The Molecular Orbital Theory (MOT) diagram of O₂ reveals two important properties that cannot be explained by simple Lewis structures — a double bond AND paramagnetism.
Lewis dot structure of O₂ shows all paired electrons (diamagnetic), but experimentally O₂ is paramagnetic. MOT correctly predicts this, making it superior to Lewis theory.
Each O atom has 8 electrons, so O₂ has 16 total electrons.
MO filling order (for O₂ and beyond): σ1s², σ1s², σ2s², σ2s², σ2p², π2p⁴, π2p², σ2p⁰
(Note: For O₂, the σ2p is lower in energy than π2p)
Bond Order = (Bonding electrons − Antibonding electrons) / 2 = (10 − 6) / 2 = 2
This confirms O₂ has a double bond (O=O).
The two electrons in π*2p orbitals are placed one in each (Hund's rule) — they are unpaired.
Because of these 2 unpaired electrons, O₂ is Paramagnetic (attracted to a magnetic field). This was experimentally proven and is a triumph of MOT over Lewis structure.
O₂⁺ has 15 electrons. Bond order = (10-5)/2 = **2.5** — stronger and shorter bond than O₂.
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