O₂ is paramagnetic because it has unpaired electrons.
- In Molecular Orbital Theory, O₂ has 16 electrons.
- When filling the molecular orbitals, the last two electrons go into two separate π orbitals*, each with one unpaired electron.
- Substances with unpaired electrons are attracted to magnets, so O₂ is paramagnetic.
In short: O₂ has unpaired electrons → it is paramagnetic.