Molecular Orbital Theory (MOT)

Molecular Orbital Theory was developed by scientists like Robert S. Mulliken and Friedrich Hund to explain bonding in molecules using quantum mechanics.

Unlike Valence Bond Theory, which considers bonds as localized between two atoms, MOT treats electrons as moving under the influence of all nuclei in a molecule.

???? Important Postulates of Molecular Orbital Theory

1?? Formation of Molecular Orbitals

• When atomic orbitals (AOs) of comparable energy and proper symmetry combine, they form molecular orbitals (MOs).

• Number of MOs formed = Number of combining AOs.

2?? Types of Molecular Orbitals

Two types are formed:

• Bonding Molecular Orbital (BMO)

  • Lower energy than parent atomic orbitals

  • Electron density between nuclei increases

  • Stabilizes molecule

• Antibonding Molecular Orbital (ABMO)

  • Higher energy

  • Electron density decreases between nuclei

  • Destabilizes molecule

  • Denoted by *

Example:
????,????σ,π → bonding
????∗,????∗σ∗,π∗ → antibonding

???? Sigma (σ) and Pi (π) Orbitals

4

• σ-orbitals: Formed by head-on overlap

• π-orbitals: Formed by sidewise overlap

3?? Filling of Molecular Orbitals

Electrons fill MOs according to:

• Aufbau Principle

• Pauli’s Exclusion Principle

• Hund’s Rule

4?? Bond Order Formula

Bond Order=????????−????????2Bond Order=2Nb?−Na??

Where:

• ????????Nb? = number of electrons in bonding orbitals

• ????????Na? = number of electrons in antibonding orbitals

Higher bond order → stronger bond → shorter bond length.

5?? Delocalization of Electrons

Electrons in MOs are delocalized over the entire molecule, not restricted between two atoms.

???? Merits of Molecular Orbital Theory

? Explains magnetic properties

• Example: O? is paramagnetic (correctly predicted by MOT).

? Explains bond order and bond strength.

? Explains existence of molecules like He??.

? Applicable to polyatomic molecules.

? Based on quantum mechanics — more accurate than VBT.

? Demerits of Molecular Orbital Theory

? Mathematical treatment is complex.

? Less intuitive for simple bonding concepts.

? Does not clearly explain molecular shapes (VSEPR does better).

? Energy level ordering varies for some molecules (e.g., B?, C?, N?).

???? Why MOT Is Important

• Accurately predicts electronic structure.

• Explains spectral properties.

• Forms the foundation for computational chemistry.

• Essential for understanding semiconductors and advanced materials.