Now that we have seen the usefulness of MO theory over valence bond theory, let’s take a look at how we can use the theory by finding the bond order of a molecule.

Previously, we have learnt how to fill the molecular orbitals following the Aufbau’s principle, Hund’s Rule and the Pauli’s Exclusion Principle. Now, we are going to use that molecular orbitals diagram to find the bond order. Read on to find out how!

Bond order

The concept of bond order in layman terms can be described as the average number of electrons involved per bond in a molecule.

It is based on the formula, B.O. = ½ [ no. of bonding electrons – no. of anti-bonding electrons].

As an example, let’s look at oxygen below.

MO Diagram of Oxygen

Bond order in O2 = ½ [ no. of bonding electrons – no. of anti-bonding electrons] = ½ [ 10 – 4 ] = 2

Hence an average of 2 electrons are involved in a bond in O2.

In reality, the dot and cross diagram does prove that to be true, as shown below.

You may want to try calculating the bond order for other molecules.

Try the example below:

Fluorine MO Diagram

Remember, bond order in O2 = ½ [ no. of bonding electrons – no. of anti-bonding electrons]

Answer: B.O.= 1

The bond order generally tells us the strength of the bond. The higher the bond order, the stronger the bond. We may make use of bond strength to further discuss about the stability of a molecule or the melting/boiling point. Yep, it’s that useful! Let me know if you find another use for it, it might be unheard of!