Understanding Molecular Orbital Theory
Molecular Orbital (MO) Theory is a method of describing the electronic structure of molecules. Unlike Valence Bond Theory, which focuses on localized bonds between atoms, MO theory considers electrons to be delocalized over the entire molecule, occupying molecular orbitals that are formed by the combination of atomic orbitals from all participating atoms.
Key Principles of MO Theory
In MO theory, atomic orbitals combine to form an equal number of molecular orbitals (MOs). These MOs can be bonding orbitals (lower energy, stabilize the molecule), antibonding orbitals (higher energy, destabilize), or non-bonding orbitals. Electrons fill these MOs according to Hund's rule and the Pauli exclusion principle, similar to how they fill atomic orbitals.
A Simple Example: Hydrogen Molecule (H₂)
For the hydrogen molecule (H₂), the 1s atomic orbitals from each hydrogen atom combine to form two molecular orbitals: a lower-energy sigma (σ) bonding orbital and a higher-energy sigma-star (σ*) antibonding orbital. The two electrons from the hydrogen atoms then occupy the lower-energy σ bonding orbital, resulting in a stable H₂ molecule with a bond order of one.
Importance in Predicting Molecular Properties
MO theory is crucial for explaining and predicting various molecular properties that Valence Bond Theory struggles with, such as magnetism (e.g., the paramagnetic nature of O₂), spectroscopic behavior, and the existence of molecules with fractional bond orders. It provides a more accurate picture of electron distribution and energy levels within molecules.