<P> A number of models for the nucleus have also been proposed in which nucleons occupy orbitals, much like the atomic orbitals in atomic physics theory . These wave models imagine nucleons to be either sizeless point particles in potential wells, or else probability waves as in the "optical model", frictionlessly orbiting at high speed in potential wells . </P> <P> In the above models, the nucleons may occupy orbitals in pairs, due to being fermions, which allows explanation of even / odd Z and N effects well - known from experiments . The exact nature and capacity of nuclear shells differs from those of electrons in atomic orbitals, primarily because the potential well in which the nucleons move (especially in larger nuclei) is quite different from the central electromagnetic potential well which binds electrons in atoms . Some resemblance to atomic orbital models may be seen in a small atomic nucleus like that of helium - 4, in which the two protons and two neutrons separately occupy 1s orbitals analogous to the 1s orbital for the two electrons in the helium atom, and achieve unusual stability for the same reason . Nuclei with 5 nucleons are all extremely unstable and short - lived, yet, helium - 3, with 3 nucleons, is very stable even with lack of a closed 1s orbital shell . Another nucleus with 3 nucleons, the triton hydrogen - 3 is unstable and will decay into helium - 3 when isolated . Weak nuclear stability with 2 nucleons (NP) in the 1s orbital is found in the deuteron hydrogen - 2, with only one nucleon in each of the proton and neutron potential wells . While each nucleon is a fermion, the (NP) deuteron is a boson and thus does not follow Pauli Exclusion for close packing within shells . Lithium - 6 with 6 nucleons is highly stable without a closed second 1p shell orbital . For light nuclei with total nucleon numbers 1 to 6 only those with 5 do not show some evidence of stability . Observations of beta - stability of light nuclei outside closed shells indicate that nuclear stability is much more complex than simple closure of shell orbitals with magic numbers of protons and neutrons . </P> <P> For larger nuclei, the shells occupied by nucleons begin to differ significantly from electron shells, but nevertheless, present nuclear theory does predict the magic numbers of filled nuclear shells for both protons and neutrons . The closure of the stable shells predicts unusually stable configurations, analogous to the noble group of nearly - inert gases in chemistry . An example is the stability of the closed shell of 50 protons, which allows tin to have 10 stable isotopes, more than any other element . Similarly, the distance from shell - closure explains the unusual instability of isotopes which have far from stable numbers of these particles, such as the radioactive elements 43 (technetium) and 61 (promethium), each of which is preceded and followed by 17 or more stable elements . </P> <P> There are however problems with the shell model when an attempt is made to account for nuclear properties well away from closed shells . This has led to complex post hoc distortions of the shape of the potential well to fit experimental data, but the question remains whether these mathematical manipulations actually correspond to the spatial deformations in real nuclei . Problems with the shell model have led some to propose realistic two - body and three - body nuclear force effects involving nucleon clusters and then build the nucleus on this basis . Three such cluster models are the 1936 Resonating Group Structure model of John Wheeler, Close - Packed Spheron Model of Linus Pauling and the 2D Ising Model of MacGregor . </P>

Does the nucleus account for most of the volume of an atom