Here’s a explanation:
Diatomic oxygen (O₂) has 16 electrons. In Molecular Orbital Theory:
- The electrons first fill the lower-energy orbitals (from 1s and 2s).
- Then they fill the 2p orbitals.
- In the 2p orbitals, two electrons are unpaired in the higher-energy anti-bonding orbitals.
These unpaired electrons explain why O₂ is paramagnetic (it is attracted to a magnetic field).
So simply: all lower orbitals are filled, and the two highest electrons are unpaired in molecular orbitals.