Question

What is the molecular orbital theory?

Answer 1

Molecular Orbital (MO) Theory tells you that any linear combination of atomic orbitals (AOs) gives you the corresponding molecular orbital(s). (Linear combination literally means moving the atomic orbitals towards each other linearly through space until they overlap.)

They can overlap either in-phase (`+` with `+`) or out-of-phase (`-` with `+`).

The linear combination of two `s` orbitals overlaps to give you a `sigma` (in-phase overlap) bonding MO or `sigma^"*"` (out-of-phase overlap) antibonding MO.

The linear combination of two `p` orbitals overlaps to give you either a `sigma` (in-phase overlap) bonding MO or `sigma^"*"` (out-of-phase overlap) antibonding MO for colinear/head-on overlap, or a `pi` (in-phase overlap) bonding MO or `pi^"*"` (out-of-phase overlap) antibonding orbital for parallel/sideways overlap.

The results of the orbital overlaps can be depicted in a Molecular Orbital Diagram. An example for `F_2` is depicted below:

Notice how the antibonding MOs are higher in energy than the bonding MOs. This is because the out-of-phase overlap creates nodes where electrons can never be, promoting nuclear repulsion, increasing the energy of the antibonding MO higher than the energy of the original AOs.

In contrast, the bonding MOs are lower in energy because the in-phase combination creates a favorable overlap that increases electron density between the two AOs, minimizing nuclear repulsion, lowering the energy.