Formula of an oxo-anion of chromium $\left(\mathrm{Cr}\right)$ in which it shows the oxidation state equal to its group number (6) is ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$
$2\mathrm{Cr}+\left(-2\times 7\right)=-2$
$2\mathrm{Cr}-14=-2$
$2\mathrm{Cr}=+12$
$\mathrm{Cr}=+6$
Oxidation state of $\mathrm{Cr}$ in ${\mathrm{Cr}}_2{\mathrm{O}}_7^{2-}$ is +6 which is equal to its group number 6 .