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.: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. 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.
- 1 History
- 2 Predicted properties
- 3 Synthesis
- 4 See also
- 5 Notes
- 6 References
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. 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. Fricke predicted the structure of the extended periodic table up to Z = 184 to be:
|Extended periodic table (Large version)|
Chemical and physical properties
Some predicted properties of elements 119 and 120 Property 119 120 Relative atomic mass   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 Boiling point 630 °C 1700 °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.
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. Its first ionization energy is predicted to be 429.4 kJ/mol, which would be lower than those of all known elements except for the alkali metals potassium, rubidium, caesium, and francium: this value is even lower than that of the period 8 alkali metal ununennium (463.1 kJ/mol). 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.
Some predicted compounds of the superactinides (X = a halogen) 121 122 123 124 125 126 132 142 143 144 145 146 148 153 154 155 Compound UbuX3 UbbX4 UbtX5 UbqX6 UbpX6 UbhF
UqpF6 UqoO6 Analogs LaX3 ThF4 UF6
UO6 Oxidation states 3 4 5 6 6 1, 2, 4, 6, 8 6 4, 6 6, 8 3, 4, 5, 6, 8 6 8 12 3 0, 2 3, 5
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.
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.
Pekka Pyykkö divides these superactinides into three series: a 5g series (elements 121 to 138), an 8p1/2 series (elements 139 to 140), and a 6f series (elements 141 to 155), although noting that there would be a great deal of overlapping between energy levels and that the 6f, 7d, or 8p1/2 orbitals could well also be occupied in the early superactinide atoms or ions. He also expects that they would behave more like "superlanthanides", in the sense that the 5g electrons would mostly be chemically inactive, similarly to how only one or two 4f electrons in each lanthanide are ever ionized in chemical compounds. He also predicted that the possible oxidation states of the superactinides might rise very high in the 6f series, to values such as +12 in element 148.
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.
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 stable +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. Nevertheless, the divalent state would be the main one in aqueous solution, and unhexquadium(II) should behave more similarly to lead than unhexquadium(IV) and unhexquadium(VI).
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 is also expected to be a soft Lewis acid and have Ahrlands softness parameter close to 4 eV. It should also have some similarities to ununoctium as well as to the other group 12 elements. Unhexquadium should be at most moderately reactive, having a first ionization energy that should be around 685 kJ/mol, comparable to that of molybdenum. Due to the lanthanide, actinide, and superactinide contractions, unhexquadium should have an metallic radius of only 158 pm, very close to that of the much lighter magnesium, despite its being expected to have an atomic weight of around 474 u, about 19.5 times as much as that of magnesium. This small radius and high weight cause it to be expected to have an extremely high density of around 46 g·cm−3, over twice that of osmium, currently the most dense element known, at 22.61 g·cm−3; unhexquadium should be the second most dense element in the first 9 periods of the periodic table, with only its neighbour unhextrium (element 163) being more dense (at 47 g·cm−3). Metallic unhexquadium should be quite stable, as the 8s and 8p1/2 electrons are very deeply buried in the electron core and only the 7d electrons are available for bonding. Metallic unhexquadium should have a very large cohesive energy due to its covalent bonds, most probably resulting in a high melting point.
Theoretical interest in the chemistry of unhexquadium is largely motivated by theoretical predictions that it, especially the isotope 482Uhq (with 164 protons and 318 neutrons), would be at the center of a hypothetical second island of stability (the first being centered around 306Ubb).
Some predicted properties of the 7d transition metals. The metallic radii and densities are first approximations. X represents a halogen. Property 156 157 158 159 160 161 162 163 164 Relative atomic mass          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, 8 1, 5, 7 2, 6, 8 3, 7 4, 8 3, 5 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 Predicted compounds UpoX6
Analogs WF6, SgF6 OsO4, HsO4 HgX2
The first island of stability is expected to be centered around unbibium-306 (with 122 protons and 184 neutrons), and the second is expected to be centered around unhexquadium-482 (with 164 protons and 318 neutrons).
The following are the expected electron configurations of the period 8 elements.
