Certain molecules exhibit fractional bond orders as a result of resonance structures, which allow for the distribution of electrons across multiple configurations.

To elaborate, bond order quantifies the number of shared electron pairs between two atoms in a molecule. Typically, bond orders are integer values: a single bond has a bond order of $1$, a double bond has a bond order of $2$, and a triple bond has a bond order of $3$. However, some molecular bonding scenarios are more intricate due to the resonance phenomenon.

Resonance occurs when a molecule can be represented by multiple valid Lewis structures—diagrams that illustrate the bonding arrangements among the atoms. These distinct representations, known as resonance forms, distribute electrons in various ways. For instance, in benzene, a molecule composed of six carbon atoms arranged in a ring, there are alternating single and double bonds. However, there are two principal ways to depict the molecule, with the positions of the double bonds swapped. Both representations qualify as resonance forms of benzene.

In reality, the electrons are not localized to just one resonance form; instead, they are delocalized throughout the entire molecule. Consequently, the actual structure is a hybrid of all possible resonance forms. This delocalization results in a fractional bond order rather than a simple integer. For benzene, the bond order is $1.5$, indicating that each bond is effectively ‘between’ a single and a double bond.

The concept of fractional bond orders arising from resonance is a fundamental aspect of molecular orbital theory, which offers a more nuanced understanding of electron behavior compared to traditional Lewis structures. This theory elucidates the properties of many molecules, including their geometries, reactivities, and the wavelengths of light they absorb or emit. Grasping this concept is essential for a comprehensive understanding of chemistry.