# Period 8 element

A period 8 element is any one of 46 hypothetical chemical elements (ununennium through unhexquadium) belonging to an eighth period of the periodic table of the elements. Sometimes, elements 169 to 172 are also considered to be in period 8 as their outermost electrons fill the 8p3/2 subshells, despite behaving chemically more like the period 9 elements. They may be referred to using IUPAC systematic element names. None of these elements have been synthesized,[note 1] and it is possible that none have isotopes with stable enough nuclei to receive significant attention in the near future. It is also probable that, due to drip instabilities, only the lower period 8 elements are physically possible and the periodic table may end soon after the island of stability at unbihexium with atomic number 126.[1]:593 The names given to these unattested elements are all IUPAC systematic names.

If it were possible to produce sufficient quantities of sufficiently long-lived isotopes of these elements that would allow the study of their chemistry, these elements may well behave very differently from those of previous periods. This is because their electronic configurations may be altered by quantum and relativistic effects, as the energy levels of the 5g, 6f, 7d and 8p1/2 orbitals are so close to each other that they may well exchange electrons with each other.[2] This would result in a large number of elements in the superactinide series that would have extremely similar chemical properties that would be quite unrelated to elements of lower atomic number.[3]

## History

There are currently seven periods in the periodic table of chemical elements, culminating with atomic number 118. If further elements with higher atomic numbers than this are discovered, they will be placed in additional periods, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements concerned. Any additional periods are expected to contain a larger number of elements than the seventh period, as they are calculated to contain elements with filled g-orbitals in their ground state. An eight-period table containing these elements was suggested by Glenn T. Seaborg in 1969.[4][5] No elements in this region have been synthesized or discovered in nature. While Seaborg's version of the extended period had the heavier elements following the pattern set by lighter elements, as it did not take into account relativistic effects, models that take relativistic effects into account do not. Pekka Pyykkö and B. Fricke used computer modeling to calculate the positions of elements up to Z = 172 (comprising periods 8 and 9), and found that several were displaced from the Madelung rule.[6][3] Fricke predicted the structure of the extended periodic table up to Z = 172 to be:

## Predicted properties

### Chemical and physical properties

#### 8s elements

Some predicted properties of elements 119 and 120[3][7]
Property 119 120
Relative atomic mass [322] [325]
Group 1 2
Valence electron configuration 8s1 8s2
Stable oxidation states 1, 3 2, 4
First ionization energy 437.1 kJ/mol 578.9 kJ/mol
Metallic radius 260 pm 200 pm
Density 3 g/cm3 7 g/cm3
Melting point 0–30 °C 680 °C

The first two elements of period 8 are expected to be ununennium and unbinilium, elements 119 and 120. Their electron configurations should have the 8s shell being filled. However, the 8s orbital is relativistically stabilized and contracted and thus, elements 119 and 120 should be more like caesium and barium than their immediate neighbours above, francium and radium. Another effect of the relativistic contraction of the 8s orbital is that the atomic radii of these two elements should be about the same of those of francium and radium. They should behave like normal alkali and alkaline earth metals, normally forming +1 and +2 oxidation states respectively, but the relativistic destabilization of the 7p3/2 subshell and the relatively low ionization energies of the 7p3/2 electrons should make higher oxidation states like +3 and +4 (respectively) possible as well.[3][7]

#### Superactinides

The superactinide series is expected to contain elements 121 to 155. In the superactinide series, the 7d3/2, 8p1/2, 6f5/2 and 5g7/2 shells should all fill simultaneously. The first superactinide, unbiunium (element 121), should be a congener of lanthanum and actinium and should have similar properties to them. In the first few superactinides, the binding energies of the added electrons are predicted to be small enough that they can lose all their valence electrons; for example, unbihexium (element 126) would usually form a +8 oxidation state, and even higher oxidation states for the next few elements may be possible. The presence of electrons in g-orbitals, which do not exist in the ground state electron configuration of any currently known element, should allow presently unknown hybrid orbitals to form and influence the chemistry of the superactinides in new ways, although the absence of g electrons in known elements makes predicting their chemistry more difficult.[3]

