Molecular Orbital Theory
Definition and meaning of Molecular Orbital Theory in chemistry.
Molecular orbital theory is a quantum mechanical model that describes electrons in molecules as occupying molecular orbitals that extend across multiple atoms, rather than being localized to individual atoms as in valence bond theory.
In more detail
In molecular orbital theory, atomic orbitals combine via constructive and destructive interference to form molecular orbitals with characteristic energies and spatial distributions. These molecular orbitals are classified as bonding (lower energy, stabilizes the molecule), antibonding (higher energy, destabilizes the molecule), or nonbonding (intermediate energy). The number of molecular orbitals formed equals the number of atomic orbitals combined. Electrons fill molecular orbitals according to the aufbau principle, and the theory accurately predicts bond order, bond strength, magnetic properties, and ionization energies that valence bond theory cannot explain.
Key facts
| Field | Physical Chemistry |
|---|---|
| Orbital types | Bonding, antibonding, nonbonding |
| Key advantage | Explains molecular magnetism and excited states accurately |
| Bond order formula | (bonding electrons - antibonding electrons) / 2 |
Oxygen gas (O2) serves as a classic example: molecular orbital theory correctly predicts it contains two unpaired electrons in antibonding pi orbitals, making it paramagnetic. This matches experimental observations, whereas simple valence bond theory would incorrectly predict a diamagnetic paired electron configuration.
Frequently asked questions
What is bond order?
Bond order is calculated as (bonding electrons - antibonding electrons) divided by 2. A bond order of 1 represents a single bond, 2 a double bond, and 3 a triple bond. Zero or fractional values indicate partial bonds.
How does molecular orbital theory differ from valence bond theory?
Valence bond theory localizes electrons between specific atom pairs, while molecular orbital theory delocalizes electrons across the entire molecule. MOT better explains paramagnetic molecules, colored compounds, and excited electronic states.