The dichromate ion

Dichromate ion is the dimer of $HCr{O}_4^{-}$

In acidic medium dichromate react with ${\mathrm{H}}_2{\mathrm{O}}_2$ forming blue ${\mathrm{CrO}}_5$ initially which decomposes to $C{r}^{3+}$ and oxygen $K_{2}C{r}_2{O}_7+4H_{2}S{O}_4+3H_2{O}_2\to K_{2}S{O}_4+C{r}_2{\left(S{O}_4\right)}_3+7H_{2}O+3O_2$ zinc in acid medium reduces $C{r}^{+6}$ to $C{r}^{+2}$.  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.