# Extended periodic table

Extended periodic table
Element 119 (highlighted) in period 8 (row 8)
marks the start of theorizations

An extended periodic table theorizes about chemical elements beyond those currently known and proven up through oganesson, which completes the seventh period (row) in the periodic table at atomic number (Z) 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 have an additional so-called g-block, containing at least 18 elements with partially filled g-orbitals in each period.

An eight-period table containing this block was suggested by Glenn T. Seaborg in 1969.[1][2] IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the nucleus to form an electron cloud.[3] No elements in this region have been synthesized or discovered in nature.[4] The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Elements in this region are likely to be highly unstable with respect to radioactive decay, and have extremely short half lives, although element 126 is hypothesized to be within an island of stability that is resistant to fission but not to alpha decay. It is not clear how many elements beyond the expected island of stability are physically possible, whether period 8 is complete, or if there is a period 9.

According to the orbital approximation in quantum mechanical descriptions of atomic structure, the g-block would correspond to elements with partially filled g-orbitals, but spin-orbit coupling effects reduce the validity of the orbital approximation substantially for elements of high atomic number. 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 Burkhard Fricke used computer modeling to calculate the positions of elements up to Z = 172, and found that several were displaced from the Madelung rule.[5][6]

As early as 1940, it was noted that a simplistic interpretation of the relativistic Dirac equation runs into problems with electron orbitals at Z > 1/α ≈ 137, suggesting that neutral atoms cannot exist beyond element 137, and that a periodic table of elements based on electron orbitals therefore breaks down at this point.[7] On the other hand, a more rigorous analysis calculates the analogous limit to be Z ≈ 173 where the 1s subshell dives into the Dirac sea, and that it is instead not neutral atoms that cannot exist beyond element 173, but bare nuclei, thus posing no obstacle to the further extension of the periodic system. Atoms beyond this critical atomic number are called supercritical atoms.

## History

It is unknown how far the periodic table might extend beyond the known 118 elements. Glenn T. Seaborg suggested that the highest possible element may be under Z = 130,[8] while Walter Greiner predicted that there may not be a highest possible element. The table below shows one possibility for the appearance of the eighth period, with placement of elements primarily based on their predicted chemistry.[9]

All of these hypothetical undiscovered elements are named by the International Union of Pure and Applied Chemistry (IUPAC) systematic element name standard which creates a generic name for use until the element has been discovered, confirmed, and an official name approved. These names are typically not used in the literature, and are referred to by their atomic numbers; hence, element 164 would usually not be called "unhexquadium" (the IUPAC systematic name), but rather "element 164" with symbol "164", "(164)", or "E164".[10]

As of April 2011, synthesis has been attempted for only ununennium, unbinilium, unbiunium, unbibium, unbiquadium, unbihexium, and unbiseptium. (Z = 119, 120, 121, 122, 124, 126, and 127)

### Aufbau model

8 119
Uue
120
Ubn
121
Ubu
122
Ubb
123
Ubt
124
Ubq
125
Ubp
126
Ubh
127
Ubs
128
Ubo
129
Ube
130
Utn
131
Utu
132
Utb
133
Utt
134
Utq
135
Utp
136
Uth
137
Uts
138
Uto
139
Ute
140
Uqn
141
Uqu
142
Uqb
143
Uqt
144
Uqq
145
Uqp
146
Uqh
147
Uqs
148
Uqo
149
Uqe
150
Upn
151
Upu
152
Upb
153
Upt
154
Upq
155
Upp
156
Uph
157
Ups
158
Upo
159
Upe
160
Uhn
161
Uhu
162
Uhb
163
Uht
164
Uhq
165
Uhp
166
Uhh
167
Uhs
168
Uho
s-block g-block f-block d-block p-block

At element 118, the orbitals 1s, 2s, 2p, 3s, 3p, 3d, 4s, 4p, 4d, 4f, 5s, 5p, 5d, 5f, 6s, 6p, 6d, 7s and 7p are assumed to be filled, with the remaining orbitals unfilled. A simple extrapolation from the Aufbau principle would predict the eighth row to fill orbitals in the order 8s, 5g, 6f, 7d, 8p; but after element 120, the proximity of the electron shells makes placement in a simple table problematic. Although a simple extrapolation of the periodic table, following Seaborg's original concept, would put the elements after 120 as follows: 121-138 form the g-block superactinides; 139-152 form the f-block superactinides, 153-161 would be transition metals; 162-166 post-transition metals; 167=halogen; 168=noble gas; 169=alkali metal; 170=alkaline earth metal, Dirac-Fock calculations predict that it will most likely go: 121-142 form the g-block superactinides; 143-156 form the f-block superactinides; 157-165 form the transition metals; 166-170 post-transition metals; 171=halogen; 172=noble gas.

