Island of stability

The island of stability is a term from nuclear physics that describes the possibility of elements with particularly stable "magic numbers" of protons and neutrons. This would allow certain isotopes of some transuranic elements to be far more stable than others, that is, decay much more slowly.

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The idea of the island of stability was first proposed by Glenn T. Seaborg. The hypothesis is that the atomic nucleus is built up in "shells" in a manner similar to the electron shells in atoms. In both cases shells are just groups of quantum energy levels that are relatively close to each other. Energy levels from quantum states in two different shells will be separated by a relatively large energy gap. So when the numbers of neutrons and protons completely fill the energy levels of a given shell in the nucleus, then the binding energy per nucleon will reach a local minimum and thus that particular configuration will have a longer lifetime than nearby isotopes that do not have filled shells^[1].

A filled shell would have "magic numbers" of neutrons and protons. One possible magic number of neutrons is 184, and some possible matching proton numbers are 114, 120 and 126 — which would mean that the most stable possible isotopes would be ununquadium-298, unbinilium-304 and unbihexium-310. Of particular note is Ubh-310, which would be "doubly magic" (both its proton number of 126 and neutron number of 184 are thought to be magic) and thus the most likely to have a very long half-life. (The next lighter doubly-magic nucleus is Lead-208, the heaviest stable nucleus and most stable heavy metal.) None of these transuranic isotopes has yet been produced, but isotopes of elements in the range between 110 through 114 are slower to decay than isotopes of nearby nuclei on the periodic table.

Half-lives of large isotopes

Fermium is the largest element that can be produced in a nuclear reactor. The stability (half-life of the longest-lived isotope) of elements generally decreases from element 101 to element 109 and then approaches an island of stability with longer-lived isotopes in the range of elements 111 and 114^[2]. This is illustrated in the following two tables.

The following table shows information about the half-lives of isotopes of elements 110 through 120.

The half lives of elements in the island are uncertain. Many physicists think they are relatively short, on the order of minutes, hours, or perhaps days. However, some theoretical calculations indicate that their half lives may be long (some calculations put it on the order of 10⁹ years)^[3]. It is possible that these elements could have unusual chemical properties, and, if long lived enough, various applications (such as targets in nuclear physics and neutron sources). However, the isotopes of several of these elements still have too few neutrons to be stable. The island of stability still hasn't been reached, since the island's "shores" have neutron richer nuclides than those produced.

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 hypothetically unstable in 2003, with an α-emission half-life of 1.9 × 10¹⁹ years for Bi-209. All other isotopes beyond bismuth are relatively or very unstable. So the main periodic table ends at bismuth, with an island at thorium and uranium. Between bismuth and thorium there is a sea trough of severe instability, which renders such elements as astatine, radon, and francium extremely short-lived relative to all but the heaviest elements found so far.

Synthesis problems

Manufacturing nuclei in the island of stability may be very difficult, because the nuclei available would not deliver the necessary sum of neutrons. So for the synthesis of isotope 298 of element 114 by using plutonium and calcium, one would require an isotope of plutonium and one of calcium, which have together a sum of at least 298 nucleons (more is better, because at the nuclei reaction some neutrons are emitted). This would require for example in the case of synthesis of element 114 the usage of calcium-50 and plutonium-248. However these isotopes (and heavier calcium and plutonium isotopes) are not available in weighable quantities. This is also the fact for other target/projectile-combinations.

However it may be possible to generate the isotope 298 of element 114, if nuclear transfer reactions would work. One of these reactions may be:

²⁰⁴Hg + ¹³⁶Xe → ²⁹⁸Uuq + ⁴⁰Ca + 2n

References

See also

• Island of stability : Ununquadium – Unbinilium – Unbihexium
• Isotope table and Isotope table (divided) - the best visualization of the island of stability
• Periodic table and Periodic table (extended)