The difference in energy between the actual structure and the lowest energy resonance structure for the given compound is

Solution

Chemistry ma’am · 3 steps

The real molecule is more stable than any single structure you draw, and that energy gap has a name.

1. Actual structure. When a molecule shows resonance, its real structure is a hybrid of all its contributing structures. Electrons are spread over more atoms, so the hybrid has lower energy than any single structure.
2. Compare energies. Even the most stable contributing structure, the one with the lowest energy, is still higher in energy than the hybrid. The difference between the two measures how much extra stability the delocalisation gives.

   Resonance energy = E_(lowest-energy resonance structure) - E_(actual structure)

3. So. This energy difference is the resonance energy, option (b). For benzene it is about 150 kJ mol⁻¹.

The idea to keep. The more stable a molecule is compared with its best single structure, the larger its resonance energy. Always compare against the lowest-energy contributor, never the average.

Answer from the NTA final answer key. Solution by PrepWiser.