### Pyykkö model

Pyykkö-displaced elements are in boldface
8  119
Uue
120
Ubn
121
Ubu
122
Ubb
123
Ubt
124
Ubq
125
Ubp
126
Ubh
127
Ubs
128
Ubo
129
Ube
130
Utn
131
Utu
132
Utb
133
Utt
134
Utq
135
Utp
136
Uth
137
Uts
138
Uto
141
Uqu
142
Uqb
143
Uqt
144
Uqq
145
Uqp
146
Uqh
147
Uqs
148
Uqo
149
Uqe
150
Upn
151
Upu
152
Upb
153
Upt
154
Upq
155
Upp
156
Uph
157
Ups
158
Upo
159
Upe
160
Uhn
161
Uhu
162
Uhb
163
Uht
164
Uhq
139
Ute
140
Uqn
169
Uhe
170
Usn
171
Usu
172
Usb
9 165
Uhp
166
Uhh
167
Uhs
168
Uho
s-block g-block f-block d-block p-block

Not all models show the higher elements following the pattern established by lighter elements. Pekka Pyykkö, for example, used computer modeling to calculate the positions of elements up to Z=172, and found that several were displaced from the Madelung energy-ordering rule.[6] He predicts that the orbital shells will fill up in this order:

• 8s,
• 5g,
• the first two spaces of 8p,
• 6f,
• 7d,
• 9s,
• the first two spaces of 9p,
• the rest of 8p.

He also suggests that period 8 be split into three parts:

• 8a, containing 8s,
• 8b, containing the first two elements of 8p,
• 8c, containing 7d and the rest of 8p.[11]

### Fricke model

Specially displaced elements are in boldface
8  119
Uue
120
Ubn
121
Ubu
122
Ubb
123
Ubt
124
Ubq
125
Ubp
126
Ubh
127
Ubs
128
Ubo
129
Ube
130
Utn
131
Utu
132
Utb
133
Utt
134
Utq
135
Utp
136
Uth
137
Uts
138
Uto
139
Ute
140
Uqn
141
Uqu
142
Uqb
143
Uqt
144
Uqq
145
Uqp
146
Uqh
147
Uqs
148
Uqo
149
Uqe
150
Upn
151
Upu
152
Upb
153
Upt
154
Upq
155
Upp
156
Uph
157
Ups
158
Upo
159
Upe
160
Uhn
161
Uhu
162
Uhb
163
Uht
164
Uhq
9 165
Uhp
166
Uhh
167
Uhs
168
Uho
169
Uhe
170
Usn
171
Usu
172
Usb
s-block g-block f-block d-block p-block

Fricke et al. also predicted the extended periodic table up to 184.[5] This model has been more widely used among scientists.

## Predicted properties of eighth-period elements

Element 118, oganesson, is the last element that has been synthesized. The next two elements, elements 119 and 120, should form an 8s series and be an alkali and alkaline earth metal respectively. Beyond element 120, the superactinide series is expected to begin, when the 8s electrons and the filling 8p1/2, 7d3/2, 6f5/2, and 5g7/2 subshells determine the chemistry of these elements. Complete and accurate CCSD calculations are not available for elements beyond 122 because of the extreme complexity of the situation: the 5g, 6f, and 7d orbitals should have about the same energy level, and in the region of element 160 the 9s, 8p3/2, and 9p1/2 orbitals should also be about equal in energy. This will cause the electron shells to mix so that the block concept no longer applies very well, and will also result in novel chemical properties that will make positioning these elements in a periodic table very difficult. For example, element 164 is expected to mix characteristics of the elements of group 10, 12, 14, and 18.[10]

### Chemical and physical properties

#### Elements 119 and 120

Some predicted properties of elements 119 and 120[5][10]
Property 119 120
Standard atomic weight [322] [325]
Group 1 2
Valence electron configuration 8s1 8s2
Stable oxidation states 1, 3 2, 4
First ionization energy 463 kJ/mol 580 kJ/mol
Metallic radius 260 pm 200 pm
Density 3 g/cm3 7 g/cm3
Melting point 0–30 °C (32–86 °F) 680 °C (1,300 °F)
Boiling point 630 °C (1,200 °F) 1,700 °C (3,100 °F)

The first two elements of period 8 will be ununennium and unbinilium, elements 119 and 120, if discovered. Their electron configurations should have the 8s orbital being filled. This orbital is relativistically stabilized and contracted and thus, elements 119 and 120 should be more like rubidium and strontium 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 as 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.[5][10]

#### Superactinides

The superactinide series is expected to contain elements 121 to 157. In the superactinide series, the 7d3/2, 8p1/2, 6f5/2 and 5g7/2 shells should all fill simultaneously:[12] this creates very complicated situations, so much so that complete and accurate CCSD calculations have been done only for elements 121 and 122.[10] The first superactinide, unbiunium (element 121), should be a congener of lanthanum and actinium and should have similar properties to them:[13] its main oxidation state should be +3, although the closeness of the valence subshells' energy levels may permit higher oxidation states, just as in elements 119 and 120.[10] Relativistic stabilization of the 8p subshell should result in a ground-state 8s28p1 valence electron configuration for element 121, in contrast to the ds2 configurations of lanthanum and actinium.[10] 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 kJ/mol). Similarly, the next superactinide, unbibium (element 122), may be a congener of cerium and thorium, with a main oxidation state of +4, but would have a ground-state 7d18s28p1 valence electron configuration, unlike thorium's 6d27s2 configuration. Hence, its first ionization energy would be smaller than thorium's (Th: 6.3 eV; Ubb: 5.6 eV) because of the greater ease of ionizing unbibium's 8p1/2 electron than thorium's 6d electron.[10]

