According to the rule, the oxidation number of 'C'r in CrO5 is +10 which is not possible as Cr has only six electrons in the outermost orbit. So, its oxidation number will assign by their structure. CrO5 has a butterfly structure in which four oxygen atoms are in peroxide linkage and one oxygen atom is in normal covalent bond with Cr atom.
Oxidation number of Cl in CrO5
Therefore, its oxidation umber is +6
Let the oxidation number of 'Cr' is 'x'
so, x + (−4) + (−2) = 0
or, x = +6