Comparison Note: VBT vs MOT

Feature	Valence Bond Theory (VBT)	Molecular Orbital Theory (MOT)
Basic concept	Bonds form from overlap of atomic orbitals	Atomic orbitals combine (LCAO) to form molecular orbitals
Electrons	Electrons localised in bond regions	Electrons delocalized over the whole molecule
Bond types	σ (head-on), π (lateral)	Bonding MOs (σ, π) and antibonding MOs (σ, π)
Hybridization	Required to explain geometry	Not required (MO directly gives geometry)
Paramagnetic $O_{2}$	FAILS — predicts diamagnetic	SUCCESS — predicts 2 unpaired $e^{-}$ in π*2p
Bond order	Directly from Lewis structure	BO = (Nb − Na)/2
Resonance	Multiple structures (hypothetical)	Single wavefunction (delocalized MOs)
Stability	Bond = orbital overlap	Bonding MOs filled → stable; antibonding MOs filled → destabilised
Limitations	Cannot predict $O_{2}$ paramagnetism	More mathematical; complex for large molecules
Successes	Geometry, bond strength, hybridization	Paramagnetism, bond order of species like $O_{2}^{-}$ , $He_{2}^{+}$

The decisive experiment: Liquid oxygen is attracted to magnets → paramagnetic. VBT predicts diamagnetic (wrong). MOT predicts paramagnetic (correct). This is MOT's greatest triumph.

Bond order comparison:

Species	VBT Bond Order	MOT Bond Order	Magnetic Nature
$O_{2}$	2	2	Paramagnetic
$O_{2}^{+}$	—	2.5	Paramagnetic
$O_{2}^{-}$	—	1.5	Paramagnetic
$O_{2}^{2-}$	1	1	Diamagnetic
$N_{2}$	3	3	Diamagnetic
$B_{2}$	1	1	Paramagnetic (!)