Question

Why is O2 paramagnetic?

Answer 1

`"O"_2` is paramagnetic because it has two unpaired electrons.

Explanation:

The Lewis structure of `"O"_2` gives a misleading impression.

It shows that all the electrons in oxygen are paired, so oxygen should be diamagnetic.

Yet oxygen is paramagnetic.

The correct explanation comes from Molecular Orbital theory.

The atomic orbitals of the `"O"` atoms overlap to form the σ and π orbitals of the `"O"_2` molecule as shown in the diagram above.

We add the 12 valence electrons according to the Aufbau principle.

The last two electrons go into separate, degenerate π orbitals, according to Hund's Rule.

Thus, oxygen has two unpaired electrons and is paramagnetic.