Why do certain molecules have fractional bond orders?

Certain molecules have fractional bond orders due to the presence of resonance structures, which distribute electrons among multiple configurations.

In more detail, bond order is a measure of the number of shared electron pairs between two atoms in a molecule. It is typically an integer value, with single bonds having a bond order of one, double bonds having a bond order of two, and so on. However, in some molecules, the bonding situation is more complex due to the phenomenon of resonance.

Resonance occurs when there are multiple valid Lewis structures (diagrams that show the bonding between atoms of a molecule) for a single molecule. These different structures, known as resonance forms, distribute the electrons in different ways. For example, in the molecule benzene, the six carbon atoms form a ring, with alternating single and double bonds. However, there are two possible ways to draw this, with the double bonds in different places. These are both resonance forms of benzene.

In reality, the electrons are not confined to one resonance form or the other, but are delocalised across the whole molecule. This means that the actual structure of the molecule is a hybrid of all its resonance forms. As a result, the bond order is not an integer, but a fraction. In the case of benzene, the bond order is 1.5, reflecting the fact that each bond is 'halfway between' a single and a double bond.

This concept of fractional bond orders due to resonance is a key part of molecular orbital theory, which provides a more accurate description of electron behaviour than simple Lewis structures. It helps to explain the properties of many molecules, including their shapes, reactivities, and the wavelengths of light they absorb or emit. Understanding this concept is crucial for a deep understanding of chemistry.