Assigning Oxidation Numbers

  1. Atoms in their elemental state have an oxidation number of zero (i.e. silver metal, Ago, and Hydrogen gas, H2o, have oxidation numbers of 0).
  2. Monoatomic ions have an oxidation number equal to their charge (i.e. Na+ and Cl- have +1 and -1 oxidation numbers, respectively).
  3. Compounds
    • Fluorine always has a -1 oxidation number.
    • Oxygen usually has a -2 oxidation number with the following exceptions:
      • Oxygen has a positive oxidation number in Oxygen-Fluorine compounds: OF2, O2F2, O3F2.
      • Oxygen has a -1 oxidation number in peroxides (i.e. hydrogen peroxide, H2O2, Hydrogen's oxidation number is +1, Oxygen's oxidation number is -1).
    • Hydrogen has a +1 oxidation number except when bonded to a metal (i.e. the H in NaH has a -1 oxidation number, H- is called the hydride ion).
  4. Atoms with unknown or variable oxidation numbers
    • Use Steps 1 - 3 to assign the oxidation numbers of the "known" atoms.
    • Sum these oxidation numbers.
    • Subtract this sum from the charge of the species.
    • Divide by the number of atoms in the formula with an unknown oxidation number
      • Example: K2Cr2O7 (Cr's oxidation number is "unknown").
        • Applying steps 1-3, K's oxidation number is +1 (K2 is +2) and O's oxidation number is -2 (O7 is -14).
        • The sum of K's and O's oxidation numbers is -12.
        • The overall charge of K2Cr2O7 is zero.   ∴  0 - (-12) = +12
        • Division by the number of Cr atoms in K2Cr2O7 gives +12 ÷ 2 = +6. The oxidation number of Cr is +6.