Chemistry / Unit 6 Chem 1141 worked example, 60 to 90 seconds
O2's Paramagnetism, Predicted by Molecular Orbital Theory
The idea
Filling oxygen's molecular orbital diagram places its final two electrons singly, by Hund's rule, into two separate degenerate antibonding pi orbitals, correctly predicting O2 is paramagnetic.
In practice
Oxygen's Lewis structure shows only paired lone pairs and gives no structural reason for magnetism. Naming the quantities from oxygen's molecular orbital diagram instead, twelve valence electrons total fill the bonding and antibonding orbitals in energy order, leaving the final two electrons to occupy two separate degenerate antibonding pi orbitals singly rather than paired, by Hund's rule. Substituting that electron count into the bond order formula from Chem 1135 gives a bond order of 2, carrying electron count through to a unitless ratio, matching O2's known double bond. Those same two unpaired electrons are what correctly predict that liquid oxygen is drawn into a magnet, a small but physically sensible attraction consistent with two unpaired electrons rather than the zero attraction a fully-paired diagram would imply.
Where people slip
A correct Lewis structure and a correct valence-bond bonding picture for O2 both still miss this magnetic prediction entirely — the gap is specific to what those models track, not an error in how they were drawn.
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