Extended periodic table

Elements beyond 118 will be placed in additional periods when discovered, laid out (as with the existing periods) to illustrate periodically recurring trends in the properties of the elements. Any additional periods are expected to contain more 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] The first element of the g-block may have atomic number 121, and thus would have the systematic name unbiunium. Despite many searches, no elements in this region have been synthesized or discovered in nature.^[3]

According to the orbital approximation in

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.^[6] On the other hand, a more rigorous analysis calculates the analogous limit to be Z ≈ 168–172 where the 1s subshell dives into the Dirac sea, and that it is instead not neutral atoms that cannot exist beyond this point, 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

Elements beyond the

The first predictions on properties of undiscovered superheavy elements were made in 1957, when the concept of nuclear shells was first explored and an island of stability was theorized to exist around element 126.^[9] In 1967, more rigorous calculations were performed, and the island of stability was theorized to be centered at the then-undiscovered flerovium (element 114); this and other subsequent studies motivated many researchers to search for superheavy elements in nature or attempt to synthesize them at accelerators.^[8] Many searches for superheavy elements were conducted in the 1970s, all with negative results. As of April 2022^[update], synthesis has been attempted for every element up to and including unbiseptium (Z = 127), except unbitrium (Z = 123),^[10][11][12] with the heaviest successfully synthesized element being oganesson in 2002 and the most recent discovery being that of tennessine in 2010.^[10]

As some superheavy elements were predicted to lie beyond the seven-period periodic table, an additional eighth period containing these elements was first proposed by Glenn T. Seaborg in 1969. This model continued the pattern in established elements and introduced a new g-block and superactinide series beginning at element 121, raising the number of elements in period 8 compared to known periods.^[1][2][8] These early calculations failed to consider relativistic effects that break down periodic trends and render simple extrapolation impossible, however. In 1971, Fricke calculated the periodic table up to Z = 172, and discovered that some elements indeed had different properties that break the established pattern,^[4] and a 2010 calculation by Pekka Pyykkö also noted that several elements might behave differently than expected.^[13] It is unknown how far the periodic table might extend beyond the known 118 elements, as heavier elements are predicted to be increasingly unstable. Glenn T. Seaborg suggested that practically speaking, the end of the periodic table might come as early as around Z = 120 due to nuclear instability.^[14]

Predicted structures of an extended periodic table

There is currently no consensus on the placement of elements beyond atomic number 120 in the periodic table.

All hypothetical elements are given an International Union of Pure and Applied Chemistry (IUPAC) systematic element name, for use until the element has been discovered, confirmed, and an official name is approved. These names are typically not used in the literature, and the elements are instead referred to by their atomic numbers; hence, element 164 is usually not called "unhexquadium" or "Uhq" (the systematic name and symbol), but rather "element 164" with symbol "164", "(164)", or "E164".^[15]

Aufbau principle

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.

Fricke

Not all models show the higher elements following the pattern established by lighter elements. Burkhard Fricke et al., who carried out calculations up to element 184 in an article published in 1971, also found some elements to be displaced from the Madelung energy-ordering rule as a result of overlapping orbitals; this is caused by the increasing role of relativistic effects in heavy elements (They describe chemical properties up to element 184, but only draw a table to element 172.)^[4][16]

Fricke et al.'s format is more focused on formal electron configurations than likely chemical behaviour. They place elements 156–164 in groups 4–12 because formally their configurations should be 7d² through 7d¹⁰. However, they differ from the previous d-elements in that the 8s shell is not available for chemical bonding: instead, the 9s shell is. Thus element 164 with 7d¹⁰9s⁰ is noted by Fricke et al. to be analogous to palladium with 4d¹⁰5s⁰, and they consider elements 157–172 to have chemical analogies to groups 3–18 (though they are ambivalent on whether elements 165 and 166 are more like group 1 and 2 elements or more like group 11 and 12 elements, respectively). Thus, elements 157–164 are placed in their table in a group that the authors do not think is chemically most analogous.^[17]