In the later superactinides, the oxidation states should become lower. By element 132, the predominant most stable oxidation state will be only +6; this is further reduced to +3 and +4 by element 144, and at the end of the superactinide series it will be only +2 (and possibly even 0) because the 6f shell, which is being filled at that point, is deep inside the electron cloud and the 8s and 8p1/2 electrons are bound too strongly to be chemically active. The 5g shell should be filled at element 144 and the 6f shell at around element 154, and at this region of the superactinides the 8p1/2 electrons are bound so strongly that they are no longer active chemically, so that only a few electrons can participate in chemical reactions. Calculations by Fricke et al. predict that at element 154, the 6f shell is full and there are no d- or other electron wave functions outside the chemically inactive 8s and 8p1/2 shells. This would cause element 154 to be very unreactive, so that it may exhibit properties similar to those of the noble gases.[3][7]

Similarly to the lanthanide and actinide contractions, there should be a superactinide contraction in the superactinide series where the ionic radii of the superactinides are smaller than expected. In the lanthanides, the contraction is about 4.4 pm per element; in the actinides, it is about 3 pm per element. The contraction is larger in the lanthanides than in the actinides due to the greater localization of the 4f wave function as compared to the 5f wave function. Comparisons with the wave functions of the outer electrons of the lanthanides, actinides, and superactinides lead to a prediction of a contraction of about 2 pm per element in the superactinides; although this is smaller than the contractions in the lanthanides and actinides, its total effect is larger due to the fact that 32 electrons are filled in the deeply buried 5g and 6f shells, instead of just 14 electrons being filled in the 4f and 5f shells in the lanthanides and actinides respectively.[3]

#### Transition metals

The transition metals in period 8 are expected to be element 156 to 164. Although the 8s and 8p1/2 electrons are bound so strongly in these elements that they should not be able to take part in any chemical reactions, the 9s and 9p1/2 levels are expected to be readily available for hybridization such that these elements will still behave chemically like their lighter homologues in the periodic table, showing the same oxidation states as they do, in contrast to earlier predictions which predicted the period 8 transition metals to have main oxidation states two less than those of their lighter congeners.[3][7]

The noble metals of this series of transition metals are not expected to be as noble as their lighter homologues, due to the absence of an outer s shell for shielding and also because the 7d shell is strongly split into two subshells due to relativistic effects. This causes the first ionization energies of the 7d transition metals to be smaller than those of their lighter congeners. Calculations predict that the 7d electrons of element 164 (unhexquadium) should participate very readily in chemical reactions, so that unhexquadium should be able to show +6 and +4 oxidation states in addition to the normal +2 state in aqueous solutions with strong ligands. Unhexquadium should thus be able to form compounds like Uhq(CO)4, Uhq(PF3)4 (both tetrahedral), and Uhq(CN)2−
2
(linear), which is very different behavior from that of lead, which unhexquadium would be a heavier homologue of if not for relativistic effects. Unhexquadium should be a soft metal like mercury, and metallic unhexquadium should have a high melting point as it is predicted to bond covalently. It should also have some similarities to ununoctium as well as to the other group 12 elements. The eighth period of the periodic table is expected to end here.[3][7]

Some predicted properties of the 7d transition metals. The metallic radii and densities are first approximations.[3][7]
Property 156 157 158 159 160 161 162 163 164
Relative atomic mass [445] [448] [452] [456] [459] [463] [466] [470] [474]
Group 4 5 6 7 8 9 10 11 12
Valence electron configuration 7d2 7d3 7d4 7d5 7d6 7d7 7d8 7d9 7d10
Stable oxidation states 2, 4 3, 5 4, 6 1, 5, 7 2, 6, 8 3, 7 4, 8 3 0, 2, 4, 6
First ionization energy 395.6 kJ/mol 453.5 kJ/mol 521.0 kJ/mol 337.7 kJ/mol 424.5 kJ/mol 472.8 kJ/mol 559.6 kJ/mol 617.5 kJ/mol 685.0 kJ/mol
Metallic radius 170 pm 163 pm 157 pm 152 pm 148 pm 148 pm 149 pm 152 pm 158 pm
Density 26 g/cm3 28 g/cm3 30 g/cm3 33 g/cm3 36 g/cm3 40 g/cm3 45 g/cm3 47 g/cm3 46 g/cm3

## Synthesis

The only period 8 elements that have had synthesis attempts were elements 119, 120, 122, 124, 126, and 127. So far, none of these synthesis attempts were successful.

