MO Diagram

Although VSEPR and the Valence Bond theory predict bond properties accurately, they do not fully explain some molecules. In developing MO diagrams, the MO theory takes into account the wave nature of electrons. MO diagrams predict a molecule’s physical and chemical properties such as shape, bond energy, bond length, and bond angle. They also aid in the prediction of a molecule’s electronic spectra and paramagnetism.

Table of Contents

• Overview of MO Diagram
• How to Draw MO Diagram?
• Relationship Between Electronic Configuration and Molecular Behaviour
• Solved Example
• Frequently Asked Questions – FAQs

Overview of MO Diagram

Molecular orbital diagrams show molecular orbital (MO) energy levels in the centre, surrounded by constituent atomic orbital (AO) energy levels for comparison, and the energy levels increase from bottom to top. Lines, which are often dashed diagonal lines, connect MO levels to their constituent AO levels. Degenerate energy levels are frequently depicted next to each other. The Pauli Exclusion Principle, represented by small vertical arrows pointing in the direction of electron spins, fills appropriate AO and MO levels with electrons.

The AO or MO shapes themselves are frequently not depicted on these diagrams.

• An MO diagram effectively shows the energetics of the bond between the two atoms, whose AO unbonded energies are shown on the sides of a diatomic molecule.
• A MO diagram may show one of the identical bonds to the central atom for simple polyatomic molecules with a “central atom,” such as methane (CH₄) or carbon dioxide (CO₂).
• An MO diagram for other polyatomic molecules may show one or more bonds of interest in the molecules while leaving others out for simplicity.
• For simplicity, even for simple molecules, the AO and MO levels of inner orbitals and their electrons may be omitted from a diagram.

A MO Diagram look like this-

How to Draw MO Diagram?

Homonuclear Molecules

Step 1 – Determine the number of valence electrons in total.

Since there are now two atoms in the molecule, the total number of valence electrons is double that of the atomic species.

Step 2 – Determine the number of electrons in each s and p orbital.

Remember that there is one s-orbital and three p-orbitals in the n=2 energy level. Determine the number of electrons in the s and p orbitals for any molecule.

For example, B has two electrons in the 2s orbital and one in the 2p orbital. F has two electrons in the 2s energy level and five electrons in the 2p orbitals.

Step 3 – Fill in the electrons in the correct MO diagram’s molecular orbitals.

All of the orbital filling principles (Hund’s Rule, Pauli Exclusion Principle, Aufbau Principle) still apply when filling in molecular orbitals.

• The Pauli Exclusion Principle states that each molecular orbital can hold two electrons.
• The Aufbau Principle states that electrons will always fill orbitals from bottom to top.
• Hund’s Rule states that orbitals on the same energy level fill singly before doubly. This applies primarily to the π and π* orbitals, where one electron will enter into each orbital before filling in the second.

Step 4 – Predict the molecule’s properties using the diagram. For example, bond order, bond angle, paramagnetism, etc.

Relationship Between Electronic Configuration and Molecular Behaviour

1. Molecule stability in terms of bonding and antibonding electrons.

The number of electrons in bonding orbitals is represented by N_b, while the number of electrons in antibonding orbitals is represented by N_a.

If N_b > N_a molecule is stable

If N_b < N_a molecule is unstable

If N_b = N_a molecule is unstable

2. Molecule stability in terms of bond order.

Bond order is defined as half the difference in the number of electrons in the bonding and antibonding orbitals.

Bond order = 1/2 (N_b – N_a)

If N_b > N_a, the molecule is stable; otherwise, it is unstable.

3. Bond nature in terms of bond order.

Bond orders 1, 2, and 3 denote single, double, and triple bonds, respectively.

4. Bond length in relation to bond order.

The length of a bond is found to be inversely proportional to its order.

5. The molecule’s diamagnetic and paramagnetic properties.

It is diamagnetic if all of the electrons in the molecule are paired.

If a molecule contains some unpaired electrons, it is paramagnetic.

Solved Example

H₂ Molecule

The electronic configuration of H₂ is σ(1s²).

N_b = 2, N_a = 0

∴ Bond order = 1, the molecule is stable since it is a positive value.

The two hydrogen atoms are connected by a single bond.

Since there are no unpaired electrons, the H₂ molecule is diamagnetic.