Molecular Orbital Theory
Molecular orbital theory describes electrons in a molecule as occupying orbitals spread over all the atoms, built by combining atomic orbitals into bonding and antibonding states.
Definition
Molecular orbital theory is a quantum-chemical model in which electrons occupy molecular orbitals, typically constructed as linear combinations of atomic orbitals, that extend over the entire molecule and determine its bonding and properties.
Scope
This topic covers the molecular orbital description of bonding: the linear combination of atomic orbitals, the formation of bonding and antibonding molecular orbitals, and the building-up of molecular electron configurations to give bond order, magnetic behaviour, and stability. It includes the treatment of diatomic and simple polyatomic molecules, sigma and pi orbitals, the relation between orbital overlap and energy, and qualitative tools such as orbital symmetry and frontier orbitals. The general variational machinery and the self-consistent Hartree-Fock method are treated in the methods topic.
Core questions
- How are molecular orbitals constructed as combinations of atomic orbitals?
- What distinguishes bonding from antibonding orbitals, and how do they affect stability?
- How does bond order follow from the occupation of molecular orbitals?
- How does molecular orbital theory explain the magnetism of molecules such as oxygen?
Key concepts
- Linear combination of atomic orbitals
- Bonding and antibonding orbitals
- Bond order
- Sigma and pi orbitals
- Frontier orbitals (HOMO and LUMO)
Key theories
- LCAO construction of molecular orbitals
- Molecular orbitals are approximated as weighted sums of atomic orbitals; constructive combinations concentrate electron density between nuclei to give bonding orbitals, while destructive combinations create nodes and antibonding orbitals.
- Aufbau filling and bond order
- Filling molecular orbitals in order of increasing energy according to the Pauli principle and Hund's rule gives the electron configuration, from which the bond order, the net number of bonding electron pairs, and magnetic properties follow.
Clinical relevance
Molecular orbital theory explains bond strengths, colours, magnetism, and reactivity, underlies frontier-orbital reasoning about reaction selectivity, and guides the design of dyes, semiconductors, photovoltaic materials, and conjugated drug molecules.
History
Molecular orbital theory was developed by Hund and Mulliken from the late 1920s as an alternative to Pauling's valence bond theory; Huckel's treatment of conjugated pi systems in the 1930s and the later frontier-orbital ideas of Fukui and the Woodward-Hoffmann rules made it central to understanding reactivity.
Key figures
- Robert S. Mulliken
- Friedrich Hund
- Erich Huckel
Related topics
Seminal works
- mcquarrie1997
- levinequantum2014
Frequently asked questions
- Why does molecular orbital theory predict that oxygen is magnetic?
- Filling the molecular orbitals of the oxygen molecule leaves two electrons unpaired in degenerate antibonding pi orbitals, making it paramagnetic; this success, which simple electron-pair pictures miss, was an early triumph of the theory.
- What are HOMO and LUMO, and why do they matter?
- They are the highest occupied and lowest unoccupied molecular orbitals; because reactions and electronic excitations usually involve these frontier orbitals, their energies and shapes strongly influence a molecule's reactivity, colour, and electronic behaviour.