Formula of an oxo-anion of chromium (Cr) in which it shows the oxidation state equal to its group number (6) is Cr2O72−
2‌Cr+(−2×7)‌=−2
2‌Cr−14‌=−2
2‌Cr‌=+12
Cr‌=+6
Oxidation state of Cr in Cr2O72− is +6 which is equal to its group number 6 .