Nefedov

Nefedov [ru], Trzhaskovskaya, and Yarzhemskii carried out calculations up to 164 (results published in 2006). They considered elements 158 through 164 to be homologues of groups 4 through 10, and not 6 through 12, noting similarities of electron configurations to the period 5 transition metals (e.g. element 159 7d⁴9s¹ vs Nb 4d⁴5s¹, element 160 7d⁵9s¹ vs Mo 4d⁵5s¹, element 162 7d⁷9s¹ vs Ru 4d⁷5s¹, element 163 7d⁸9s¹ vs Rh 4d⁸5s¹, element 164 7d¹⁰9s⁰ vs Pd 4d¹⁰5s⁰). They thus agree with Fricke et al. on the chemically most analogous groups, but differ from them in that Nefedov et al. actually place elements in the chemically most analogous groups. Rg and Cn are given an asterisk to reflect differing configurations from Au and Hg (in the original publication they are drawn as being displaced in the third dimension). In fact Cn probably has an analogous configuration to Hg, and the difference in configuration between Pt and Ds is not marked.^[18]

Pyykkö

Pekka Pyykkö used computer modeling to calculate the positions of elements up to Z = 172 and their possible chemical properties in an article published in 2011. He reproduced the orbital order of Fricke et al., and proposed a refinement of their table by formally assigning slots to elements 121–164 based on ionic configurations.^[13]

In order to bookkeep the electrons, Pyykkö places some elements out of order: thus 139 and 140 are placed in groups 13 and 14 to reflect that the 8p_1/2 shell needs to fill, and he distinguishes separate 5g, 8p_1/2, and 6f series.^[13] Fricke et al. and Nefedov et al. do not attempt to break up these series.^[17][18]

Kulsha

Computational chemist Andrey Kulsha has suggested two forms of the extended periodic table up to 172 that build on and refine Nefedov et al.'s versions up to 164 with reference to Pyykkö's calculations.^[19] Based on their likely chemical properties, elements 157–172 are placed by both forms as eighth-period congeners of yttrium through xenon in the fifth period;^[19] this extends Nefedov et al.'s placement of 157–164 under yttrium through palladium,^[18] and agrees with the chemical analogies given by Fricke et al.^[17]

Kulsha suggested two ways to deal with elements 121–156, that lack precise analogues among earlier elements. In his first form (2011, after Pyykkö's paper was published),^[19] elements 121–138 and 139–156 are placed as two separate rows (together called "ultransition elements"), related by the addition of a 5g¹⁸ subshell into the core, as according to Pyykkö's calculations of oxidation states,^[13] they should respectively mimic lanthanides and actinides.^[19][20] In his second suggestion (2016), elements 121–142 form a g-block (as they have 5g activity), while elements 143–156 form an f-block placed under actinium through nobelium.^[21]

Thus, period 8 emerges with 54 elements, and the next noble element after 118 is 172.^[22]

Smits et al.

In 2023 Smits, Düllmann, Indelicato, Nazarewicz, and Schwerdtfeger made another attempt to place elements from 119 to 170 in the periodic table based on their electron configurations. A few elements (121–124 and 168) could not be placed unambiguously. Element 145 appears twice, some places have double occupancy, and others are empty.^[23]

Searches for undiscovered elements

Synthesis attempts

Unsuccessful attempts have been made to synthesise the period 8 elements up to unbiseptium, except unbitrium. Attempts to synthesise ununennium, the first period 8 element, are ongoing as of 2024^[update].

