Here’s a explanation of how Molecular Orbital Theory (MOT) applies to polyatomic molecules:
1. MOT in Polyatomic Molecules
- In polyatomic molecules (more than two atoms), atomic orbitals from all atoms combine to form molecular orbitals.
- These molecular orbitals extend over the whole molecule, not just between two atoms.
- Electrons occupy these MOs according to the Pauli exclusion principle and Hund’s rule, just like in diatomic molecules.
2. Delocalization of Electrons
- MOT explains electron delocalization in molecules with conjugation or resonance.
- Example: Benzene (C₆H₆)
- Six carbon p orbitals combine to form six π molecular orbitals.
- The π electrons are delocalized over all six carbons, giving resonance stabilization.
3. Predicting Molecular Properties
- MOT can explain:
- Bonding and antibonding interactions in multi-atom systems
- Stability (more filled bonding MOs → more stable)
- Magnetic properties
- Molecules with unpaired electrons in MOs are paramagnetic.
- Molecules with all electrons paired are diamagnetic.
- Reactivity
- HOMO-LUMO interactions help predict where chemical reactions will occur.
4. Example: Water (H₂O)
- Oxygen has 2s and 2p orbitals; hydrogen has 1s orbitals.
- All combine to form bonding and antibonding MOs.
- MOT explains why H₂O has a bent shape and strong O–H bonds, consistent with experimental observations.
5. Key Points
- MOT for polyatomic molecules uses linear combinations of atomic orbitals (LCAO) to make MOs.
- Electrons can be delocalized over many atoms, explaining resonance, stability, magnetism, and spectra.