The actual delocalized electronic structure, an average of the contributing resonance forms.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
The resonance hybrid is the actual electronic structure of the molecule, a weighted average of its contributing resonance forms. Contributors with lower formal charges, full octets and negative charges on electronegative atoms carry more weight in the average. The resonance energy is the extra stability of the real molecule compared with the lowest-energy hypothetical contributor, and it can be estimated from thermochemical data. The hybrid is a single averaged structure, not a rapidly flipping mixture of forms. Delocalized bonds in the hybrid therefore have fractional bond orders.
Key ideas: Rank contributors by: full octets, minimal charge separation, negative charge on the more electronegative atom, and more bonds. Average bond order is the total bonds divided by the number of equivalent positions, so benzene is 1.5 and carbonate is 1.33. Resonance energy is estimated by comparing measured and calculated enthalpies of hydrogenation, as for benzene.
Common mistakes and connections: Students say the hybrid is a mixture of the structures, but it is a single structure that none of the drawings fully shows. Unequal contributors do not weigh equally. Also, the nuclei do not move, only the electron depiction changes. Connects to formal charge, delocalization, bond lengths and carboxylate stability.
In carbonate all three C-O bonds are identical at about 129 pm, between single and double, and each oxygen carries two-thirds of a negative charge. The interactive lesson shows this as a live animation or chart.
Carbonate has three equivalent resonance forms, so each C-O bond has a bond order of 4/3 and each oxygen carries an average charge of -2/3.
The actual delocalized electronic structure, an average of the contributing resonance forms.
Resonance Hybrid is a intermediate topic in Bonding & Molecular Structure. It builds on the vocabulary of beginner topics in this area.
Study these earlier topics in Bonding & Molecular Structure first: Lone Pairs, Formal Charge, Resonance.
The next topics in Bonding & Molecular Structure are Expanded Octet and Incomplete Octet.
Yes. The interactive lesson includes a self-check question on Resonance Hybrid, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Resonance Hybrid is topic 13 of 104 in Bonding & Molecular Structure.