Ununennium (E119)

The synthesis of element 119 (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
₉₉Es
+ ⁴⁸
₂₀Ca
→ ³⁰²119* → no atoms

No atoms were identified, leading to a limiting cross section of 300 nb.^[24] Later calculations suggest that the cross section of the 3n reaction (which would result in ²⁹⁹119 and three neutrons as products) would actually be six hundred thousand times lower than this upper bound, at 0.5 pb.^[25]

From April to September 2012, an attempt to synthesize the isotopes ²⁹⁵119 and ²⁹⁶119 was made by bombarding a target of berkelium-249 with titanium-50 at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany.^[26][27] 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.^[28] Moreover, as berkelium-249 decays to californium-249 (the next element) with a short half-life of 327 days, this allowed elements 119 and 120 to be searched for simultaneously.^[29]

²⁴⁹
₉₇Bk
+ ⁵⁰
₂₂Ti
→ ²⁹⁹119* → no atoms

The experiment was originally planned to continue to November 2012,^[30] but was stopped early to make use of the ²⁴⁹Bk target to confirm the synthesis of tennessine (thus changing the projectiles to ⁴⁸Ca).^[31] This reaction between ²⁴⁹Bk and ⁵⁰Ti was predicted to be the most favorable practical reaction for formation of element 119,^[27] as it is rather asymmetrical,^[28] though also somewhat cold.^[31] (The reaction between ²⁵⁴Es and ⁴⁸Ca would be superior, but preparing milligram quantities of ²⁵⁴Es for a target is difficult.)^[28] Nevertheless, the necessary change from the "silver bullet" ⁴⁸Ca to ⁵⁰Ti divides the expected yield of element 119 by about twenty, as the yield is strongly dependent on the asymmetry of the fusion reaction.^[28]

Due to the predicted short half-lives, the GSI team used new "fast" electronics capable of registering decay events within microseconds.^[27] No atoms of element 119 were identified, implying a limiting cross section of 70 fb.^[31] The predicted actual cross section is around 40 fb, which is at the limits of current technology.^[28]

The team at RIKEN in Wakō, Japan began bombarding curium-248 targets with a vanadium-51 beam in January 2018^[32] to search for element 119. Curium was chosen as a target, rather than heavier berkelium or californium, as these heavier targets are difficult to prepare.^[33] The ²⁴⁸Cm targets were provided by Oak Ridge National Laboratory. RIKEN developed a high-intensity vanadium beam.^[34] The experiment began at a cyclotron while RIKEN upgraded its linear accelerators; the upgrade was completed in 2020.^[35] Bombardment may be continued with both machines until the first event is observed; the experiment is currently running intermittently for at least 100 days per year.^[36][33] The RIKEN team's efforts are being financed by the Emperor of Japan.^[37] The team at the JINR plans to attempt synthesis of element 119 in the future, probably using the ²⁴³Am + ⁵⁴Cr reaction, but a precise timeframe has not been publicly released.^[38][39]

Unbinilium (E120)

Following their success in obtaining

The Russian team planned to upgrade their facilities before attempting the reaction again.[44]

In April 2007, the team at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany, attempted to create element 120 using uranium-238 and nickel-64:^[45]

²³⁸
₉₂U
+ ⁶⁴
₂₈Ni
→ ³⁰²120* → no atoms

No atoms were detected providing a limit of 1.6 pb for the cross section at the energy provided. The GSI repeated the experiment with higher sensitivity in three separate runs in April–May 2007, January–March 2008, and September–October 2008, all with negative results, reaching a cross section limit of 90 fb.^[45]

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:^[46]

²⁴⁸
₉₆Cm
+ ⁵⁴
₂₄Cr
→ ³⁰²120 → no atoms

It was expected that the change in reaction would quintuple the probability of synthesizing element 120,^[47] as the yield of such reactions is strongly dependent on their asymmetry.^[28] Three correlated signals were observed that matched the predicted alpha decay energies of ²⁹⁹120 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.^[48][49][46]

In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:^[50][31]

