why oxygen molecule is paramagnetic in nature

hi sohail,

  The order in which O₂ will fill the orbitals is:

sigma2s, sigma2s*, sigma2p, sigma2p*

Two electrons can occupy each s orbital, while 6 electrons can occupy each p orbital. Following the Exclusion Principle, two electrons will fill both the 2s and 2s* orbitals, 6 electrons will fill the 2p orbital, and that leaves 2 electrons to fill the 2p* orbital. These two electrons will only partially fill this orbital, and will have parallel spins. Since the rest of the electrons are all paired, the remaining two electrons in the 2p* orbital give the diatomic molecule a net total spin (it does not matter if they are 1/2 or -1/2 spins, they will both be the same). Since there is a net spin, O₂ is paramagnetic.