Look at the three resonance structures of naphthalene shown...

Look at the three resonance structures of naphthalene shown in Section $9-5,$ and account for the fact that not all carbon-carbon bonds have the same length. The C1-C2 bond is 136 pm long, whereas the C2-C3 bond is $139 \mathrm{pm}$ long.

Key Concepts

Resonance Structures

Resonance structures are different Lewis structures for a molecule that depict alternate arrangements of electrons, especially ?-electrons, within a conjugated or aromatic system. They are used to show the delocalization of electrons and help explain molecular stability by providing multiple contributing forms rather than a single fixed structure.

Resonance Hybrid

The resonance hybrid is the true representation of a molecule’s electronic structure, derived from the averaging of all possible resonance structures. This concept explains why certain bonds in molecules, particularly in aromatic systems, exhibit bond lengths that are intermediate between those expected for strictly single and double bonds.

Bond Order

Bond order is a measure of the number of bonding electrons between two atoms. In molecules where resonance plays a role, the bond order may be fractional, reflecting the averaged contributions from different resonance forms, which in turn affects the bond lengths and strengths.

Aromaticity

Aromaticity is a property of cyclic, planar molecules with a continuous overlap of p-orbitals, which leads to a delocalized ?-electron system following Hückel's rule. It imparts special stability and results in uniformity of bond characteristics, such as intermediate bond lengths, distinct from typical isolated double or single bonds.

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