What is Molecular Orbital Theory?
Molecular orbital theory explains how electrons are distributed across a molecule by treating the whole molecule as having orbitals, not just individual atoms. Electrons occupy bonding (lower energy) and antibonding (higher energy) orbitals, and the HOMO–LUMO gap determines reactivity.
Molecular orbital (MO) theory describes molecules using delocalized orbitals formed by combining atomic orbitals. Bonding MOs stabilise molecules; antibonding MOs destabilise them. The HOMO–LUMO gap predicts reactivity.
Step-by-step worked examples
In H₂, how many electrons occupy bonding orbitals?
H₂ has 2 valence electrons (1 from each H) Both electrons pair in the σ bonding orbital (lower energy) Result: 2 electrons in bonding orbital
Explain why O₂ is paramagnetic using molecular orbital theory.
O₂ has 12 valence electrons After filling bonding π and σ orbitals, two π* antibonding orbitals have unpaired electrons Unpaired electrons = paramagnetic (attracts magnetic field)
Define the HOMO–LUMO gap.
HOMO = highest occupied molecular orbital (where top electrons sit) LUMO = lowest unoccupied molecular orbital (empty orbital above) Gap = energy difference between them; smaller gap = more reactive
Flashcards
Quick quiz
Q1.In molecular orbital theory, bonding orbitals…
Q2.What does HOMO stand for?
Q3.Antibonding orbitals form from…
Q4.Which factor increases chemical reactivity in MO theory?
The full card deck, worked steps and AI-tutor support for “What is Molecular Orbital Theory?” are in Notek — study by hand before your exam.
Common mistakes
Thinking molecular orbitals belong to single atoms. — Correct: MOs are delocalized across the entire molecule; electrons belong to the whole molecule.
Confusing antibonding orbitals with being non-existent. — Correct: Antibonding orbitals exist and can be occupied; they weaken bonding and increase energy.
Assuming all molecules are diamagnetic. — Correct: Molecules with unpaired electrons in MOs (like O₂) are paramagnetic.
Ignoring that HOMO–LUMO gap varies with molecule type. — Correct: Different molecules have different gaps; smaller gaps = higher reactivity.
FAQ
What is molecular orbital theory in simple terms?
It treats a whole molecule as having orbitals shared across all atoms, rather than each atom having its own orbitals. Electrons fill the lowest-energy orbitals first.
How is MO theory different from Lewis structures?
MO theory accounts for electron delocalization and shows why some molecules are paramagnetic; Lewis structures only show valence bonds.
Why do some molecules with even electrons become paramagnetic?
When antibonding orbitals receive electrons with unpaired spins (like O₂), the molecule is paramagnetic despite having an even electron count.
What determines if a molecule will be stable?
If more electrons occupy bonding orbitals than antibonding ones, the molecule is stable. Bond order = (bonding − antibonding) / 2.