Question

why O2 is paramagnetic?

Answer 1

O2 is paramagnetic because it has two unpaired electrons.

Answer 2

Let's first check the electron configuration of oxygen atom. Notice that Molecular Orbital Theory predicts that O2 has two unparied electrons in 2px and 2py degenerate orbitals. Unparied electrons spin in the same direction as each other, which increases the magnetic field effect. So O2 becomes paramagnetic.