Element | Oxidation Number | Exceptions |
---|---|---|
Fluorine |
–1 |
none |
Group 1 or 2 metal |
+1 or +2, respectively |
none |
Hydrogen |
+1 |
If paired with Group 1A or 2A metal to form a metal hydride such as LiH and CaH2 |
Oxygen |
–2 |
If paired with any element listed above that would violate rules 1-4 such as H2O2 or KO2 |
Group 17 (excluding Fluorine) |
–1 |
If paired with any element listed above that would violate rules 1-4 such as ClF, BrO4–, and IO3– |
Rules for Oxidation Numbers
Oxidation number rules
- Oxidation number = 0 for any atom in its pure, elemental state.
- Zn(s): Zn = 0
- H2(g): H = 0
- Oxidation number = charge on a monatomic ion
- Na+: Na = 1
- Cl–: Cl = –1
- Sum of all oxidation numbers for the atoms in a polyatomic ion = charge on the polyatomic ion
- OH–: O + H = –2 + 1 = –1
- Sum of all oxidation numbers = charge on the species
- H2O: O + H + H = –2 + 1 + 1 = 0
- NaCl: Na + Cl = 1 + –1 = 0
Additionally, the rules below are prioritized in the following order: