The dichromate ion

Dichromate ion is the dimer of HCrO4−In acidic medium dichromate react with H2O2 forming blue CrO5 initially which decomposes to Cr3+ and oxygen K2Cr2O7+4H2SO4+3H2O2→K2SO4+Cr2SO43+7H2O+3O2 zinc in acid medium reduces Cr+6 to Cr+2.  All O−Cr−O bond angles are not same because the terminal Cr-O bonds have double bond character due to resonance but the bridge bond have single bond character.