MO theory shows O2 has two unpaired electrons in degenerate pi-star orbitals, making it paramagnetic.
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In the molecular orbital picture, the last two electrons of O2 enter the two degenerate pi* 2p orbitals. Hund's rule places them with parallel spins in separate orbitals, giving two unpaired electrons. This makes O2 paramagnetic, a property that Lewis structures with all electrons paired cannot explain. A simple demonstration is that liquid oxygen is attracted into a magnetic field and can be held between the poles of a strong magnet. The explanation of this property was an important early success of molecular orbital theory.
Key ideas: The O2 configuration from MO theory has 12 valence electrons: sigma 2s squared, sigma star 2s squared, sigma 2p squared, pi 2p to the fourth, pi star 2p squared, with parallel spins. Spin-only moment is square root of 2 times 4, about 2.83 Bohr magnetons, close to the measured value. Bond order is 2, matching the O=O bond length of 121 pm.
Common mistakes and connections: A frequent mistake is to draw O2 as O=O with all electrons paired and so claim the molecule should be diamagnetic. Lewis structures cannot predict this. Another is assuming paramagnetism means the molecule is a free radical in the sense of being highly reactive: triplet O2 is relatively unreactive toward organic molecules at room temperature. Links to MO theory and singlet oxygen.
Molecular orbital theory puts the last two electrons of O2 singly in two degenerate pi* orbitals, so liquid oxygen is attracted to a magnet. The interactive lesson shows this as a live animation or chart.
Liquid oxygen poured between the poles of a strong magnet is held in place by its attraction to the magnetic field.
MO theory shows O2 has two unpaired electrons in degenerate pi-star orbitals, making it paramagnetic.
Paramagnetism of Oxygen is a advanced topic in Bonding & Molecular Structure. It builds on beginner and intermediate topics, so review those first if needed.
Study these earlier topics in Bonding & Molecular Structure first: Bonding Molecular Orbital, Antibonding Molecular Orbital, MO Diagram of Homonuclear Diatomics.
The next topics in Bonding & Molecular Structure are Diamagnetism and Paramagnetism.
Yes. The interactive lesson includes a self-check question on Paramagnetism of Oxygen, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Paramagnetism of Oxygen is topic 63 of 104 in Bonding & Molecular Structure.