The reason for different molecular formulas for oxides of the same metals is the difference in oxidation number of metals in different metal oxides.
An example of this is the iron oxides, which can have molecular formulas of FeO and `Fe_2O_3` . Metal oxides are named as 'metal name (N) oxide'. In this case, these iron oxides are called iron (II) oxide and iron (III) oxide or as Fe(II) oxide and Fe(III) oxides or Ferrous oxide and ferric oxide. Iron atoms have oxidation numbers of +2 and +3 (respectively) in these oxides. There are a number of other similar examples such as Mn(II) oxide, Mn (III) oxide and Mn (VIII) oxide with oxidation numbers of +2, +3 and +7, respectively.
Kindly note that metals that have a fixed oxidation number will be simply named as 'metal name oxide' and they will always have same molecular formula.
The group-d elements can have variable oxidation states and thus can have different valency.
Thus, the molecular formula of the oxides of the same metals are different.
There are two oxides of copper.
1) Cu2O = Cuporous oxide, Cu has a valency of 1
2) CuO= Cupric oxide, Cu has a valency of 2
Similarly, there are 2 oxides of iron:
1) FeO = Iron(II)oxide or Ferrous oxide
2) Fe2O3 = Iron(III)oxide or Ferric oxide
Again, Manganese has:
1) MnO2 = Manganese dioxide
2) KMnO4 = Potassium Permanganate
3) Mn2O3 = Manganese (III)oxide
D-block elements, also known as transition metals, are placed from group-3 to group-12 in periodic table. The valence shell electrons in these metals are located in d-orbital and are loosely bound and contribute to high electrical conductivity of the transition metals. They have low ionization potential and show a wide range of oxidation. Due to variable oxidation states of transition metals, they can form various oxides.
Scandium and Zinc are exceptions in d block metals which have only one oxidation state.
Apart from d block metals, some of the p-block metals like Lead and Aluminium also exhibit variable oxidation states.
So we have , Lead(II) Oxide PbO , Lead (IV) Oxide `PbO_2` , Aluminium (II) Oxide (Aluminium Monoxide ) AlO , Aluminium (III) Oxide`Al_2O_3.