ionic-covalent resonance

In chemistry, resonance describes a molecule whose actual state cannot be represented by a single structure but is a combination of several distinct, alternative structures. The molecule is said to "resonate" among these structures, and its true form is a resonance hybrid. This resonance stabilizes the molecule, lowering its energy compared to any individual structure. The difference in energy is known as resonance energy.1

A key concept related to resonance is ionic-covalent resonance. This occurs when a bond has characteristics of both ionic and covalent bonding. It can be viewed as a blend between a purely covalent form (shared electrons) and a purely ionic form (electrons fully transferred to one atom). As the difference in electronegativity between bonded atoms increases, the bond leans more towards ionic character. For example, in hydrogen chloride (HCl), the bond is a resonance hybrid of covalent and ionic forms, with a greater contribution from the ionic structure (H⁺Cl⁻) due to the electronegativity difference, resulting in a polar bond.2

Even bonds between identical atoms, like in chlorine gas (Cl₂), can be described by ionic-covalent resonance. However, because the contributions of the ionic forms (Cl⁻Cl⁺ and Cl⁺Cl⁻) are equal and their dipoles cancel, the bond is considered nonpolar and predominantly covalent. Resonance also explains why molecules like ozone (O₃) have equivalent bonds, even though simple Lewis structures might suggest a mix of single and double bonds.23