The number of physically possible elements is unknown. A low estimate is that the periodic table may end soon after the island of stability,[8] which is expected to center on Z = 126, as the extension of the periodic and nuclides tables is restricted by the proton and the neutron drip lines;[21] some, such as Walter Greiner, predict that there may not be an end to the periodic table.[9] Other predictions of an end to the periodic table include Z = 128 (John Emsley) and Z = 155 (Albert Khazan).[22]Richard Feynman noted[7] that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at Z > 1/α ≈ 137 as described in the sections below, suggesting that neutral atoms cannot exist beyond untriseptium, and that a periodic table of elements based on electron orbitals therefore breaks down at this point. On the other hand, a more rigorous analysis calculates the limit to be Z ≈ 173.
The first island of stability is expected to be centered on unbibium-306 (with 122 protons and 184 neutrons),[16] and the second is expected to be centered on unhexquadium-482 (with 164 protons and 318 neutrons).[17][18] This second island of stability should confer additional stability on elements 152–168.[12]
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 310Ubh and 322Ubh, for these might be considerably longer-lived than other isotopes. Unbihexium, 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.[27]
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