Chemical Bonding and Molecular Structure · Class 11
Molecular Orbital Theory (MOT)
Molecular orbital theory for Class 11 / JEE: bonding and antibonding orbitals, bond order, magnetism of O₂ and N₂, and MOT vs VBT.
Direct answer
Molecular Orbital Theory describes electrons in molecules as occupying molecular orbitals formed by combination of atomic orbitals, spread over the molecule.
Bonding vs antibonding
Constructive overlap → bonding MO (lower energy). Destructive overlap → antibonding MO (higher energy, often marked with *).
Electrons fill MOs by Aufbau, Pauli, and Hund rules — similar to atomic orbitals.
Bond order
Bond order = ½ (Nᵦ − Nₐ), where Nᵦ = electrons in bonding MOs and Nₐ = electrons in antibonding MOs.
Higher bond order → generally stronger bond and shorter bond length (for comparable systems).
Bond order 0 → molecule is typically unstable (e.g. He₂).
Classic exam molecules
O₂: MOT predicts two unpaired electrons → paramagnetic (a famous result VBT alone does not explain well).
N₂: high bond order (3) → very strong triple bond character; diamagnetic.
Learn the MO energy order carefully for B₂, C₂, N₂ vs O₂, F₂ (order changes after N₂).
Formulae
Bond order = ½ (N_bonding − N_antibonding)
Predicts relative bond strength and whether a species is likely stable.