Chemical element Chemical series Electron configuration
119 Uue Ununennium Alkali metal [Uuo] 8s1 120 Ubn Unbinilium Alkaline earth metal [Uuo] 8s2 121 Ubu Unbiunium Superactinide [Uuo] 8s2 8p1
122 Ubb Unbibium Superactinide [Uuo] 7d1 8s2 8p1
123 Ubt Unbitrium Superactinide [Uuo] 6f1 7d1 8s2 8p1
124 Ubq Unbiquadium Superactinide [Uuo] 6f3 8s2 8p1
125 Ubp Unbipentium Superactinide [Uuo] 5g1 6f3 8s2 8p1
126 Ubh Unbihexium Superactinide [Uuo] 5g2 6f2 7d1 8s2 8p1
127 Ubs Unbiseptium Superactinide [Uuo] 5g3 6f2 8s2 8p2
128 Ubo Unbioctium Superactinide [Uuo] 5g4 6f2 8s2 8p2
129 Ube Unbiennium Superactinide [Uuo] 5g5 6f2 8s2 8p2
130 Utn Untrinilium Superactinide [Uuo] 5g6 6f2 8s2 8p2
131 Utu Untriunium Superactinide [Uuo] 5g7 6f2 8s2 8p2
132 Utb Untribium Superactinide [Uuo] 5g8 6f2 8s2 8p2
133 Utt Untritrium Superactinide [Uuo] 5g8 6f3 8s2 8p2
134 Utq Untriquadium Superactinide [Uuo] 5g8 6f4 8s2 8p2
135 Utp Untripentium Superactinide [Uuo] 5g9 6f4 8s2 8p2
136 Uth Untrihexium Superactinide [Uuo] 5g10 6f4 8s2 8p2
137 Uts Untriseptium Superactinide [Uuo] 5g11 6f3 7d1 8s2 8p2
138 Uto Untrioctium Superactinide [Uuo] 5g12 6f3 7d1 8s2 8p2
139 Ute Untriennium Superactinide [Uuo] 5g13 6f2 7d2 8s2 8p2
140 Uqn Unquadnilium Superactinide [Uuo] 5g14 6f3 7d1 8s2 8p2
141 Uqu Unquadunium Superactinide [Uuo] 5g15 6f2 7d2 8s2 8p2
142 Uqb Unquadbium Superactinide [Uuo] 5g16 6f2 7d2 8s2 8p2
143 Uqt Unquadtrium Superactinide [Uuo] 5g17 6f2 7d2 8s2 8p2
144 Uqq Unquadquadium Superactinide [Uuo] 5g18 6f1 7d3 8s2 8p2
145 Uqp Unquadpentium Superactinide [Uuo] 5g18 6f3 7d2 8s2 8p2
146 Uqh Unquadhexium Superactinide [Uuo] 5g18 6f4 7d2 8s2 8p2
147 Uqs Unquadseptium Superactinide [Uuo] 5g18 6f5 7d2 8s2 8p2
148 Uqo Unquadoctium Superactinide [Uuo] 5g18 6f6 7d2 8s2 8p2
149 Uqe Unquadennium Superactinide [Uuo] 5g18 6f6 7d3 8s2 8p2
150 Upn Unpentnilium Superactinide [Uuo] 5g18 6f6 7d4 8s2 8p2
151 Upu Unpentunium Superactinide [Uuo] 5g18 6f8 7d3 8s2 8p2
152 Upb Unpentbium Superactinide [Uuo] 5g18 6f9 7d3 8s2 8p2
153 Upt Unpenttrium Superactinide [Uuo] 5g18 6f11 7d2 8s2 8p2
154 Upq Unpentquadium Superactinide [Uuo] 5g18 6f12 7d2 8s2 8p2
155 Upp Unpentpentium Superactinide [Uuo] 5g18 6f13 7d2 8s2 8p2
156 Uph Unpenthexium Transition metal [Uuo] 5g18 6f14 7d2 8s2 8p2
157 Ups Unpentseptium Transition metal [Uuo] 5g18 6f14 7d3 8s2 8p2
158 Upo Unpentoctium Transition metal [Uuo] 5g18 6f14 7d4 8s2 8p2
159 Upe Unpentennium Transition metal [Uuo] 5g18 6f14 7d4 8s2 8p2
160 Uhn Unhexnilium Transition metal [Uuo] 5g18 6f14 7d5 8s2 8p2
161 Uhu Unhexunium Transition metal [Uuo] 5g18 6f14 7d6 8s2 8p2
162 Uhb Unhexbium Transition metal [Uuo] 5g18 6f14 7d8 8s2 8p2
163 Uht Unhextrium Transition metal [Uuo] 5g18 6f14 7d9 8s2 8p2
164 Uhq Unhexquadium Transition metal [Uuo] 5g18 6f14 7d10 8s2 8p2
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.
No atoms were identified, leading to a limiting yield of 300 nb. 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:
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. 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.
The Russian team are planning to upgrade their facilities before attempting the reaction again.
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.
In June–July 2010, scientists at the GSI attempted the fusion reaction:
They were unable to detect any atoms but exact details are not currently available.
In August–October 2011, a different team at the GSI using the TASCA facility tried the new reaction:
Results from this experiment are not yet available. 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:
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. 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.
No atoms were detected and a yield limit of 5 mb (5,000,000,000 pb) was measured. Current results (see flerovium) have shown that the sensitivity of this experiment was too low by at least 6 orders of magnitude.
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.
The two attempts in the 1970s to synthesize unbibium were caused by research investigating whether superheavy elements could potentially be naturally occurring. 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. 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.
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:
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.
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.
- Extended periodic table: extension of the table beyond the 7th period
- Nucleon drip line
- Period 9 element
- Period 10 element
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- Pyykkö, Pekka (2011). "A suggested periodic table up to Z≤ 172, based on Dirac–Fock calculations on atoms and ions". Physical Chemistry Chemical Physics 13 (1): 161–8. Bibcode:2011PCCP...13..161P. doi:10.1039/c0cp01575j. PMID 20967377.
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- Natowitz, Joseph (2008). "How stable are the heaviest nuclei?". Physics 1: 12. Bibcode:2008PhyOJ...1...12N. doi:10.1103/Physics.1.12.
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- see slide 11 in Future Plan of the Experimental Program on Synthesizing the Heaviest Element at RIKEN
- see Flerov lab annual reports 2000–2004 inclusive http://www1.jinr.ru/Reports/Reports_eng_arh.html
|Extended periodic table (Large version)|