Oxidation state of Cr in K2Cr2O7 is
+6
The oxidation number of oxygen is -2 in most of its compounds , except in peroxides like H2O2, BaO2 etc. where it is -1
1.Another interesting exception is found in the compound OF2 (oxygen difluoride) where the oxidation number of oxygen is +2
2.This is due to the fact that fluorine being the most electronegative element known has always an oxidation number of -1.
3. In compounds formed by union of metals with non-metals, the metal atoms will have positive oxidation numbers and the non-metals will have negative oxidation numbers.
4. The oxidation number of alkali metals (Li, Na, K etc.) is always +1 and those of alkaline earth metals (Be, Mg, Ca etc) is +2.
Therefore, O.N. (O) = -2
O.N. (K) = +1 (alkali metal)
K2Cr2O7⟹ 2×(1) +2×n + 7×(-2) = 0
⟹ n =6