to find out the oxidation number of chromium in the given compound....u've to look into the lewis structure of the compound....
however , by the general method, the O.N. comes out to b +10....but itz not correct....
in itz real structure....one Cr atom is attached to 4 oxygen atoms....double-bonded with two O-atoms and single bonded with the rest two O-atoms....
oxygen being more electronegative attracts the shared electron towards itself....the double-bonded O-atoms retains 4 electrons while the single-bonded retains 2 electrons.....
in that case...one Cr-atom loses 6 electrons and it retains a positive charge of +6......
so, itz O.N. becomes +6....
hope it aids....
approve it yss...