Molecular Orbital Theory (MOT) and Valence Bond Theory (VBT) are both models used to explain chemical bonding, but they differ fundamentally in how they describe electrons in molecules. Here’s a breakdown:
1. Electron description
- Valence Bond Theory (VBT): Electrons are localized between two atoms, forming bonds by the overlap of atomic orbitals.
- Molecular Orbital Theory (MOT): Electrons are delocalized over the entire molecule, occupying molecular orbitals that belong to the whole molecule.
2. Bond formation
- VBT: Bonds form when half-filled atomic orbitals overlap. Bond strength depends on the extent of this overlap.
- MOT: Bonds form by filling molecular orbitals, which are combinations of atomic orbitals. Bonding and antibonding orbitals determine the bond order.
3. Magnetic properties
- VBT: Cannot always explain paramagnetism or diamagnetism accurately.
- MOT: Can predict magnetic behavior (e.g., O₂ is paramagnetic because of unpaired electrons in molecular orbitals).
4. Resonance explanation
- VBT: Uses the concept of resonance (multiple structures) to explain delocalization.
- MOT: Delocalization is built-in; electrons naturally occupy orbitals spread over the molecule.
5. Applicability
- VBT: Works well for simple molecules and covalent bonds.
- MOT: Better for molecules with delocalized electrons, like O₂, NO, or benzene, and for explaining electronic spectra.
In short:
VBT = electrons are shared between atoms (localized).
MOT = electrons are spread over the molecule (delocalized).