The Periodic Table of the Elements - ACS Publications - American

Chapter 2

The Periodic Table of the Elements: A Review of the Future Downloaded by UNIV OF FLORIDA on March 27, 2018 | https://pubs.acs.org Publication Date (Web): October 30, 2017 | doi: 10.1021/bk-2017-1263.ch002

Paul J. Karol* Chemistry Department, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States *E-mail: [email protected].

Early in 2017, the remaining four elements of the Periodic Table were officially added, completing the seventh row with eka-radon, now oganesson. A perfectly natural vision of what the future holds in store is the subject of this appraisal. How will the Periodic Table continue to evolve? How much further will it go and how fast will new elements be found? Will any new elements be found? Where? Are there any in nature? How will new elements be synthesized? How will the synthesized products be measured? And for our particular interests, what chemistries are to be anticipated? The answers to these questions will constitute a review of our expectations – the future – based on various studies. Emphasis is on qualitative predictions rather than on the theoretical details that address both the nuclear and electronic aspects of structure and reactions.

The Periodic Table We are obligated to begin with a definition of the Periodic Table. In chemistry, the most respected source and official compendium of definitions is found in the “Gold Book” of the International Union of Pure and Applied Chemistry (IUPAC), i.e. the 2014 “Compendium of Chemical Terminology”. Ironically, there is no definition of the Periodic Table. Nevertheless, we proceed under the reasonable assumption that the readership is comfortable with their understanding of the Periodic Table, perhaps the quintessence of chemistry.

© 2017 American Chemical Society Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Downloaded by UNIV OF FLORIDA on March 27, 2018 | https://pubs.acs.org Publication Date (Web): October 30, 2017 | doi: 10.1021/bk-2017-1263.ch002

Our focus is on the upper limit, “superheavy elements”, if any, on the number of elements that might be discovered or synthesized beyond those currently named (1–6). Physicist John Wheeler used the term “very heavy nuclei” in 1955 and subsequently used “superheavy nuclei” that same year for nuclei with a higher atomic number Z than 100, the highest known at the time. Many nuclear chemists define superheavy elements as being transactinides, Z >104 but there are arguments as to why they should refer to a mass number A >280 instead. A very good monograph on our subject is the 1990 publication “The Elements Beyond Uraniuim” by Glenn Seaborg and Walter Loveland. As an aside, note that both theorists and experimentalists in the entire heavy and superheavy element community shun the unwarranted IUPAC Greco-roman systematic naming system that extends to Z=999 (7). It is interesting to look back and review some predictions on the anticipated upper limit to the Table. The first fifty years of speculation on the upper limit are tabulated below in Table 1 based partially on Kragh (1). Note the Z=137 limit which will be discussed more thoroughly later.

Table 1. History of Predictions about the Upper Limit of the Periodic Table Year

Proposed by

Z limit

J. Newlands

1878

A → 480

E. Mills

1884

92

V. Meyer

1889

100

C. Baskerville

1901

93

S. Losanitsch

1906

>92

W. Tilden

1910

92

N. Bohr

1922

hundreds or thousands

S. Rosseland

1923

92

S. Rosseland and N. Bohr

1923

137

A. Sommerfeld

1924

137

The most recent fifty years shown in Table 2 continue the general lack of agreement as to where the end of the Periodic Table will emerge: soon or later, but likely well short of Z = 200. As of this writing, the number of known elements is 118 (8–10). The rate of discovery, disregarding the first dozen or so ancient elements, has been nearly constant over the past two and a half centuries at an average of one element every two and a half years as shown in Figure 1. Whether or not this rate of discovery will continue is implicitly discussed in the following sections. Assuming the discovery rate will continue as displayed, two centuries will elapse before a hypothetical 200th element (not necessarily Z=200) will be discovered, an unlikely achievement. 42 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Table 2. History of Predictions about the Upper Limit of the Periodic Table Year


Proposed by

Z limit

G. Seaborg

1969

126

B. Fricke and W. Greiner

1971

172

A. Migdal

1974

> (137)3/2 ~ 1600

J. Berger et al.

2001

≈300

V. Nefedov

2006

164

A. Khazan

2007

155

J. Emsley

2011

128

W. Brodziński and J. Skalski

2013

126

Y. K. Gambhir et al.

2015

146

Figure 1. The cumulative number of elements as a function of time.

