Overview
This lecture introduces molecular orbital (MO) theory, comparing it to valence bond (VB) theory and explaining the basic mathematical reasoning and concepts behind MO theory.
Valence Bond Theory (VB Theory)
- VB theory focuses on the hybridization of atomic orbitals to explain molecular shapes and bonding.
- It accounts for observed shapes, such as methane (CHâ‚„) being tetrahedral, and explains rigidity in double bonds.
- Electrons in VB theory are assigned to quantum mechanical orbitals calculated for individual atoms.
- These atomic orbitals are determined by solving the Schrödinger equation for each atom.
Limitations of Valence Bond Theory
- VB theory confines electrons to the individual atoms forming the bond.
- In reality, electrons are delocalized over the whole molecule rather than being localized.
Introduction to Molecular Orbital Theory
- MO theory describes electrons as occupying molecular orbitals that extend over the entire molecule.
- For even simple molecules, the Schrödinger equation for the whole molecule is too complex to solve exactly.
- Instead, scientists use a trial function (an educated guess) to approximate solutions.
Linear Combination of Atomic Orbitals (LCAO)
- The simplest trial functions in MO theory are linear combinations of atomic orbitals (LCAO).
- An LCAO is a weighted linear sum (like an average) of the valence atomic orbitals from all atoms in the molecule.
- The main difference: VB theory uses atomic orbitals from individual atoms, while MO theory (LCAO) combines orbitals from every atom in the molecule.
Key Terms & Definitions
- Valence Bond (VB) Theory — A theory where electrons are localized in atomic orbitals, often hybridized, to form bonds.
- Molecular Orbital (MO) Theory — A theory where electrons occupy orbitals delocalized over the whole molecule.
- Schrödinger Equation — A mathematical equation used to determine the allowed energy states of quantum mechanical systems.
- Linear Combination of Atomic Orbitals (LCAO) — A method where molecular orbitals are constructed by combining atomic orbitals from all atoms.
- Valence Orbitals — The outermost electron orbitals of an atom involved in bonding.
Action Items / Next Steps
- Watch the next video covering molecular orbitals for Hâ‚‚ and definitions of bonding and antibonding molecular orbitals.