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) could easily form a +8 oxidation state, and even higher oxidation states for the next few elements may be possible. Unbihexium is also predicted to display a variety of other oxidation states: recent calculations have suggested a stable monofluoride UbhF may be possible, resulting from a bonding interaction between the 5g orbital on unbihexium and the 2p orbital on fluorine.[14] Other predicted oxidation states include +2, +4, and +6; +4 is expected to be the most usual oxidation state of unbihexium.[12] 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 superactinide chemistry more difficult.[5]

Some predicted compounds of the superactinides (X = a halogen)[11][15]
121 122 123 124 125 126 132 142 143 144 145 146 148 153 154 155 156 157
Compound UbuX3 UbbX4 UbtX5 UbqX6 UbpX6
UbpO2+
2
UbhF
UbhF6
UbhO4
UqbX4
UqbX6
UqtF6 UqqX6
UqqO2+
2

UqqF8
UqqO4
UqpF6 UqoO6
Analogs LaX3
AcX3
CeX4
ThX4
NpO2+
2
ThF4 UF6
UO2+
2

PuF8
PuO4
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 2 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.[5][10]

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.[5]

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.[11]

As an example from the late superactinides, element 156 is expected to exhibit mainly the +2 oxidation state. Its first ionization energy should be about 395.6 kJ/mol and its metallic radius should be about 170 picometers. It should be a very heavy metal with a density of around 26 g/cm3. Its relative atomic mass should be around 445 u.[5]

#### Elements 157 to 166

The transition metals in period 8 are expected to be elements 157 to 165 (or perhaps with element 121 replacing 157, similarly to the dispute on whether lanthanum or lutetium is better placed as the first 5d transition metal). To these element 166 may be added to complete the 7d subshell, although like its lighter group 12 homologues it is questionable if it would show transition metal character. 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.[5][10]

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.[5][10][12]

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).[10][12]

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 oganesson as well as to the other group 12 elements.[10] Unhexquadium should be at most moderately reactive, having a first ionization energy that should be around 685 kJ/mol, comparable to that of molybdenum.[5][12] Due to the lanthanide, actinide, and superactinide contractions, unhexquadium should have a 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.[5] 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 172 elements in the periodic table, with only its neighbor unhextrium (element 163) being more dense (at 47 g·cm−3).[5] 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 (enthalpy of crystallization) due to its covalent bonds, most probably resulting in a high melting point.[12]

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 on copernicium, particularly the isotopes 291Cn, 293Cn, and 296Cn which are expected to have half-lives of centuries or millennia).[16][17][18]

Elements 165 (unhexpentium) and 166 (unhexhexium), the last two 7d metals, should behave similarly to alkali and alkaline earth metals when in the +1 and +2 oxidation states respectively. The 9s electrons should have ionization energies comparable to those of the 3s electrons of sodium and magnesium, due to relativistic effects causing the 9s electrons to be much more strongly bound than non-relativistic calculations would predict. Elements 165 and 166 should normally exhibit the +1 and +2 oxidation states respectively, although the ionization energies of the 7d electrons are low enough to allow higher oxidation states like +3 for element 165. The oxidation state +4 for element 166 is less likely, creating a situation similar to the lighter elements in groups 11 and 12 (particularly gold and mercury).[5][10] As with mercury but not copernicium, ionization of element 166 to Uhh2+ is expected to result in a 7d10 configuration corresponding to the loss of the s-electrons but not the d-electrons, making it more analogous to the lighter "less relativistic" group 12 elements zinc, cadmium, and mercury, which have essentially no transition-metal character.[11]

Some predicted properties of elements 157–166
The metallic radii and densities are first approximations.[5][11][10]
Most analogous group is given first, followed by other similar groups.[12]
Property 157 158 159 160 161 162 163 164 165 166
Standard atomic weight [448] [452] [456] [459] [463] [466] [470] [474] [477] [481]
Group 3
(5)
4
(6)
5
(7)
6
(8)
7
(9)
8
(10)
9
(11)
10
(12, 14, 18)
11
(1, 13)
12
(2, 14)
Valence electron configuration 7d3 7d4 7d4 9s1 7d5 9s1 7d6 9s1 7d7 9s1 7d8 9s1 7d10 7d10 9s1 7d10 9s2
Stable oxidation states 3 4 1 2 3 4 5 2, 4, 6 1, 3 2
First ionization energy 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 521.0 kJ/mol 627.2 kJ/mol
Metallic radius 163 pm 157 pm 152 pm 148 pm 148 pm 149 pm 152 pm 158 pm 250 pm 200 pm
Density 28 g/cm3 30 g/cm3 33 g/cm3 36 g/cm3 40 g/cm3 45 g/cm3 47 g/cm3 46 g/cm3 7 g/cm3 11 g/cm3

