II) and (III) are the oxidation states of the iron in the two compounds: +2 and +3 respectively. That tells you that they contain Fe2+ and Fe3+ ions.
Example
Formula
Name of the compound
FeCl2
Iron(II) chloride
FeCl3
Iron (III) chloride
MnO2
Manganese(IV) oxide
Mn(NO3)2
Manganese (II) nitrate
PbCl2
Lead(II) chloride
Transition metals always show difference oxidation state as shown in the table below.
Metal
Oxidation state
Fe
+2, +3
Cu
+1, +2
Mn
+2, +4, +6, +7
Non-metal elements (except fluorine) usually have more than one oxidation state.