Explain why there are very few odd-odd nuclei (both Z and N odd) which are beta stable? Solution According to paulis exclusive principle, each nuclear energy level contain two nucleons of opposite sign. The proton and neutrons accupy different sets of energy levels. When both Z and N event all the levels completely filled or half filled. so This nucleus doesnot want to participate nuclear reactions to gain or lose electrons. When both Z and N are odd, these are much more likely want to participate nuclear reactions or nuclear decay due to the reason of incompletely filled shells..