#### Elements 167 to 172

The next six elements on the periodic table should be the last main-group elements closing their period.[11] In elements 167 to 172, the 9p1/2 and 8p3/2 shells will be filled. Their energy eigenvalues are so close together that they behave as one combined p shell, similar to the non-relativistic 2p and 3p shells. Thus, the inert pair effect does not occur and the most common oxidation states of elements 167 to 170 should be +3, +4, +5, and +6 respectively. Element 171 (unseptunium) is expected to show some similarities to the halogens, showing various oxidation states ranging from −1 to +7, although its physical properties should be closer to that of a metal. Its electron affinity should be 3.0 eV, allowing it to form HUsu, analogous to a hydrogen halide. The Usu ion is expected to be a soft base, comparable to iodide (I). Element 172 (unseptbium) should be a noble gas with chemical behaviour similar to that of xenon, as their ionization energies should be very similar (Xe, 1170.4 kJ/mol; Usb, 1090.3 kJ/mol). The only main difference between them is that element 172, unlike xenon, is expected to be a liquid or a solid at standard temperature and pressure due to its much higher atomic weight.[5] Unseptbium should be a strong Lewis acid, forming fluorides and oxides, similarly to its lighter congener xenon.[12] Because of this analogy of elements 165–172 to periods 2 and 3, Fricke et al. considered them to form a ninth period of the periodic table, while the eighth period was taken by them to end at the noble metal element 164. This ninth and final period would be similar to the second and third period in that it should have no transition metals.[12]

Some predicted properties of elements 167–172
The metallic or covalent radii and densities are first approximations.[5][10]
Most analogous group is given first, followed by other similar groups.[12]
Property 167 168 169 170 171 172
Standard atomic weight [485] [489] [493] [496] [500] [504]
Group 13 14 15 16 17 18
Valence electron configuration 9s2 9p1 9s2 9p2 9s2 9p2 8p1 9s2 9p2 8p2 9s2 9p2 8p3 9s2 9p2 8p4
Stable oxidation states 3 4 5 6 −1, 3, 7 0, 4, 6, 8
First ionization energy 617.5 kJ/mol 723.6 kJ/mol 800.8 kJ/mol 887.7 kJ/mol 984.2 kJ/mol 1090.3 kJ/mol
Metallic or covalent radius 190 pm 180 pm 175 pm 170 pm 165 pm 220 pm
Density 17 g/cm3 19 g/cm3 18 g/cm3 17 g/cm3 16 g/cm3 9 g/cm3

#### Beyond element 172

Immediately after element 172 (unseptbium, the last period 8 element), the first noble gas after oganesson (the last period 7 element), it was originally expected that another long transition series like the superactinides should begin, filling at least the 6g, 7f, and 8d shells (with 10s, 10p1/2, and 6h11/2 too high in energy to contribute early in the series). These electrons would be very loosely bound, potentially rendering extremely high oxidation states reachable, though the electrons would become more tightly bound as the ionic charge rises.[12]

In element 173 (unsepttrium), the last electron would enter the 6g7/2 subshell. Because spin-orbit interactions would create a very large energy gap between the 8p3/2 and 6g7/2 subshells, this outermost electron is expected to be very loosely bound and very easily lost to form a Ust+ cation. As a result, element 173 is expected to behave chemically like an alkali metal, and one by far more reactive than even caesium (francium and element 119 being less reactive than caesium due to relativistic effects).[19][20]

Element 184 (unoctquadium) was significantly targeted in early predictions, as it was originally speculated that 184 would be a proton magic number: it is predicted to have an electron configuration of [Usb] 6g5 7f4 8d3, with at least the 7f and 8d electrons chemically active. Its chemical behaviour is expected to be similar to uranium and neptunium, as further ionisation past the +6 state (corresponding to removal of the 6g electrons) is likely to be unprofitable; the +4 state should be most common in aqueous solution, with +5 and +6 reachable in solid compounds.[5][12][21]

### End of the periodic table

The number of physically possible elements is unknown. A low estimate is that the periodic table may end soon after the island of stability,[8] which is expected to center on Z = 126, as the extension of the periodic and nuclides tables is restricted by the proton and the neutron drip lines;[22] some, such as Walter Greiner, predicted that there may not be an end to the periodic table.[9] Other predictions of an end to the periodic table include Z = 128 (John Emsley) and Z = 155 (Albert Khazan).[23]

