<P> where m is the rest mass of the electron . For Z> 137, the wave function of the Dirac ground state is oscillatory, rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox . More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc for Z> Z ≈ 173 . For Z> Z, if the innermost orbital (1s) is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron . The precise details of what happens to atoms with Z> 173 are not known yet, but they probably should not survive long enough as such to be considered elements . </P> <P> The first island of stability is expected to be centered on unbibium - 306 (with 122 protons and 184 neutrons), and the second is expected to be centered on unhexquadium - 482 (with 164 protons and 318 neutrons). This second island of stability should confer additional stability on elements 152--168; on the other hand, due to the enormously greater forces of electromagnetic repulsion that must be overcome by the strong force at this second island, it is possible that nuclei around this region only exist as resonances and cannot stay together for a meaningful amount of time . It is also possible that some of the superactinides between these series may not actually exist because they are too far from both islands, in which case the periodic table would quite possibly end around Z = 130 instead of 173, with nuclear properties imposing an end earlier than the electronic properties . </P> <P> Calculations according to the Hartree--Fock--Bogoliubov Method using the non-relativistic Skyrme interaction have proposed Z = 126 as a closed proton shell . In this region of the periodic table, N = 184 and N = 196 have been suggested as closed neutron shells . Therefore, the isotopes of most interest are Ubh and Ubh, for these might be considerably longer - lived than other isotopes . Element 126, having a magic number of protons, is predicted to be more stable than other elements in this region, and may have nuclear isomers with very long half - lives . </P> <P> The following are the expected electron configurations of elements 118--173 . Beyond element 122, no complete calculations are available and hence the data in this table must be taken as tentative . </P>

Where are the most massive elements found on the periodic table