How do you determine if a molecule exhibits resonance?

A molecule exhibits resonance if it has delocalised electrons within a conjugated system of alternating single and double bonds.

Resonance is a concept in chemistry that describes the delocalisation of electrons within molecules. It is a phenomenon that occurs in certain molecules where the actual structure of the molecule cannot be accurately represented by a single Lewis structure. Instead, the molecule exists as a hybrid of multiple structures, known as resonance structures.

To determine if a molecule exhibits resonance, you need to look for a conjugated system. A conjugated system is a system in which a single electron occupies a p orbital and is adjacent to a double bond. This system allows for the delocalisation of electrons, which is a key characteristic of resonance.

The most common example of a molecule that exhibits resonance is benzene. Benzene has a six-membered carbon ring with alternating single and double bonds. The double bonds can be moved around the ring, creating different resonance structures. However, the actual structure of benzene is a hybrid of these structures, with the electrons delocalised around the ring.

Another way to identify resonance is to look for molecules that have a charge that is spread over two or more atoms. For example, the nitrate ion (NO3-) has a negative charge that is spread over the three oxygen atoms. This is another indication of resonance.

In summary, to determine if a molecule exhibits resonance, look for a conjugated system of alternating single and double bonds, or a charge that is spread over two or more atoms. These are indications of the delocalisation of electrons, which is a key characteristic of resonance.