#### Feynmanium and elements above the atomic number 137

It is a "folk legend" among physicists that Richard Feynman suggested that neutral atoms could not exist for atomic numbers greater than Z = 137, on the grounds that the relativistic Dirac equation predicts that the ground-state energy of the innermost electron in such an atom would be an imaginary number. Here, the number 137 arises as the inverse of the fine-structure constant. By this argument, neutral atoms cannot exist beyond untriseptium (alternatively called "feynmanium"), and therefore a periodic table of elements based on electron orbitals breaks down at this point. However, this argument presumes that the atomic nucleus is pointlike. A more accurate calculation must take into account the small, but nonzero, size of the nucleus, which is predicted to push the limit further to Z ≈ 173.[9]

##### Bohr model

The Bohr model exhibits difficulty for atoms with atomic number greater than 137, for the speed of an electron in a 1s electron orbital, v, is given by

${\displaystyle v=Z\alpha c\approx {\frac {Zc}{137.036}}}$

where Z is the atomic number, and α is the fine structure constant, a measure of the strength of electromagnetic interactions.[24] Under this approximation, any element with an atomic number of greater than 137 would require 1s electrons to be traveling faster than c, the speed of light. Hence the non-relativistic Bohr model is clearly inaccurate when applied to such an element.

##### Relativistic Dirac equation

The relativistic Dirac equation gives the ground state energy as

${\displaystyle E={\frac {mc^{2}}{\sqrt {1+{\dfrac {Z^{2}\alpha ^{2}}{n-\left(j+{\frac {1}{2}}\right)+{\sqrt {\left(j+{\frac {1}{2}}\right)^{2}-Z^{2}\alpha ^{2}}}}}}}},}$

where m is the rest mass of the electron. For Z > 137, the wave function of the Dirac ground state is oscillatory, rather than bound, and there is no gap between the positive and negative energy spectra, as in the Klein paradox.[25] More accurate calculations taking into account the effects of the finite size of the nucleus indicate that the binding energy first exceeds 2mc2 for Z > Zcr ≈ 173. For Z > Zcr, if the innermost orbital (1s) is not filled, the electric field of the nucleus will pull an electron out of the vacuum, resulting in the spontaneous emission of a positron.[26][27] This diving of the 1s subshell into the negative continuum has often been taken to constitute an "end" to the periodic table, although more detailed treatments suggest a less bleak outcome.[11][9][28]

Atoms with atomic numbers above Zcr ≈ 173 have been termed supercritical atoms. Supercritical atoms cannot be totally ionised because their 1s subshell would be filled by spontaneous pair creation in which an electron-positron pair is created from the negative continuum, with the electron being bound and the positron escaping. However, the strong field around the atomic nucleus is restricted to a very small region of space, so that the Pauli exclusion principle forbids further spontaneous pair creation once the subshells that have dived into the negative continuum are filled. Elements 173–184 have been termed weakly supercritical atoms as for them only the 1s shell has dived into the negative continuum; the 2p1/2 shell is expected to join around element 185 and the 2s shell around element 245. Unfortunately, experiments have so far not succeeded in detecting spontaneous pair creation from assembling supercritical charges through the collision of heavy nuclei (e.g. colliding lead with uranium to momentarily give an effective Z of 174; uranium with uranium gives effective Z = 184 and uranium with californium gives effective Z = 190). Supercritical atoms are hence expected to pose no difficulties with their electronic structure, so that the end of the periodic table may be determined instead by nuclear instability rather than electron shell instabilities.[29]

### Nuclear properties

The first island of stability is expected to be centered on unbibium-306 (with 122 protons and 184 neutrons),[16] and the second is expected to be centered on unhexquadium-482 (with 164 protons and 318 neutrons).[17][18] This second island of stability should confer additional stability on elements 152–168; on the other hand, due to the enormously greater forces of electromagnetic repulsion that must be overcome by the strong force at this second island, it is possible that nuclei around this region only exist as resonances and cannot stay together for a meaningful amount of time. It is also possible that some of the superactinides between these series may not actually exist because they are too far from both islands, in which case the periodic table would quite possibly end around Z = 130.[12]

Calculations according to the Hartree–Fock–Bogoliubov Method using the non-relativistic Skyrme interaction have proposed Z = 126 as a closed proton shell. In this region of the periodic table, N = 184 and N = 196 have been suggested as closed neutron shells. Therefore, the isotopes of most interest are 310Ubh and 322Ubh, for these might be considerably longer-lived than other isotopes. Element 126, having a magic number of protons, is predicted to be more stable than other elements in this region, and may have nuclear isomers with very long half-lives.[30]

### Electron configurations

The following are the expected electron configurations of elements 118–173. Beyond element 122, no complete calculations are available and hence the data in this table must be taken as tentative.[12][19]

