What is the oxidation number of "Cr" in Cs2Cr2O7?

Answer 1

To find the oxidation number of Cr in Cs₂Cr₂O₇

treat it as a completely ionic compound

(even though the Cr–O bonds are not completely ionic)

So Cs₂Cr₂O₇ "becomes" 2 Cs⁺ + 2 Crⁿ + 7 O^2–

where n is the oxidation number of Cr that balances the overall charge.

Adding Up Charges

2 Cs⁺ = +2

7 O²⁻ = –14

total: –12

So 2 Crⁿ must have a charge of +12

which means that

each Crⁿ has a charge of +12/2 = +6

so n = +6

Answer

The oxidation number of Cr in Cs₂Cr₂O₇ is +6