Molecular Orbital Theory — Definition

Definition

Imagine atoms coming together to form a molecule. In the traditional Valence Bond Theory (VBT), we think of atomic orbitals (like s, p, d) on individual atoms overlapping to form specific bonds between two atoms.

It's like each atom brings its own 'house' (atomic orbital) and they share a 'room' (overlapping orbital) to form a bond. However, Molecular Orbital Theory (MOT) offers a different, more holistic perspective.

Instead of thinking about individual atomic orbitals overlapping to form localized bonds, MOT suggests that when atoms combine, their atomic orbitals lose their individual identity and merge to form entirely new orbitals called 'molecular orbitals'.

Think of it this way: when two atoms approach each other, their electron clouds interact. According to MOT, these interactions lead to the formation of new 'molecular houses' that are no longer specific to one atom but belong to the entire molecule.

These molecular orbitals are spread out, or 'delocalized', over all the nuclei in the molecule. Just like atomic orbitals describe the probability of finding an electron around a single nucleus, molecular orbitals describe the probability of finding an electron around all the nuclei in a molecule.

The key idea is the 'Linear Combination of Atomic Orbitals' (LCAO). This mathematical approach states that molecular orbitals are formed by adding or subtracting the wave functions of the atomic orbitals.

When atomic orbitals combine 'constructively' (like waves adding up), they form 'bonding molecular orbitals'. These orbitals have lower energy than the original atomic orbitals, meaning the electrons in them are more stable and hold the atoms together.

When atomic orbitals combine 'destructively' (like waves cancelling out), they form 'antibonding molecular orbitals'. These orbitals have higher energy, and electrons in them actually destabilize the molecule, pushing the atoms apart.

Electrons then fill these molecular orbitals following the same rules as atomic orbitals: the Aufbau principle (fill lowest energy first), Pauli exclusion principle (max two electrons per orbital with opposite spins), and Hund's rule (maximize unpaired spins in degenerate orbitals).

By calculating the 'bond order' (half the difference between bonding and antibonding electrons), MOT can predict the stability of a molecule, its bond strength, and even its magnetic properties, which VBT often struggles with.