Rules for Oxidation Numbers

Oxidation number rules

  1. Oxidation number = 0 for any atom in its pure, elemental state.
    • Zn(s):   Zn = 0
    • H2(g):   H = 0
  2. Oxidation number = charge on a monatomic ion
    • Na+:   Na = 1
    • Cl:   Cl = –1
  3. Sum of all oxidation numbers for the atoms in a polyatomic ion = charge on the polyatomic ion
    • OH:   O + H = –2 + 1 = –1
  4. 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:

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