Rules to assign oxidation numbers

1. The oxidation number for an atom of any free (uncombined) element is ZERO.
   • Examples: Na, Ca have zero oxidation number.
2. The oxidation number of an element in self-combination is always ZERO.
   • Examples: H₂, O₂, P4have zero oxidation number.
3. In most hydrogen containing compounds, oxidation number of hydrogen is + 1.
   • (Exception is when H combines with alkali metals or alkaline earth to form hydrides of metals such as: NaH, LiH, CaH₂. Then, the oxidation number of H is -1).
4. In compounds involving the alkali metals, the elements are assigned oxidation number of +1.
5. In compounds involving the alkaline earth metals, the elements are asasigned oxidation number of +2.
6. Oxygen is usually assigned an oxidation number of -2 for oxides. It has an oxidation number of -1 in peroxides (H₂O₂).
7. Fluorine always has oxidation number of -1 in compounds. The other elements in that group is usually -1 in compounds with elements of low electronegativity.
8. The sum of oxidation numbers of all the atoms in the formula for a neutral compound is ZERO.
9. The sum of oxidation numbers of an ion or complex ion is the same as the charge on that ion.
10. Negative oxidation number in compounds of two unlike atoms are assigned to the more electronegative atom.
    • Examples: Oxidation numbers
      • HCl: H is assigned +1, Cl is assigned -1
      • H₂O: H is assigned +1, O is assigned -2
11. In combinations of nonmetals not involving hydrogen and oxygen, the nonmetal that is more electronegative is considered negative.
    • Examples: Oxidation numbers
      • CCl₄ C is assigned +4, Cl is assigned -1
      • SF₆ S is assigned +6, F is assigned -1
      • CS₂ C is assigned +4, S is assigned -2