The oxidation number of chromium in the dichromate ion, Cr2O7 2-, is
- A. plus 3
- B. plus 12
- C. plus 6
- D. plus 7
Explanation
Seven oxygens contribute minus 14, and the overall charge is minus 2, so the two chromium atoms must total plus 12, giving plus 6 each. Quoting plus 12 forgets to divide between the two atoms, which is the commonest slip. Acidified dichromate is reduced to the green chromium three ion, and that colour change is used as a test for a reducing agent.