In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the oxidation number (or oxidation state). It represents the effective charge of an atom or element, which is assigned using a set of rules.
In the case of an ionic compound, oxidation numbers are assigned based on the number of electrons transferred between reacting species. For example, in the formation of calcium chloride (CaCl2), calcium loses two valence electrons, and the two chlorine atoms gain one electron each. In CaCl2, calcium’s oxidation state is +2, and each chlorine’s oxidation state is −1.
In the case of covalent compounds, electrons are not gained or lost but instead are shared between the atoms. The atom with a greater attraction for electrons pulls the shared pair more strongly. Reactions involving covalent compounds are identified as redox by applying the concept of oxidation number to track electron movements. Oxidation states help us easily identify the species being oxidized and reduced in redox reactions.
Oxidation numbers can be positive, negative, or zero. They are assigned based on the following rules:
This text is adapted from Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions.
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