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The Periodic Table. Geoff Rayner-Canham
Читать онлайн.Название The Periodic Table
Год выпуска 0
isbn 9789811218507
Автор произведения Geoff Rayner-Canham
Жанр Биографии и Мемуары
Издательство Ingram
Two postlead elements for which only radioactive isotopes exist, uranium and thorium, are found quite abundantly on Earth because the half-lives of some of their isotopes — 108 to 109 years — are almost as great as the age of Earth itself.
Synthesis of New Elements
A goal of both chemists and physicists has been the synthesis of atoms of new chemical elements. Such atoms generally have very short half-lives. In fact, in order to claim synthesis of a new element, the isotope must have a half-life longer than 10−14 seconds, thus excluding “quasi-atoms,” briefly existing species formed during nuclear collisions [18]. Seaborg designed a plot of the stability of elements using a geographical analogy of islands in a sea [19]. The goal was to reach the “island of stability” (Figure 1.7). Many variations and updates have appeared since this first one, all based upon the same theme.
Figure 1.7 The original geographic plot by Seaborg of isotope stability (from Ref. [19]).
To accomplish such syntheses, a target of a high atomic number element is bombarded with atoms of a neutron-rich element whose combined atomic number is that, or greater than that, of the desired element. Up to p = 112, the more common route for the synthesis of postactinoid elements was the use of “doubly magic” lead-208 or “singly magic” bismuth-209 as targets and stable neutron-rich nickel-64 or zinc-70 as projectiles [20]. However, to synthesize the “doubly magic” hessium-270, californium-248 was bombarded with magnesium-26 [21].
Beyond P = 112, a different route was chosen. This method involved taking atoms of one of the later actinoid elements and bombarding them with calcium-48 as a projectile. About 0.2% of natural calcium is neutron-rich “doubly magic” calcium-48 (20P, 28N). With a neutron–proton ratio of 1:1.4, calcium-48 has been the key to synthesizing many new elements. Using calcium-48 nuclei as projectiles, nuclear physicists have claimed the synthesis of isotopes of element 114 (Fl) from plutonium-244; element 115 (Mc) from americium-243; element 116 (Lv) from curium-248; element 117 (Ts) from berkelium-249; and element 118 (Og) from californium-249.
Now the aim is to make the first elements of the next period [22]. “Quo Vadis?” stated Karol, as he reviewed the nuclear challenges [23]. There are no long-lived target isotopes with even higher atomic number, while the most probable higher atomic number projectile would be titanium-50. Titanium-50 (abundance 5.2%) has the same magic number of neutrons as calcium-48 with two more protons. Therefore, impacting californium-249 should enable the synthesis of an isotope of element 120. Unfortunately, with a P:N ratio of only 1:1.27, it is less likely that long half-life atoms of the desired atomic number would be produced. Similarly, an isotope of element 119 might be expected from the impact of titanium-50 on berkelium-249. But to raise the probability of success, the focus is on the even-proton-numbered element 120.
Island of Stability
Much of the interest in the synthesis of new elements is the belief that, approaching the next set of magic numbers of protons and neutrons, the trend for ever-shorter half-lives will be reversed. As mentioned earlier, this P-N region has been called the island of stability. There have been predictions that the island is centered around P = 114 and N = 184 and encompass combinations of proton and neutron values around that. Many proposals have been made about the island’s precise location. The major difficulty is in the synthesis of nuclei with high enough numbers of neutrons to confer longer half-lives. Some of the more optimistic calculations have suggested half-lives in days, years, and even millions of years [24].
It has been claimed that other islands of stability may exist. The first of these would be around P = 126, N = 216 or 228; and the second near P = 164, N = 308 or 318. Only time and nuclear experimentation will tell whether these nuclei are forever beyond the limits of synthesis. The key to reaching any of the islands is, of course, adding enough neutrons to generate a high enough P:N ratio [25].
Commentary
Chemists so often overlook the fascinating world of nuclear structure. So much can be explained. In fact, knowing about P:N ratios and magic numbers is key to understanding the difficulties of synthesizing new elements.
References
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4.“Even and odd atomic nuclei,” Wikipedia, https://en.wikipedia.org/wiki/Even_and_odd_atomic_nuclei, accessed 28 June 2019.
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