43 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Transuranium Elements in Nature Since one of the co-sponsors of the Symposium on the Periodic Table is the Environmental Division, an appropriate exploration is to review the search for the existence in nature of the heaviest elements: the transuraniums. A summary is shown in Table 3 In 1948, G. Seaborg and I. Perlman extracted chemically purified neptunium and plutonium from a Canadian pitchblende ore (11). The measured alpha activity corresponded to 10-14 g 239Pu/g, most likely from neutron capture on natural uranium over the years. W. Grimm reports that in 1969 S. G. Thompson et al. searched for element 110, eka-Pt, in platinum ores but obtained negative results with a variety of detection methods (12), Also, that year (13), G. N. Flerov et al. found positive evidence for element 114, eka-lead in some lead-bearing samples by the observation of strongly ionizing events identifying fission in proportional counters and of fission tracks in plastics and glasses. Another attempt to discover superheavy elements involved minerals showing giant radiation damage halos hypothetically produced by high-energy alpha particles from transuraniums (14). Alternative explanations for the long-range tracks repudiated those interpretations. Characteristic X-rays were subsequently measured with PIXE (proton induced x-ray emission analysis) (15). Several of the observed x-rays were interpreted as originating from superheavy atoms with Z ranging from 116 to 126. However, this interpretation was refuted by subsequent independent experiments. In 1971, D. C. Hoffman et al. reported on 244Pu in nature (16). The best estimate for terrestrial abundance was 10-18 g 244Pu /g. This was consistent with both nucleosynthetic production 4.7 billion years ago and cosmic-ray influx. M. Jaschek and E. Brandi in 1972 observed spectral lines for Pu, Am and Cm in “peculiar A stars” (17). (A stars are young, main sequence stars.) Evidence for the possible existence of long-lived superheavy nucleus ekathorium with atomic mass number A=292 and atomic number Z ≈ 122 in natural Th was reported by A. Marinov et al. in 2007 (18). An abundance of about 1 x 10-11 relative to 232Th was reported in this work based on mass spectrometric measurements. R. Barber and J. De Laeter disputed the validity of the technique’s asserted sensitivity (19). Existence of long-lived isotopes of superheavy elements in natural Au was claimed by A. Marinov et al. (2011) (20). In essence they claimed to have observed 261Rg and 265Rg at an abundance level of (1-10) x 10-10 in gold. Rg is eka-Au. J. Lachner in 2012 instituting another search for 244Pu found an upper limit 15% of that of the positive finding by Hoffman’s 1971 study (21). Chemically etched radiation damage tracks of heavy nuclei in olivine from pallasite meteorites was used by Bagulya et al. in 2013 to assign approximately 6000 nuclear charges greater than 55 from galactic cosmic rays (22). Three superheavy nuclei were detected whose charge was within the range 105 < Z < 130. 44 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Extensive accelerator mass spectrometry searches for 42 superheavy nuclides (A= 288-310) around the much discussed “island of stability” (Z=114, N=184) in natural Pt, Au, Pb, Bi materials were reviewed in 2015 by Korschinek (23). No positive evidence for the existence of long-lived superheavies (t1/2 > 108 yr) with abundance limits of 10-12 to 10-16 was found. Data on the composition of galactic cosmic-rays with Z > 70 were obtained in experiments by Ter-Akopian (2015) with the aid of Lexan-polycarbonate sheets used to detect the radiation damage tracks of incident heavy cosmic-ray nuclei (24). The high probability obtained for the existence, even at extreme trace levels, of Pu (and possibly Cm) nuclei in galactic cosmic-rays indicates a component of “freshly synthesized” (< 108 years old) nucleosynthetic transuranium matter. Ter-Akopian also discusses experiments carried out in Dubna, Russia found that flerovium is similar to noble gases in its chemical behavior (24). A 140-gram sample of xenon gas extracted from the atmosphere to look for rare events of spontaneous fission provided either by the flerovium nuclei or by their daughters was measured. The attainable limit of the flerovium concentration on the Earth amounts to 10-20 g/g assuming a billion year flerovium lifetime. Belli (2015) noted that searches by others for superheavy elements in Os, Pt and PbF2 established limit 3 were observed corresponding to a hassium limit concentration of < 10-14 g Hs/g Os with the standard assumption that the lifetime of eka-Os is ≈109 yr. Another search based on the similarity of the chemical properties of seaborgium to those of tungsten follows W in chemical purification processes including the growth of a ZnWO4 scintillation detector crystal allowing near 100% radiation detection efficiency. Signals induced in ZnWO4 by alpha particles are different from those caused by beta particles or gamma rays. The limit for a billion year seaborgium life time was 137 because E acquires an imaginary value. In Figure 15, the exact total E’s for the first few atomic orbitals are shown as a function of the nucleus atomic number with the cutoffs implied by the end of each energy curve. One-electron total energies (including rest mass, mc2) for a point nucleus in the figure correspond to total energy relative to mc2.

Figure 15. Total orbital energy (including inertial rest mass) of one-electron systems as a function of nuclear charge for a point nucleus. Below zero, the total E becomes imaginary. The 1s at Z=1 corresponds to E=-13.6 eV when the rest mass energy is ignored, for example. 59 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


In 1969 it was recognized that the Z=137 cutoff could be circumvented by introducing a finite-sized nucleus rather than a point nucleus (30). The one-electron total energies now followed the pattern shown in Figure 16 and place the 1s electron in the negative energy continuum in the presence of a nuclear charge >172.

