Define mesomers with an example?

Mesomers, also known as resonance structures or resonance forms, are a concept in chemistry used to describe the possible ways in which electrons can be delocalized within a molecule. These structures are not distinct molecules but rather theoretical representations of a molecule's electronic structure.

In organic chemistry, mesomers are particularly important in conjugated systems where electrons are able to move freely along a series of adjacent atoms. The actual electronic structure of the molecule is often a hybrid or combination of these resonance structures.

An example of mesomers can be found in the resonance forms of benzene (C6H6). Benzene is a planar, cyclic molecule with alternating single and double bonds between carbon atoms. However, benzene does not exhibit the characteristics expected of a molecule with alternating single and double bonds. Instead, benzene is found to be highly stable and exhibits equal bond lengths and strengths between all carbon-carbon bonds.