The concept of shells allows one to understand why some nuclei are bound more tightly than others. This is because two nucleons of the same kind cannot be in the same state (Pauli exclusion principle). Werner Heisenberg extended the principle of Pauli exclusion to nucleons, via the introduction of the iso-spin concept. Nucleons are thought to be composed of two kind of particles, the neutron and the proton that differ through their intrinsic property, associated with their iso-spin quantum number. This concept enables the explanation of the bound state of Deuterium, in which the proton and neutron can couple their spin and iso-spin in two different manners. So the lowest-energy state of the nucleus is one where nucleons fill all energy levels from the bottom up to some level. Nuclei that exhibit an odd number of either protons or neutrons are less bound than nuclei with even number. A nucleus with full shells is exceptionally stable, as will be explained.