Allocating electrons to the molecular orbitals, according to the aufbau principle (Section A2), we find that once all the lower orbitals have been filled, we are left with two electrons. By the standard rules, these should be placed in p* orbitals. There are two of these, which are of equal energy (i.e. degenerate). These two orbitals arise because side-on overlap is possible between two pairs of atomic p-orbitals. The lowest energy distribution of these two electrons is to place one in each of these p* orbitals, with their spins parallel. This is due to Hund’s rule (Section A2). This treatment predicts that the O2 molecule behaves as a di-radical (i.e. two unpaired electrons, see Section I1). The molecule should also be paramagnetic, which is a form of magnetism that is due to the presence of unpaired electrons in a molecule. These predictions turn out to be correct. The two unpaired electrons mean that the molecule is highly reactive.