Question

How can I read molecular orbital diagram?

Answer 1

WARNING. This is a long answer.

Let's analyze a molecular orbital (MO) diagram for CO.

Points to Note:

1. Individual atomic orbitals (AOs) are on the far left (C) and far right (O) of the diagram.

2. These AOs overlap to produce the MOs of CO in the middle. The MOs are either σ or π orbitals. They are bonding, nonbonding, or antibonding.

Conservation of Orbitals:

The overlap of two AOs produces two MOs, one bonding and one antibonding.

a. The overlap of two s AOs produces a σ MO and a σ* MO.

b. The end-on overlap of two p AOs produces a σ MO and a σ* MO.

c. The side-on overlap of two p AOs produces a π MO and a π* MO.

Interpreting the Diagram:

1. The 2s orbitals of C and O overlap to form the σ(2s) and σ*(2s) orbitals of CO.

2. The `2p_z` orbitals of C and O overlap end-on to form the σ(2p) and σ*(2p) orbitals of CO.

3. The `2p_x` orbitals of C and O overlap side-on to form one set of π(2p) and π*(2p) orbitals for CO.

4. The `2p_y` orbitals of C and O overlap side-on to form a second set of π(2p) and π(2p) orbitals of CO. These two sets of orbitals are degenerate* (they have the same energy).

Adding the Electrons:

1. We place the 4 valence electrons of C in its AOs on the left hand side according to the Pauli Exclusion Principle and Hund's Rule.

2. We place the 6 valence electrons of O in its AOs on the right hand side according to the Pauli Exclusion Principle and Hund's Rule.

3. We place the 10 valence electrons in the 5 lowest MOs of CO.). Again we use the Pauli Exclusion Principle and Hund's Rule.

And we're done.