Island of Relative Stability
232Th (thorium), 235U and 238U (uranium) are the only naturally occurring isotopes beyond bismuth that are relatively stable over the current lifespan of the universe. Bismuth was found to be unstable in 2003, with an α-emission half-life of 1.9×1019 years for 209Bi. All other isotopes beyond bismuth are relatively or very unstable. So the main periodic table ends there (by geographical analogy, the shore edge of a continent; a continental shelf continues however, with shallows beginning at radium that rapidly drop off again after californium, with significant islands at thorium and uranium, as well as minor ones at e.g. plutonium, all of which is surrounded by a "sea of instability") which renders such elements as astatine, radon, and francium extremely short-lived relative to all but the heaviest elements found so far.
Current theoretical investigation indicates that in the region Z=106–108 and N≈160–164, a small ‘island/peninsula’ might be stable with respect to fission and beta decay, such superheavy nuclei undergoing only alpha decay. Also, 298Fl is not the center of the magic island as predicted earlier. On the contrary, the nucleus with Z=110, N=183 (293Ds) appears to be near the center of a possible 'magic island' (Z=104–116, N≈176–186). In the N≈162 region the beta-stable, fission survived 268Sg is predicted to have alpha-decay half-life ~3.2 hours that is greater than that (~28 s) of the deformed doubly-magic 270Hs. The superheavy nucleus 268Sg has not been produced in the laboratory as yet (2009). For superheavy nuclei with Z>116 and N≈184 the alpha-decay half-lives are predicted to be less than one second. The nuclei with Z=120, 124, 126 and N=184 are predicted to form spherical doubly-magic nuclei and be stable with respect to fission. Calculations in a quantum tunneling model show that such superheavy nuclei would undergo alpha decay within microseconds or less.
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