Rules for Assigning Oxidation Numbers

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1- The oxidation number of a free element is always 0.

The atoms in He and N2, for example, have oxidation numbers of 0.

2- The oxidation number of a monatomic ion equals the charge of the ion.

For example, the oxidation number of Na+ is +1; the oxidation number of N3- is -3.

3- The usual oxidation number of hydrogen is +1.

The oxidation number of hydrogen is -1 in compounds containing elements that are less electronegative than hydrogen, as in CaH2.

4- The oxidation number of oxygen in compounds is usually -2.

Exceptions include OF2, since F is more electronegative than O, and BaO2, due to the structure of the peroxide ion, which is [O-O]2-.

5- The oxidation number of a Group IA element in a compound is +1.

6- The oxidation number of a Group IIA element in a compound is +2.

7- The oxidation number of a Group VIIA element in a compound is -1, except when that element is combined with one having a higher electronegativity.

The oxidation number of Cl is -1 in HCl, but the oxidation number of Cl is +1 in HOCl.

8- The sum of the oxidation numbers of all of the atoms in a neutral compound is 0.

9- The sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

For example, the sum of the oxidation numbers for SO42- is -2.