Figure 16. One-electron total energy (including rest mass, mc2) for a finite-size nucleus. At E below –mc2, the negative energy continuum is breached and positron-electron pair production is predicted to occur.

The fate of the “vacuum” surrounding a completely ionized atom in overcritical Z cases evinces spontaneous pair production (which requires an energy of two electron rest masses = 1022 keV). The “vacuum” becomes negatively charged. Theorists find no stable atomic structures conceivable above the critical nuclear charge of 172. The previous illustration was for one-electron systems. A germane question is how the scheme might change when more than one electron is present. In Figure 17 below, the finite-size nucleus electron total energy (including rest mass, mc2) results are shown for atomic orbitals containing the 18 electrons of an argon-like system (31). Zcrit is essentially unchanged from the single electron result. 60 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 17. Total orbital energies for systems containing eighteen electrons as a function of non-zero-sized nuclear charge, Z. The 1s level dives into the negative energy continuum (-2mec2 = -511 keV) at Zcrit. The dashed extension curves are for Z > Zcrit.

Figure 18. Comparison of (theoretical) orbital radial density distributions with (solid) and without (dashed) relativistic inclusion in copernicium (Z=112). 61 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

Radial density distributions are shown in Figure 18 for the non-relativistic (solid curves) and relativistic (dashed curves) 7s, 7p, and 8s atomic orbitals calculated for 112Cn (31). These demonstrate the effects of relativity that include contraction of the s and p orbital sizes. Those reductions are accompanied by reciprocal expansions in the d and f and higher angular momentum orbitals. Figures 16 and 17 also reflect the increased spin-orbit splitting of the p, d, (f…) atomic orbitals. Theoretical approaches continue to improve but all still lead to electronic instabilities above Zcrit ≈ 172.


The Future A number of promising attempts to synthesize superheavy elements further out on the Periodic Table, eka-francium and eka-radium, have been made and were not successful. These are indicated below in Table 4.

Table 4. Unsuccessful Routes to New Elements 1985

254Es

+ 48Ca → 119X183-x + xn

2007

244Pu

+ 58Fe → 120Y182-x + xn

2009

238U

2011

248Cm

2011

249Cf

+ 50Ti → 120Y179-x + xn

2011

249Bk

+ 50Ti → 119X180-x + xn

+ 64Ni → 120Y182-x + xn + 54Cr → 120Y182-x + xn

So, what’s the problem? Neutron population. Fission and alpha-decay of the intermediate compound nuclei sidetrack the reaction pathway. But what if, in the future, laboratories are successful at overcoming technical challenges such as increased beam intensities (as seems on the horizon) and decay detection systems (which are already quite efficient). Suppose higher mass number and atomic number to Z=164 nuclei and maybe slightly beyond could be synthesized or their existence from astrophysical sources confirmed? What about the positioning of further superheavy elements onto the Periodic Table, the subject of this report? Three major attemptsat calculating electronic structure incorporating relativistic effects have appeared (32–34). These are reviewed in the depictions below in Figures 19, 20 and 21. 62 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


Figure 19. The Mendeleev-Seaborg extended Periodic Table prediction through Z=168 displayed in the spdf (shell partitioned display format) (35) following Seaborg (32).

Figure 20. The extended Periodic Table of Fricke et al. through Z=170 (33).

Figure 21. The extended Periodic Table through Z=172 following Pyykko (34). 63 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.


In Figure 19, Z = 164 would be a p-block element; in Figure 20, Z = 164 would be an s-block element; in Figure 21, Z=164 would be a d-block element. Moreover, in all three variations of the Periodic Table, the element with atomic number Z = 125 has the valence electron configuration of 8s25g5. But to illustrate the emerging issue of assigning valence electrons, Nefedov calculated that the valence electrons occupy a mixed configuration describable as 81% 8s25g6f28p2 + 17% 8s25g6f7d28p + 2% 8s26f37d8p (36). If location on the Periodic Table is determined by valence electron configuration, where would mixed configuration assignments go? That question is currently unanswered. The author has a suggestion: Continue with the spdf (shell partitioned display format) Table structure, Figure 19, and allow for exceptions in electron configurations as is already explicit in the current Periodic Table. Despite the implied complexity on the horizon, peeking into the immediate future allows us to conclude with Figure 22, asserting some reasonable degree of confidence in its realization and correctness. Particularly likely are the alkali and alkaline earth chemistries of eka-francium and eka-radium. Beyond that expectations remain highly provisional (34, 37).

Figure 22. The extended Periodic Table in the near future through eka-radium (Z=120).

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Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

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66 Benvenuto and Williamson; Elements Old and New: Discoveries, Developments, Challenges, and Environmental Implications ACS Symposium Series; American Chemical Society: Washington, DC, 2017.

The Periodic Table of the Elements - ACS Publications - American

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