²⁴⁹
₉₈Cf
+ ⁵⁰
₂₂Ti
→ ²⁹⁹120* → no atoms

This was also tried unsuccessfully the next year during the aforementioned attempt to make element 119 in the ²⁴⁹Bk+⁵⁰Ti reaction, as ²⁴⁹Bk decays to ²⁴⁹Cf. Because of its asymmetry,^[51] 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.^[31] Jens Volker Kratz predicted the actual maximum cross-section for producing element 120 by any of these reactions to be around 0.1 fb;^[52] in comparison, the world record for the smallest cross section of a successful reaction was 30 fb for the reaction ²⁰⁹Bi(⁷⁰Zn,n)²⁷⁸Nh,^[28] and Kratz predicted a maximum cross-section of 20 fb for producing the neighbouring element 119.^[52] If these predictions are accurate, then synthesizing element 119 would be at the limits of current technology, and synthesizing element 120 would require new methods.^[52]

In May 2021, the JINR announced plans to investigate the ²⁴⁹Cf+⁵⁰Ti reaction in their new facility. However, the ²⁴⁹Cf target would have had to be made by the Oak Ridge National Laboratory in the United States,^[53] and after the Russian invasion of Ukraine began in February 2022, collaboration between the JINR and other institutes completely ceased due to sanctions.^[54] Consequently, the JINR now plans to try the ²⁴⁸Cm+⁵⁴Cr reaction instead. A preparatory experiment for the use of ⁵⁴Cr projectiles was conducted in late 2023, successfully synthesising ²⁸⁸Lv in the ²³⁸U+⁵⁴Cr reaction,^[55] and the hope is for experiments to synthesise element 120 to begin by 2025.^[56]

Starting from 2022,^[34] plans have also been made to use 88-inch cyclotron in the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California, United States to attempt to make new elements using ⁵⁰Ti projectiles. The plan is to first test them on a plutonium target to create livermorium (element 116) in late 2023. If that is successful, an attempt to make element 120 in the ²⁴⁹Cf+⁵⁰Ti reaction will begin, probably in 2024 at the earliest.^[57][58]

Unbiunium (E121)

The synthesis of

No atoms were identified.[11]

Unbibium (E122)

The first attempts to synthesize

These experiments were motivated by early predictions on the existence of an island of stability at N = 184 and Z > 120. No atoms were detected and a yield limit of 5 nb (5,000 pb) was measured. Current results (see flerovium) have shown that the sensitivity of these experiments were too low by at least 3 orders of magnitude.^[12]

In 2000, the

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 in the future for more quality results.

Another unsuccessful attempt to synthesize element 122 was carried out in 1978 at the GSI Helmholtz Center, where a natural

In particular, the reaction between ¹⁷⁰Er and ¹³⁶Xe was expected to yield alpha-emitters with half-lives of microseconds that would decay down to isotopes of flerovium with half-lives perhaps increasing up to several hours, as flerovium is predicted to lie near the center of the island of stability. After twelve hours of irradiation, nothing was found in this reaction. Following a similar unsuccessful attempt to synthesize element 121 from ²³⁸U and ⁶⁵Cu, it was concluded that half-lives of superheavy nuclei must be less than one microsecond or the cross sections are very small.^[59] More recent research into synthesis of superheavy elements suggests that both conclusions are true.^[28][60] The two attempts in the 1970s to synthesize element 122 were both propelled by the research investigating whether superheavy elements could potentially be naturally occurring.^[10]

Several experiments studying the fission characteristics of various superheavy compound nuclei such as ³⁰⁶122* were performed between 2000 and 2004 at the

Scientists at

The fission of the compound nucleus ³¹²124 was also studied in 2006 at the tandem ALPI heavy-ion accelerator at the Laboratori Nazionali di Legnaro (Legnaro National Laboratories) in Italy:^[62]

²³²
₉₀Th
+ ⁸⁰
₃₄Se
→ ³¹²124* → fission

Similarly to previous experiments conducted at the JINR (

No atoms were detected, and a cross section limit of 5 nb was determined. This experiment was motivated by the possibility of greater stability for nuclei around Z ~ 126 and N ~ 184,[12] though more recent research suggests the island of stability may instead lie at a lower atomic number (such as copernicium, Z = 112), and the synthesis of heavier elements such as element 125 will require more sensitive experiments.^[28]

