Final answer:
A molecule exhibits resonance when it has at least one double or triple bond, as these allow for delocalized electrons, which means multiple Lewis structures, representing the electron distribution, contribute to the overall structure known as the resonance hybrid.
Step-by-step explanation:
In order for a molecule to exhibit resonance, the Lewis structure must contain at least one double or triple bond. This allows for the possibility of delocalized electrons moving between different arrangements where the atoms are in the same positions but the distribution of electrons varies. This concept is shown in resonance forms or resonance structures, where each form contributes to the overall structure of the molecule, known as the resonance hybrid. In essence, the true structure of the molecule is a hybrid or average of all the resonance structures depicted, which often results in fractional bonds and charges that cannot be fully described by a single Lewis structure.
For example, in benzene and ozone (O3), electrons are delocalized throughout the molecule, creating a situation where no single Lewis structure can describe the electron distribution accurately. The bonding in such molecules can be better expressed through multiple Lewis structures which reflect the stable possible ways those electrons could be distributed.