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.

Oxidation Number (Oxidation State)

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.  

The Rules for Assigning Oxidation Number

Oxidation numbers can be positive, negative, or zero. They are assigned based on the following rules:

1. All free elements have an oxidation number zero. The elements could be monoatomic, diatomic, or polyatomic. 
2. In a compound, group 1A elements (all alkali metals) have an oxidation number of +1, while group 2A elements (all alkaline earth metals) have an oxidation number of +2. 
3. Halogens usually have an oxidation number of −1, except in their compounds with oxygen, where they have a positive oxidation state. 
   Fluorine is the most electronegative element. It has a −1 oxidation state in all its compounds.
4. For monoatomic ions, the oxidation number is the same as the charge on the ion. 
5. Oxygen always has an oxidation number of −2, except in peroxides, where its oxidation number is −1.
6. Hydrogen has an oxidation state of +1 with nonmetals and −1 with metals.
7. The sum of the oxidation number for a neutral compound is zero, while for a polyatomic ion, it is equal to the charge on the ion.

This text is adapted from Openstax, Chemistry 2e, Section 4.2: Classifying Chemical Reactions.