(after lead) have stable isotopes. Nevertheless, because of reasons not very well understood yet, there is a slight increased nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted. In this region of the periodic table, N = 184 has been suggested as a closed neutron shell, and various atomic numbers have been proposed as closed proton shells, such as Z = 114, 120, 122, 124, and 126. The island of stability would be characterized by longer half-lives of nuclei located near these magic numbers, though the extent of stabilizing effects is uncertain due to predictions of weakening of the proton shell closures and possible loss of double magicity. More recent research predicts the island of stability to instead be centered at beta-stable copernicium isotopes 291Cn and 293Cn, which would place unbibium well above the island and result in short half-lives regardless of shell effects. The increased stability of elements 112–118 has also been attributed to the oblate shape of such nuclei and resistance to spontaneous fission. The same model also proposes 306Ubb as the next spherical doubly magic nucleus, thus defining the true island of stability for spherical nuclei. A quantum tunneling model predicts the alpha-decay half-lives of unbibium isotopes 284–322Ubb to be on the order of microseconds or less for all isotopes lighter than 315Ubb, highlighting a significant challenge in experimental observation of this element. This is consistent with many predictions, though the exact location of the 1 microsecond border varies by model. Additionally, spontaneous fission is expected to become a major decay mode in this region, with half-lives on the order of femtoseconds predicted for some even–even isotopes due to minimal hindrance resulting from nucleon pairing and loss of stabilizing effects farther away from magic numbers. A 2016 calculation on the half-lives and probable decay chains of isotopes 280–339Ubb yields corroborating results: 280–297Ubb will be proton unbound and possibly decay by proton emission, 298–314Ubb will have alpha half-lives on the order of microseconds, and those heavier than 314Ubb will predominantly decay by spontaneous fission with short half-lives. For the lighter alpha emitters that may be populated in fusion-evaporation reactions, some long decay chains leading down to known or reachable isotopes of lighter elements are predicted. Additionally,