why oxygen is paramagnetic in nature by molecular orbital theory
The electronic configuration O-atom is ; 1s22s22p4 and the O2 molecule has total 16 electrons.
Now, according to MO theory, all the 16 electrons will be counted during formation of MO-diagram and so the electronic configuration of O2 molecule will be:σπ
[σ(1s)]2 [σ*(1s)]2 [σ(2s)]2 [σ*(2s)]2 [σ(2Pz)]2 [π(2Px)]2 [π(2Py)]2 [π*(2Px)]1 [π*(2Py)]1
Since the core orbitals are full occupied , we will consider for the valence shell (2nd shell). Here we see that, in the antibonding shell, having highest energy, the last two electrons fill singly ,because both the [π*(2Px)]1 [π*(2Py)]1 orbitals have nearly equal energy. So, according to Hund's rule the last two electrons are filled singly. Due to this the ,O2 molecule contains unpaired electrons, which characterizes its paramagnetic character.
