Calculation of oxidation number: 

• O. S. of all the elements in their elemental form (in standard state) is taken as zero O. S. of elements in Cl₂, F₂, O₂, P₄, O₃, Fe(s), H₂, N₂, C(graphite) is zero.
• Common O. S. of elements of group one (1st) is one. Common O. S. of elements of group two (2nd) is two.
• For ions composed of only one atom, the oxidation number is equal to the charge on the ion.
• The oxidation number of oxygen in most compounds is –2. While in peroxides (e.g., H₂O₂, Na₂O₂), each oxygen atom is assigned an oxidation number of –1, in super oxides (e.g., KO₂,RbO₂) each oxygen atom is assigned an oxidation number of –(½).
• In oxygen difluoride (OF₂) and dioxygen difluoride (O₂F₂), the oxygen is assigned an oxidation number of +2 and +1,respectively.
• The oxidation number of hydrogen is +1 but in metal hydride its oxidation no. is –1.
• In all its compounds, fluorine has an oxidation number of –1.
• The algebraic sum of the oxidation number of all the atoms in a compound must be zero.
• In polyatomic ion, the algebraic sum of all the oxidation numbers of atoms of the ion must equal the charge on the ion.