RULES FOR ASSIGNING OXIDATION NUMBERS

  1. The oxidation number of an atom in its elemental state is zero . . . Li0, Ag0, Ar0, H02, O02, N02, Cl02, F02, I02, Br02, P04, S08
  2. The oxidation number of a monatomic ion is equal to the ion's charge . . .   Cs1+= Cs+1       Mg2+= Mg+2       O2-= O-2
  3. The oxidation number of a nonmetal . . . .
    1. Hydrogen:
      1. +1 when combined with nonmetals . . . H+12O, NH+14, H+1Br
      2. -1 when combined with metals . . . NaH-1, AlH-13
    2. Fluorine: -1 always
    3. Oxygen: -2 except when
      1. combined with fluorine . . . O+1F-1, O+2F-12
      2. present in the peroxide ion (O22–   ) . . . H+12O-12, Na+12O-12
    4. Halogens (Cl, Br, I): -1 except when combined with oxygen or another halogen . . . Cl+12O-2
  4. The oxidation number of a metal . . . .
    1. The oxidation numbers of Li, K, Na, Rb, Cs and Be, Mg, Ca, Sr, Ba metals are always +1 and +2, respectively.
    2. The oxidation number of any remaining metal is assigned by creating a simple equation based on one of the following facts:
      1. the sum of the oxidation numbers in a molecule equals zero . . .

        for KMnO4, we can assign K (Rule 4a) and O (Rule 3c) . . . K+1MnO-24

        however, the total oxidation number for four oxygens is not -2, it's -8 . . . K+1+1MnO-2-84

        now, we can create an equation to solve for the oxidation number of manganese . . . K+1+1Mn+x+xO-2-84      = 0

        solving for x gives K+1+1Mn+x+7O-2-84      = 0

        the oxidation number of Mn is +7

      2. the sum of the oxidation numbers in an ion equals the ion's charge . . .

        for the dichromate ion, Cr2O72–, we can assign O (Rule 3c) . . . Cr2O-272–

        however, the total oxidation number for seven oxygens is not -2, it's -14 . . . Cr2O-2-1472–

        now, we can create an equation to solve for the oxidation number of chromium . . . Cr+x+2x2O-2-147      = -2

        solving for x gives Cr+62(+6)2O-2-147      = -2

        the oxidation number of Cr is +6