### Ununennium

The synthesis of ununennium was attempted in 1985 by bombarding a target of einsteinium-254 with calcium-48 ions at the superHILAC accelerator at Berkeley, California:

$\,^{254}_{99}\mathrm{Es} + \,^{48}_{20}\mathrm{Ca} \to \,^{302}_{119}\mathrm{Uue} ^{*}$

No atoms were identified, leading to a limiting yield of 300 nb.[8] As of May 2012, plans are under way to attempt to synthesize the isotopes 295Uue and 296Uue by bombarding a target of berkelium with titanium at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany:[9][10]

$\,^{249}_{97}\mathrm{Bk} + \,^{50}_{22}\mathrm{Ti} \to \,^{296}_{119}\mathrm{Uue} \,+3\,^{1}_{0}\mathrm{n}$
$\,^{249}_{97}\mathrm{Bk} + \,^{50}_{22}\mathrm{Ti} \to \,^{295}_{119}\mathrm{Uue} \,+4\,^{1}_{0}\mathrm{n}$

#### Target-projectile combinations leading to Z=119 compound nuclei

The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with an atomic number of 119.

Target Projectile CN Attempt result
254Es 48Ca 302Uue Failure to date
249Bk 50Ti 299Uue Planned reaction

### Unbinilium

Attempts to date to synthesize the element using fusion reactions at low excitation energy have met with failure, although there are reports that the fission of nuclei of unbinilium at very high excitation has been successfully measured, indicating a strong shell effect at Z=120. In March–April 2007, the synthesis of unbinilium was attempted at the Flerov Laboratory of Nuclear Reactions in Dubna by bombarding a plutonium-244 target with iron-58 ions.[11] Initial analysis revealed that no atoms of element 120 were produced providing a limit of 400 fb for the cross section at the energy studied.[12]

$\,^{244}_{94}\mathrm{Pu} + \,^{58}_{26}\mathrm{Fe} \to \,^{302}_{120}\mathrm{Ubn} ^{*} \to \ \mathit{fission\ only}$

The Russian team are planning to upgrade their facilities before attempting the reaction again.[13]

In April 2007, the team at GSI attempted to create unbinilium using uranium-238 and nickel-64:[13]

$\,^{238}_{92}\mathrm{U} + \,^{64}_{28}\mathrm{Ni} \to \,^{302}_{120}\mathrm{Ubn} ^{*} \to \ \mathit{fission\ only}$

No atoms were detected providing a limit of 1.6 pb on the cross section at the energy provided. The GSI repeated the experiment with higher sensitivity in three separate runs from April–May 2007, Jan–March 2008, and Sept–Oct 2008, all with negative results and providing a cross section limit of 90 fb.[13]

In June-July 2010, scientists at the GSI attempted the fusion reaction:[13]

$\,^{248}_{96}\mathrm{Cm} + \,^{54}_{24}\mathrm{Cr} \to \,^{302}_{120}\mathrm{Ubn} ^{*}$

They were unable to detect any atoms but exact details are not currently available.[13]

In August-October 2011, a different team at the GSI using the TASCA facility tried the new reaction:[13]

$\,^{249}_{98}\mathrm{Cf} + \,^{50}_{22}\mathrm{Ti} \to \,^{299}_{120}\mathrm{Ubn} ^{*}$

Results from this experiment are not yet available.[13] In 2008, the team at GANIL, France, described the results from a new technique which attempts to measure the fission half-life of a compound nucleus at high excitation energy, since the yields are significantly higher than from neutron evaporation channels. It is also a useful method for probing the effects of shell closures on the survivability of compound nuclei in the super-heavy region, which can indicate the exact position of the next proton shell (Z=114, 120, 124, or 126). The team studied the nuclear fusion reaction between uranium ions and a target of natural nickel:

$\,^{238}_{92}\mathrm{U} + \,^{nat}_{28}\mathrm{Ni} \to \,^{296,298,299,300,302}\mathrm{Ubn} ^{*} \to \ \mathit{fission}.$

The results indicated that nuclei of unbinilium were produced at high (~70 MeV) excitation energy which underwent fission with measurable half-lives > 10−18 s. Although very short, the ability to measure such a process indicates a strong shell effect at Z=120. At lower excitation energy (see neutron evaporation), the effect of the shell will be enhanced and ground-state nuclei can be expected to have relatively long half-lives. This result could partially explain the relatively long half-life of 294Uuo measured in experiments at Dubna. Similar experiments have indicated a similar phenomenon at Z=124 (see unbiquadium) but not for flerovium, suggesting that the next proton shell does in fact lie at Z>120.[14][15] The team at RIKEN have begun a program utilizing 248Cm targets and have indicated future experiments to probe the possibility of Z=120 being the next magic number using the aforementioned nuclear reactions to form 302Ubn.[16]

#### Target-projectile combinations leading to Z=120 compound nuclei

The below table contains various combinations of targets and projectiles which could be used to form compound nuclei with an atomic number of 120.

