Oxidation NumberOxidation Number

This is a signed (+ or –) number that This is a signed (+ or –) number that indicates the indicates the actual chargeactual charge or the or the apparent chargeapparent charge that an atom of an that an atom of an element would have in a particular element would have in a particular situation.situation.

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #1:Rule #1:

The oxidation number of a The oxidation number of a free elementfree element is zero (0). is zero (0).

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #2:Rule #2:

The oxidation number of a The oxidation number of a monatomic ionmonatomic ion is equal is equal to the charge on the ion.to the charge on the ion.

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #3:Rule #3:

The oxidation number of The oxidation number of hydrogenhydrogen when in a when in a compound is +1; except when combined with a compound is +1; except when combined with a metal, then it is – 1.metal, then it is – 1.

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #4:Rule #4:

The oxidation number of The oxidation number of oxygenoxygen when in a when in a compound is – 2; except when it would violate compound is – 2; except when it would violate other rules, then it is – 1.other rules, then it is – 1.

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #5:Rule #5:

The oxidation numbers of the atoms of a The oxidation numbers of the atoms of a compoundcompound must add up to zero (0). must add up to zero (0).

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #6:Rule #6:

The oxidation numbers of the atoms of a The oxidation numbers of the atoms of a polyatomic ionpolyatomic ion must add up to equal the must add up to equal the chargecharge on on the ion.the ion.

Rules for Assigning Oxidation NumbersRules for Assigning Oxidation Numbers

Rule #7:Rule #7:

The oxidation numbers of The oxidation numbers of Group IA elementsGroup IA elements in a in a compound is always +1; compound is always +1; Group IIA elementsGroup IIA elements in a in a compound always have an oxidation of +2; compound always have an oxidation of +2; Group VIIA Group VIIA elementselements when written last in a binary compound will when written last in a binary compound will always have an oxidation number of – 1.always have an oxidation number of – 1.