In the redox conversion of Ni2+ to NiO4−, the oxidation number of Ni goes from (−2, 0, +2) to (−1, +1, +7, +9). Recall that the oxidation number of oxygen is typically −2.

1. 0; +9
2. +2; +7
3. +2; −1
4. −2; +9
5. −2; +7
6. 0; +1
7. 0; +7
8. +2; +9
9. −2; +1