Question

What is the oxidation state of Cr in Cr₂O₇²^-

Answer 1

Oxygen is more electronegative than chromium, so it'll take its usual −2 charge here. There are seven oxygen atoms, so the total charge is −2⋅7=−14.

Let x be the oxidation number of two chromium atoms. So, we got:

x+(−14)=−2

x−14=−2

x=12

So, two chromium atoms have a total oxidation number of +12. Since one is not more electronegative than the other, we can divide the number by two to get one chromium's oxidation number.
12/6 =6

So, a chromium atom here has an oxidation number of +6.