Oxidation States

Oxidation states (also called oxidation numbers) provide a systematic way of tracking electron distribution in chemical species. They are essential for identifying redox reactions, naming compounds, and balancing redox equations. Mastering the rules for assigning oxidation states is a fundamental A-Level skill.

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What Is an Oxidation State?

An oxidation state is a number assigned to an atom in a chemical species that represents the number of electrons it has lost, gained, or shared unequally compared to its elemental form. It is essentially a bookkeeping tool for tracking electrons.

• A positive oxidation state means the atom has (effectively) lost electrons.
• A negative oxidation state means the atom has (effectively) gained electrons.
• An oxidation state of zero means the atom is in its elemental form or has not been oxidised/reduced.

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Rules for Assigning Oxidation States

Apply these rules in order of priority:

Rule 1: The oxidation state of an uncombined element is 0. Examples: Fe(s) = 0, O₂(g) = 0, S₈(s) = 0, Na(s) = 0

Rule 2: The oxidation state of a monatomic ion equals its charge. Examples: Na⁺ = +1, Cl⁻ = −1, Fe³⁺ = +3, O²⁻ = −2