Chemical element Chemical series Predicted electron configuration[10][12][19][31]
118 Og Oganesson Noble gas [Rn] 5f14 6d10 7s2 7p6
119 Uue Ununennium Alkali metal [Og] 8s1
120 Ubn Unbinilium Alkaline earth metal [Og] 8s2
121 Ubu Unbiunium Superactinide [Og] 8s2 8p1
1/2
122 Ubb Unbibium Superactinide [Og] 7d1 8s2 8p1
1/2
123 Ubt Unbitrium Superactinide [Og] 6f2 8s2 8p1
1/2
124 Ubq Unbiquadium Superactinide [Og] 6f3 8s2 8p1
1/2
125 Ubp Unbipentium Superactinide [Og] 5g1 6f2 8s2 8p2
1/2
126 Ubh Unbihexium Superactinide [Og] 5g2 6f3 8s2 8p1
1/2
127 Ubs Unbiseptium Superactinide [Og] 5g3 6f2 8s2 8p2
1/2
128 Ubo Unbioctium Superactinide [Og] 5g4 6f2 8s2 8p2
1/2
129 Ube Unbiennium Superactinide [Og] 5g4 6f3 7d1 8s2 8p1
1/2
130 Utn Untrinilium Superactinide [Og] 5g5 6f3 7d1 8s2 8p1
1/2
131 Utu Untriunium Superactinide [Og] 5g6 6f3 8s2 8p2
1/2
132 Utb Untribium Superactinide [Og] 5g7 6f3 8s2 8p2
1/2
133 Utt Untritrium Superactinide [Og] 5g8 6f3 8s2 8p2
1/2
134 Utq Untriquadium Superactinide [Og] 5g8 6f4 8s2 8p2
1/2
135 Utp Untripentium Superactinide [Og] 5g9 6f4 8s2 8p2
1/2
136 Uth Untrihexium Superactinide [Og] 5g10 6f4 8s2 8p2
1/2
137 Uts Untriseptium Superactinide [Og] 5g11 6f4 8s2 8p2
1/2
138 Uto Untrioctium Superactinide [Og] 5g12 6f3 7d1 8s2 8p2
1/2
139 Ute Untriennium Superactinide [Og] 5g13 6f2 7d2 8s2 8p2
1/2
140 Uqn Unquadnilium Superactinide [Og] 5g14 6f3 7d1 8s2 8p2
1/2
141 Uqu Unquadunium Superactinide [Og] 5g15 6f2 7d2 8s2 8p2
1/2
142 Uqb Unquadbium Superactinide [Og] 5g16 6f2 7d2 8s2 8p2
1/2
143 Uqt Unquadtrium Superactinide [Og] 5g17 6f2 7d2 8s2 8p2
1/2
1/2
145 Uqp Unquadpentium Superactinide [Og] 5g18 6f3 7d2 8s2 8p2
1/2
146 Uqh Unquadhexium Superactinide [Og] 5g18 6f4 7d2 8s2 8p2
1/2
147 Uqs Unquadseptium Superactinide [Og] 5g18 6f5 7d2 8s2 8p2
1/2
148 Uqo Unquadoctium Superactinide [Og] 5g18 6f6 7d2 8s2 8p2
1/2
149 Uqe Unquadennium Superactinide [Og] 5g18 6f6 7d3 8s2 8p2
1/2
150 Upn Unpentnilium Superactinide [Og] 5g18 6f7 7d3 8s2 8p2
1/2
151 Upu Unpentunium Superactinide [Og] 5g18 6f8 7d3 8s2 8p2
1/2
152 Upb Unpentbium Superactinide [Og] 5g18 6f9 7d3 8s2 8p2
1/2
153 Upt Unpenttrium Superactinide [Og] 5g18 6f10 7d3 8s2 8p2
1/2
154 Upq Unpentquadium Superactinide [Og] 5g18 6f11 7d3 8s2 8p2
1/2
155 Upp Unpentpentium Superactinide [Og] 5g18 6f12 7d3 8s2 8p2
1/2
156 Uph Unpenthexium Superactinide [Og] 5g18 6f13 7d3 8s2 8p2
1/2
157 Ups Unpentseptium Superactinide [Og] 5g18 6f14 7d3 8s2 8p2
1/2
158 Upo Unpentoctium Transition metal [Og] 5g18 6f14 7d4 8s2 8p2
1/2
159 Upe Unpentennium Transition metal [Og] 5g18 6f14 7d4 8s2 8p2
1/2
9s1
160 Uhn Unhexnilium Transition metal [Og] 5g18 6f14 7d5 8s2 8p2
1/2
9s1
161 Uhu Unhexunium Transition metal [Og] 5g18 6f14 7d6 8s2 8p2
1/2
9s1
162 Uhb Unhexbium Transition metal [Og] 5g18 6f14 7d7 8s2 8p2
1/2
9s1
163 Uht Unhextrium Transition metal [Og] 5g18 6f14 7d8 8s2 8p2
1/2
9s1
164 Uhq Unhexquadium Transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
165 Uhp Unhexpentium Transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
9s1
166 Uhh Unhexhexium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
9s2
167 Uhs Unhexseptium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
9s2 9p1
1/2
168 Uho Unhexoctium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
9s2 9p2
1/2
169 Uhe Unhexennium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
8p1
3/2
9s2 9p2
1/2
170 Usn Unseptnilium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
8p2
3/2
9s2 9p2
1/2
171 Usu Unseptunium Post-transition metal [Og] 5g18 6f14 7d10 8s2 8p2
1/2
8p3
3/2
9s2 9p2
1/2
172 Usb Unseptbium Noble gas [Og] 5g18 6f14 7d10 8s2 8p2
1/2
8p4
3/2
9s2 9p2
1/2
173 Ust Unsepttrium Alkali metal [Usb] 6g1

## Attempts to synthesize still undiscovered elements

Period 8 elements that have had synthesis attempts were elements 119, 120, 121, 122, 124, 126, and 127. So far, none of these synthesis attempts have been successful.