Target Projectile CN Attempt result
208Pb 88Sr 296Ubn Reaction yet to be attempted[7]
238U 64Ni 302Ubn Failure to date, σ < 94 fb
244Pu 58Fe 302Ubn Failure to date, σ < 0.4 pb
248Cm 54Cr 302Ubn Failure to date, not all details available
250Cm 54Cr 304Ubn Reaction yet to be attempted
249Cf 50Ti 299Ubn Results are not yet available
252Cf 50Ti 302Ubn Reaction yet to be attempted
257Fm 48Ca 305Ubn Reaction yet to be attempted

### Unbibium

The first attempt to synthesize unbibium was performed in 1972 by Flerov et al. at JINR, using the hot fusion reaction:[1]

$\,^{238}_{92}\mathrm{U} + \,^{66}_{30}\mathrm{Zn} \to \,^{304}_{122}\mathrm{Ubb} ^{*} \to \ \mbox{no atoms}.$

No atoms were detected and a yield limit of 5 mb (5,000,000 pb)[dubious ] was measured. Current results (see flerovium) have shown that the sensitivity of this experiment was too low by at least 6 orders of magnitude.[citation needed]

In 2000, the Gesellschaft für Schwerionenforschung (GSI) performed a very similar experiment with much higher sensitivity:[1]

$\,^{238}_{92}\mathrm{U} + \,^{70}_{30}\mathrm{Zn} \to \,^{308}_{122}\mathrm{Ubb} ^{*} \to \ \mbox{no atoms}.$

These results indicate that the synthesis of such heavier elements remains a significant challenge and further improvements of beam intensity and experimental efficiency is required. The sensitivity should be increased to 1 fb.[citation needed]

Another unsuccessful attempt to synthesize unbibium was carried out in 1978 at the GSI, where a natural erbium target was bombarded with xenon-136 ions:[1]

$\,^{nat}_{68}\mathrm{Er} + \,^{136}_{54}\mathrm{Xe} \to \,^{298,300,302,303,304,306}\mathrm{Ubb} ^{*} \to \ \mbox{no atoms}.$

The two attempts in the 1970s to synthesize unbibium were caused by research investigating whether superheavy elements could potentially be naturally occurring.[1] Several experiments have been performed between 2000-2004 at the Flerov laboratory of Nuclear Reactions studying the fission characteristics of the compound nucleus 306Ubb. Two nuclear reactions have been used, namely 248Cm + 58Fe and 242Pu + 64Ni.[1] The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as 132Sn (Z=50, N=82). It was also found that the yield for the fusion-fission pathway was similar between 48Ca and 58Fe projectiles, indicating a possible future use of 58Fe projectiles in superheavy element formation.[17]

In a series of experiments, scientists at GANIL have attempted to measure the direct and delayed fission of compound nuclei of elements with Z=114, 120, and 124 in order to probe shell effects in this region and to pinpoint the next spherical proton shell. This is because having complete nuclear shells (or, equivalently, having a magic number of protons or neutrons) would confer more stability on the nuclei of such superheavy elements, thus moving closer to the island of stability. In 2006, with full results published in 2008, the team provided results from a reaction involving the bombardment of a natural germanium target with uranium ions:

$\,^{238}_{92}\mathrm{U} + \,^{nat}_{32}\mathrm{Ge} \to \,^{308,310,311,312,314}\mathrm{Ubq} ^{*} \to \ fission.$

The team reported that they had been able to identify compound nuclei fissioning with half-lives > 10−18 s. A compound nucleus is a loose combination of nucleons that have not arranged themselves into nuclear shells yet. It has no internal structure and is held together only by the collision forces between the target and projectile nuclei. It is estimated that it requires around 10−14 s for the nucleons to arrange themselves into nuclear shells, at which point the compound nucleus becomes an nuclide, and this number is used by IUPAC as the minimum half-life a claimed isotope must have to potentially be recognised as being discovered. Thus, the GANIL experiments do not count as a discovery of element 124.[1]

### Unbihexium

The first and only attempt to synthesize unbihexium, which was unsuccessful, was performed in 1971 at CERN by René Bimbot and John M. Alexander using the hot fusion reaction:[1]

232
90
Th
+ 84
36
Kr
316
126
Ubh
* → no atoms

A high energy alpha particle was observed and taken as possible evidence for the synthesis of unbihexium. Recent research[which?] suggests that this is highly unlikely as the sensitivity of experiments performed in 1971 would have been several orders of magnitude too low according to current understanding.

