By applying various rules of calculating the oxidation number of the desired element in a compound , the oxidation number of each metallic element in its compound is as follows :
`HAuCl_(4) to Au` has 3
`Tl_(2)O to Tl` has 1
`Fe O to Fe` has 2
`Fe_(2)O_(3) to Fe` has 3
`Cul to Cu` has 1
`CuO to Cu` has 2
`MnO to Mn` has 2
`MnO_(2) to Mn` has 4
Therefore , these compounds may be represents as :
`HAu (III) Cl_(4) , Tl_(2)(I)O , Fe (II) O , Fe_(2)(III)O_(3) , Cu(I) I , Cu(II) O , Mn(II) O , Mn(IV)O_(2)`