### Ununennium

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

254
99
Es
+ 48
20
Ca
302
119
Uue
* → no atoms

No atoms were identified, leading to a limiting cross section of 300 nb.[32] Later calculations suggest that the cross section of the 3n reaction (which would result in 299Uue and three neutrons as products) would actually be six hundred thousand times lower than this upper bound, at 0.5 pb.[33]

As ununennium is the lightest undiscovered element, it has been the target of synthesis experiments by both German and Russian teams in recent years.[34][35] The Russian experiments were conducted in 2011, and no results were released, strongly implying that no ununennium atoms were identified. From April to September 2012, an attempt to synthesize the isotopes 295Uue and 296Uue was made by bombarding a target of berkelium-249 with titanium-50 at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany.[36][37] Based on the theoretically predicted cross-section, it was expected that an ununennium atom would be synthesized within five months of the beginning of the experiment.[38]

249
97
Bk
+ 50
22
Ti
299
119
Uue
* → 296
119
Uue
+ 3 1
0

n
249
97
Bk
+ 50
22
Ti
299
119
Uue
* → 295
119
Uue
+ 4 1
0

n

The experiment was originally planned to continue to November 2012,[39] but was stopped early to make use of the 249Bk target to confirm the synthesis of tennessine (thus changing the projectiles to 48Ca).[40] This reaction between 249Bk and 50Ti was predicted to be the most favorable practical reaction for formation of ununennium,[37] as it is rather asymmetrical,[38] though also somewhat cold.[40] (The reaction between 254Es and 48Ca would be superior, but preparing milligram quantities of 254Es for a target is difficult.)[38] Nevertheless, the necessary change from the "silver bullet" 48Ca to 50Ti divides the expected yield of ununennium by about twenty, as the yield is strongly dependent on the asymmetry of the fusion reaction.[38]

Due to the predicted short half-lives, the GSI team used new "fast" electronics capable of registering decay events within microseconds.[37] No ununennium atoms were identified, implying a limiting cross-section of 70 fb.[40] The predicted actual cross-section is around 40 fb, which is at the limits of current technology.[38]

The team at the Joint Institute for Nuclear Research in Dubna, Russia, is planning to begin new experiments on the synthesis of ununennium and unbinilium using the 249Bk+50Ti and 249Cf+50Ti reactions in 2019 using a new experimental complex.[41][42] The team at RIKEN in Japan also plans to make attempts on these elements around the same time with 248Cm targets using the 248Cm+51V and 248Cm+54Cr reactions.[43]

### Unbinilium

Following their success in obtaining oganesson by the reaction between 249Cf and 48Ca in 2006, the team at the Joint Institute for Nuclear Research (JINR) in Dubna started similar experiments in hope of creating unbinilium (element 120) from nuclei of 58Fe and 244Pu.[44] Isotopes of unbinilium are predicted to have alpha decay half-lives of the order of microseconds.[45][46] In March–April 2007, the synthesis of unbinilium was attempted at the JINR by bombarding a plutonium-244 target with iron-58 ions.[47] 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.[48]

244
94
Pu
+ 58
26
Fe
302
120
Ubn
* → no atoms

The Russian team planned to upgrade their facilities before attempting the reaction again.[49]

In April 2007, the team at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany attempted to create unbinilium using uranium-238 and nickel-64:[49]

238
92
U
+ 64
28
Ni
302
120
Ubn
* → no atoms

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, January–March 2008, and September–Oct 2008, all with negative results and providing a cross section limit of 90 fb.[49]

In June–July 2010, and again in 2011, after upgrading their equipment to allow the use of more radioactive targets, scientists at the GSI attempted the more asymmetrical fusion reaction:[49]

248
96
Cm
+ 54
24
Cr
302
120
Ubn
* → no atoms

It was expected that the change in reaction would quintuple the probability of synthesizing unbinilium,[50] as the yield of such reactions is strongly dependent on their asymmetry.[38] Three correlated signals were observed that matched the predicted alpha decay energies of 299Ubn and its daughter 295Og, as well as the experimentally known decay energy of its granddaughter 291Lv. However, the lifetimes of these possible decays were much longer than expected, and the results could not be confirmed.[51][52][53]

In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:[49]

