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An oxidation number is a hypothetical charge assigned to an atom in a compound that represents the effective charge if shared electrons in heteronuclear bonds were completely transferred to the more electronegative atom. Oxidation numbers track electron movement in oxidation reduction reactions electron transfer reactions, helping identify which atoms are oxidized or reduced during chemical reactions.

According to the first rule for assigning oxidation numbers, all free elements have an oxidation number of zero, regardless of whether they are monoatomic, diatomic, or polyatomic. This applies to elemental forms like O₂, S₈, or individual atoms before they form compounds.

In ionic compounds, oxidation numbers are assigned based on the number of electrons transferred between reacting species. In covalent compounds, electrons are shared rather than transferred, so oxidation numbers are assigned hypothetically by assuming complete transfer to the more electronegative atom. Both allow identification of redox reactions.

Oxygen has an oxidation number of −2 in most compounds because it is highly electronegative and typically gains two electrons when bonding. The exception is in peroxides, where oxygen has an oxidation number of −1 due to the O-O bond structure.

A reaction is a redox reaction if the oxidation numbers of atoms change during the reaction. If oxidation numbers remain unchanged, it is not a redox reaction. For example, in the formation of SO₂, sulfur's oxidation number increases from zero to +4 while oxygen decreases from zero to −2, indicating electron transfer and confirming it as a redox reaction.

For polyatomic ions, the sum of oxidation numbers equals the charge on the ion. For neutral compounds, the sum of oxidation numbers equals zero. This rule helps determine unknown oxidation numbers when other atoms' oxidation states are known in complex compounds.

In covalent bonds, the more electronegative atom is assigned the electrons in the bond when calculating oxidation numbers. For example, in hydrogen chloride, chlorine is more electronegative, so it receives hydrogen's electron, giving chlorine an oxidation number of −1 and hydrogen +1, even though the bond is covalent.