The oxidation number indicates how much electrons have moved during a reaction. It can be found mechanically according to a set of rules.
Basic Knowledge
Rules for oxidation numbers:
① Atoms in an elementary substance → 0 (Examples: H2, O2, Cu, and Fe are all 0)
② Monatomic ions → the charge of the ion (Examples: Na+ is +1, Cl- is -1)
③ H = +1 in compounds (Exception: -1 in metal hydrides such as NaH)
④ O = -2 in compounds (Exceptions: -1 in the peroxide H2O2, and +2 in OF2)
⑤ The sum of the oxidation numbers of an entire compound = 0; for a polyatomic ion, it equals the charge of the ion.
Example: For S in H2SO4: (+1)×2 + S + (-2)×4 = 0 → S = +6.
Oxidation number（A numerical value expressing the distribution of electrons possessed by an atom as an integer. Always include + or -）
Elementary substance（H2, O2, Cu, and others all have an oxidation number of 0）
Oxidation number of hydrogen（Basically +1; -1 in metal hydrides）
Oxidation number of oxygen（Basically -2; -1 in peroxides and +2 in OF2）
Total for a compound（A neutral compound = 0; a polyatomic ion = its charge）
Change in oxidation number（Increase → oxidation; decrease → reduction）
Further Study (Background・Significance)
By looking at changes in oxidation numbers, we can immediately determine which atoms have been oxidized or reduced in a reaction.
Example: Cu + 2H2SO4 → CuSO4 + SO2 + 2H2O (hot concentrated sulfuric acid)
・Cu: 0 → +2 (oxidized, reducing agent)
・S (some H2SO4): +6 → +4 (reduced, oxidizing agent)
An increase in oxidation number means oxidation, and a decrease means reduction. Remembering just this allows you to understand the flow of electrons even in complex reactions.
Nitrogen, sulfur, halogens, and metals in particular readily undergo large changes in oxidation number (N: -3〜+5, S: -2〜+6, etc.).
- Elementary substance = 0
- H = +1 (exception: NaH and others are -1)
- O = -2 (exception: O in the peroxide H2O2 is -1)
- Entire compound = 0; polyatomic ion = charge
- Increase in oxidation number → oxidation
- Decrease in oxidation number → reduction
- S in H2SO4 = +6; N in HNO3 = +5
Points to Note (Easily Confused)
① There are exceptions: O in the peroxide (H2O2) is -1, and O in the fluoride compound (OF2) is +2. ② H in metal hydrides (NaH, CaH2) is -1. ③ Oxidation numbers are not always integers, but only integers are studied in high school. ④ Always include the +/- sign. Do not write “6” alone for “+6.”
Practice
- Find the oxidation number of S in H2SO4.
- Find the oxidation number of Mn in KMnO4. (K=+1, O=-2)
- In
Cl2 + 2KBr → 2KCl + Br2, which atom was oxidized?