249
98
Cf
+ 50
22
Ti
299
120
Ubn
* → no atoms

Because of its asymmetry,[54] the reaction between 249Cf and 50Ti was predicted to be the most favorable practical reaction for synthesizing unbinilium, although it is also somewhat cold. No unbinilium atoms were identified, implying a limiting cross-section of 200 fb.[40] Jens Volker Kratz predicted the actual maximum cross-section for producing unbinilium by any of these reactions to be around 0.1 fb;[16] in comparison, the world record for the smallest cross section of a successful reaction was 30 fb for the reaction 209Bi(70Zn,n)278Nh,[38] and Kratz predicted a maximum cross-section of 20 fb for producing the neighbouring ununennium.[16] If these predictions are accurate, then synthesizing ununennium would be at the limits of current technology, and synthesizing unbinilium would require new methods.[16]

The team at the Joint Institute for Nuclear Research in Dubna, Russia, is planning to begin new experiments on the synthesis of ununennium and unbinilium using the 249Bk+50Ti and 249Cf+50Ti reactions in 2019 using a new experimental complex.[55][56] The team at RIKEN in Japan also plans to make attempts on these elements around the same time with 248Cm targets using the 248Cm+51V and 248Cm+54Cr reactions.[43]

### Unbiunium

The synthesis of unbiunium was first attempted in 1977 by bombarding a target of uranium-238 with copper-65 ions at the Gesellschaft für Schwerionenforschung in Darmstadt, Germany:

238
92
U
+ 65
29
Cu
303
121
Ubu
* → no atoms

No atoms were identified.[57]

### Unbibium

The first attempt to synthesize unbibium was performed in 1972 by Flerov et al. at JINR (Joint Institute for Nuclear Research), using the hot fusion reaction:[23]

238
92
U
+ 66
30
Zn
304
122
Ubb
* → no atoms

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.[citation needed]

In 2000, the Gesellschaft für Schwerionenforschung (GSI) performed a very similar experiment with much higher sensitivity:[23]

238
92
U
+ 70
30
Zn
308
122
Ubb
* → 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:[23]

nat
68
Er
+ 136
54
Xe
298,300,302,303,304,306
Ubb
* → no atoms

The two attempts in the 1970s to synthesize unbibium were caused by research investigating whether superheavy elements could potentially be naturally occurring.[23] 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.[23] 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.[58]

In a series of experiments, scientists at GANIL (Grand Accélérateur National d'Ions Lourds) 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
U
+ nat
32
Ge
308,310,311,312,314
Ubq
* → fission

The team reported that they had been able to identify compound nuclei fissioning with half-lives > 10−18 s. This result suggests a strong stabilizing effect at Z=124 and points to the next proton shell at Z>120, not at Z=114 as previously thought. 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 a 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.[23]

### Unbihexium

The first and only attempt to synthesize unbihexium, which was unsuccessful, was performed in 1971 at CERN (European Organization for Nuclear Research) by René Bimbot and John M. Alexander using the hot fusion reaction:[23]

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:[23]

nat
73
Ta
+ 136
54
Xe
316,317
Ubs
* → no atoms

## Possible natural occurrence

On April 24, 2008, a group led by Amnon Marinov at the Hebrew University of Jerusalem claimed to have found single atoms of unbibium-292 in naturally occurring thorium deposits at an abundance of between 10−11 and 10−12, relative to thorium.[59] The claim of Marinov et al. was criticized by a part of the scientific community, and Marinov says he has submitted the article to the journals Nature and Nature Physics but both turned it down without sending it for peer review.[60] The unbibium-292 atoms were claimed to be superdeformed or hyperdeformed isomers, with a half-life of at least 100 million years.[23]

A criticism of the technique, previously used in purportedly identifying lighter thorium isotopes by mass spectrometry,[61] was published in Physical Review C in 2008.[62] A rebuttal by the Marinov group was published in Physical Review C after the published comment.[63]

A repeat of the thorium-experiment using the superior method of Accelerator Mass Spectrometry (AMS) failed to confirm the results, despite a 100-fold better sensitivity.[64] This result throws considerable doubt on the results of the Marinov collaboration with regard to their claims of long-lived isotopes of thorium,[61] roentgenium[65] and unbibium.[59] It is still possible that traces of unbibium might only exist in some thorium samples, although this is unlikely.[23]

It was suggested in 1976 that primordial superheavy elements (mainly livermorium, unbiquadium, unbihexium, and unbiseptium) could be a cause of unexplained radiation damage in minerals. This prompted many researchers to search for them in nature from 1976 to 1983. Some[who?] claimed that they had detected alpha particles with the right energies to cause the damage observed, supporting the presence of these elements, while some claimed that none had been detected.

The possible extent of primordial superheavy elements on Earth today is uncertain. Even if they are confirmed to have caused the radiation damage long ago, they might now have decayed to mere traces, or even be completely gone.[30]

A recent hypothesis tries to explain the spectrum of Przybylski's Star by naturally occurring flerovium, unbinilium and unbihexium.[66][67][68]

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