### Unbiseptium

Unbiseptium has had one failed attempt at synthesis in 1978 at the Darmstadt UNILAC accelerator by bombarding a natural tantalum target with xenon ions:[1]

$\,^{nat}_{73}\mathrm{Ta} + \,^{136}_{54}\mathrm{Xe} \to \,^{316, 317}\mathrm{Ubs} ^{*} \to \mbox{no atoms}.$

#### Target-projectile combinations leading to Z=127 compound nuclei

The below table shows various combinations of targets and projectiles leading to compound nuclei with an atomic number of 127.

Target Projectile CN Attempt result
180mTa 136Xe 316Ubs Failure to date
181Ta 136Xe 317Ubs Failure to date

## Notes

1. ^ The heaviest element that has been synthesized to date is ununoctium with atomic number 118, which is the last period 7 element.

## References

1. Emsley, John (2011). Nature's Building Blocks: An A-Z Guide to the Elements (New ed.). New York, NY: Oxford University Press. ISBN 978-0-19-960563-7.
2. ^ Waber, J. T. (1969). "SCF Dirac–Slater Calculations of the Translawrencium Elements". The Journal of Chemical Physics 51 (2): 664–661. doi:10.1063/1.1672054. edit
3. Fricke, B.; Greiner, W.; Waber, J. T. (1971). "The continuation of the periodic table up to Z = 172. The chemistry of superheavy elements". Theoretica chimica acta (Springer-Verlag) 21 (3): 235–260. doi:10.1007/BF01172015. Retrieved 28 November 2012.
4. ^ Seaborg, Glenn (August 26, 1996). "An Early History of LBNL".
5. ^ Frazier, K. (1978). "Superheavy Elements". Science News 113 (15): 236–238. doi:10.2307/3963006. JSTOR 3963006.
6. ^
7. Haire, Richard G. (2006). "Transactinides and the future elements". In Morss; Edelstein, Norman M.; Fuger, Jean. The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Dordrecht, The Netherlands: Springer Science+Business Media. ISBN 1-4020-3555-1.
8. ^ R. W. Lougheed, J. H. Landrum, E. K. Hulet, J. F. Wild, R. J. Dougan, A. D. Dougan, H. Gäggeler, M. Schädel, K. J. Moody, K. E. Gregorich, and G. T. Seaborg (1985). "Search for superheavy elements using 48Ca + 254Esg reaction". Physical Reviews C 32 (5): 1760–1763. Bibcode:1985PhRvC..32.1760L. doi:10.1103/PhysRevC.32.1760.
9. ^
10. ^ http://fias.uni-frankfurt.de/kollo/Duellmann_FIAS-Kolloquium.pdf
11. ^ THEME03-5-1004-94/2009
12. ^ Oganessian, Yu. Ts.; Utyonkov, V.; Lobanov, Yu.; Abdullin, F.; Polyakov, A.; Sagaidak, R.; Shirokovsky, I.; Tsyganov, Yu.; Voinov, A. et al. (2009). "Attempt to produce element 120 in the 244Pu+58Fe reaction". Phys. Rev. C 79 (2): 024603. Bibcode:2009PhRvC..79b4603O. doi:10.1103/PhysRevC.79.024603.
13. ^ Natowitz, Joseph (2008). "How stable are the heaviest nuclei?". Physics 1: 12. Bibcode:2008PhyOJ...1...12N. doi:10.1103/Physics.1.12.
14. ^ Morjean, M. et al. (2008). "Fission Time Measurements: A New Probe into Superheavy Element Stability". Phys. Rev. Lett. 101 (7): 072701. Bibcode:2008PhRvL.101g2701M. doi:10.1103/PhysRevLett.101.072701. PMID 18764526.
15. ^
16. ^ see Flerov lab annual reports 2000–2004 inclusive http://www1.jinr.ru/Reports/Reports_eng_arh.html