The oxidation number represents the charge of an element in the compound. Oxidation number is a positive or negative number assigned to an atom according to a set of rules. The oxidation number is also called as the oxidation state.
Rules for Assigning Oxidation Numbers:
- The oxidation number of any free element is 0.
- The oxidation number of a monatomic ion is equal to the charge on the ion.
- Some atoms have several possible oxidation numbers. For example, iron can be Fe+2 or Fe+3.
- The oxidation number of each hydrogen atom in most of its compounds is +1, except in metal hydrides (e.g., NaH, LiH) where it is -1.
- The oxidation number of hydrogen is +1 when it is combined with a nonmetal. Example: CH4, NH3, H2O, and HCl.
- The oxidation number of hydrogen is -1 when it is combined with a metal. Example: LiH, NaH, and CaH2.
- The oxidation number of oxygen in a compound is -2, except in peroxides (H2O2) where it has oxidation number of -1.
- In polyatomic ions, the algebraic sum of the oxidation numbers of all the atoms must equal to the charge of that polyatomic ion.
- In all neutral compounds, the algebraic sum of the oxidation numbers of all the atoms is 0.
- All the elements of Group 1 have positive oxidation number of +1 except hydrogen.
- All the elements of Group 2 have positive oxidation number of +2.
The atoms in Na, O2, N2, Pb, He, H2, Ne, Zn, for example, have oxidation numbers of 0.
For example, the oxidation number of chlorine in the Cl- ion is -1. The oxidation number of bromine in the Br- ion is -1. The oxidation number of sodium in the Na+ ion is +1. List of monoatomic ions oxidation numbers