In Chapter 9 we saw that the resonance concept is useful for dealing with species such as the benzene molecule and the carbonate ion. How does molecular orbital theory deal with these species?

Benzene, with its ring-like structure, has been a subject of fascination for chemists due to its unusual stability. According to Molecular Orbital Theory, the secret behind this lies in the concept of delocalized electrons. Benzene consists of six carbon atoms, bonded in a hexagonal arrangement where each carbon atom contributes one electron to a shared cloud that hovers above and below the plane of the molecule.

This cohesive electron cloud is spread over all the carbon atoms, minimizing the electron's energy by maximizing their space. Once the atomic orbitals overlap and combine, it generates molecular orbitals. Among these, the bonding molecular orbitals are filled with the pi (π) electrons, creating a uniform distribution because the energy levels of these orbitals are identical. As a result, Benzene exhibits a resonating structure without a fixed single or double bond, explaining its remarkable stability and symmetry.

Considering the carbonate ion (CO3^2-), resonance is the traditional way of explaining the electrons' arrangement. However, Molecular Orbital Theory provides a more in-depth and expansive explanation. It posits that the electrons associated with forming the bonds between the carbon and the oxygen atoms are, instead, delocalized over the entire ion.

The orbitals from the carbon atom and the oxygen atoms combine to form molecular orbitals that are spread out across the whole molecule. This distribution of molecular orbitals allows the electrons to move freely among the oxygen atoms, resulting in none of the oxygens being singly or doubly bonded to the carbon atom exclusively. Consequently, the resonance illustrated by the multiple Lewis structures is elegantly rationalized by this theory as a single stable form, where the delocalized electrons are the actual representation of electron distribution in the carbonate ion.

Delocalized electrons play a central role in Molecular Orbital Theory. Unlike the classical description of electrons that are confined to specific bonds between two atoms, delocalized electrons are spread out across several atoms. This concept is crucial in explaining the chemical properties of molecules such as the benzene molecule and the carbonate ion.

Delocalized electrons afford greater stability to a molecule because their charge and bonding influence is distributed over a greater number of atoms. This results in structures that don't conform to the simple one-line or two-dots representations of single or double bonds, respectively. Moreover, the distribution of these electrons can affect their energy states, and in the case of benzene, leads to an overall reduction in energy due to the formation of a full set of bonding molecular orbitals. The delocalization concept illuminates the malleability and resilience of certain molecular structures in the natural world.