How Do Fractional Bond Orders Arise in Resonant Molecular Structures?

[mahela007]

What is resonance when it comes to molecular structure? I understand that the concept of resonance arises when there is more than on possible lewis structure. However, everything I read says the atoms don't oscillate between structures and that in fact, they assume a constant shape.
My question in, how do bonds of intermediate length form between these atoms? for example bonds with a bond order of 1.33?

 

[Ygggdrasil]

Resonance comes from the fact that although we think of bonds as the sharing of electrons between two atoms, some chemical bonds are formed from the sharing of electrons between three or more atoms. In this case, the molecular orbital(s) representing the bond do span the three or more atoms and the (average) number of electrons between two atoms is indeed the intermediate number of electrons predicted by the resonance hybrid.

 

[quicknote]

Resonance in molecular structure refers to the phenomenon where a molecule can have multiple Lewis structures that contribute to its overall structure. This occurs when there are delocalized electrons, meaning the electrons are not confined to a specific bond or atom, but rather spread out over several atoms.

To understand this concept, it is important to first understand the concept of bond order. Bond order is a measure of the number of bonds between two atoms in a molecule. In a single bond, the bond order is 1, in a double bond it is 2, and in a triple bond it is 3. However, in molecules with resonance, the bond order can be fractional, such as 1.33 as you mentioned.

This fractional bond order occurs because the electrons are delocalized and can be found in different positions around the molecule, resulting in a partial bond between the atoms. This allows for the formation of intermediate bond lengths between the single and double bond lengths.

It is important to note that these bonds are not constantly switching between single and double bonds, but rather exist as a hybrid of the two states. This is why the atoms do not oscillate between structures, but rather assume a constant shape.

In summary, resonance in molecular structure refers to the presence of multiple Lewis structures and delocalized electrons, resulting in fractional bond orders and intermediate bond lengths. This concept is crucial in understanding the stability and reactivity of molecules and plays a significant role in many chemical reactions.