Unbihexium (E126)

The first and only attempt to synthesize element 126 (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:^[10]

²³²
₉₀Th
+ ⁸⁴
₃₆Kr
→ ³¹⁶126* → no atoms

On April 24, 2008, a group led by Amnon Marinov at the Hebrew University of Jerusalem claimed to have found single atoms of ²⁹²122 in naturally occurring thorium deposits at an abundance of between 10⁻¹¹ and 10⁻¹² relative to thorium.^[65] The claim of Marinov et al. was criticized by a part of the scientific community. Marinov claimed that he had submitted the article to the journals Nature and Nature Physics but both turned it down without sending it for peer review.^[66] The ²⁹²122 atoms were claimed to be superdeformed or hyperdeformed isomers, with a half-life of at least 100 million years.^[10]

A criticism of the technique, previously used in purportedly identifying lighter

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.^[70] This result throws considerable doubt on the results of the Marinov collaboration with regard to their claims of long-lived isotopes of thorium,^[67] roentgenium^[71] and element 122.^[65] It is still possible that traces of unbibium might only exist in some thorium samples, although this is unlikely.^[10]

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.^[72] It is also uncertain if such superheavy nuclei may be produced naturally at all, as spontaneous fission is expected to terminate the r-process responsible for heavy element formation between mass number 270 and 290, well before elements beyond 120 may be formed.^[73]

A recent hypothesis tries to explain the spectrum of Przybylski's Star by naturally occurring flerovium and element 120.^[74][75][76]

Predicted properties of eighth-period elements

Element 118,

Chemical and physical properties

Elements 119 and 120

The first two elements of period 8 will be ununennium and unbinilium, elements 119 and 120. Their electron configurations should have the 8s orbital being filled. This orbital is relativistically stabilized and contracted; 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 (albeit less reactive than their immediate vertical neighbours), normally forming +1 and +2 oxidation states respectively, but the relativistic destabilization of the 7p_3/2 subshell and the relatively low ionization energies of the 7p_3/2 electrons should make higher oxidation states like +3 and +4 (respectively) possible as well.^[4][15]

Superactinides

The superactinides may be considered to range from elements 121 through 157, which can be classified as the 5g and 6f elements of the eighth period, together with the first 7d element.

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 8p_1/2 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 8p_1/2 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 8p_1/2 shells. This may cause element 154 to be rather unreactive with noble gas-like properties.^[4][15] Calculations by Pyykkö nonetheless expect that at element 155, the 6f shell is still chemically ionisable: 155³⁺ should have a full 6f shell, and the fourth ionisation potential should be between those of terbium and dysprosium, both of which are known in the +4 state.^[13]

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.^[4]

Pekka Pyykkö divides these superactinides into three series: a 5g series (elements 121 to 138), an 8p_1/2 series (elements 139 to 140), and a 6f series (elements 141 to 155), also noting that there would be a great deal of overlapping between energy levels and that the 6f, 7d, or 8p_1/2 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.^[13]

Andrey Kulsha has called the thirty-six elements 121 to 156 "ultransition" elements and has proposed to split them into two series of eighteen each, one from elements 121 to 138 and another from elements 139 to 156. The first would be analogous to the lanthanides, with oxidation states mainly ranging from +4 to +6, as the filling of the 5g shell dominates and neighbouring elements are very similar to each other, creating an analogy to uranium, neptunium, and plutonium. The second would be analogous to the actinides: at the beginning (around elements in the 140s) very high oxidation states would be expected as the 6f shell rises above the 7d one, but after that the typical oxidation states would lower and in elements in the 150s onwards the 8p_1/2 electrons would stop being chemically active. Because the two rows are separated by the addition of a complete 5g¹⁸ subshell, they could be considered analogues of each other as well.^[19][20]

As an example from the late superactinides, element 156 is expected to exhibit mainly the +2 oxidation state, on account of its electron configuration with easily removed 7d² electrons over a stable [Og]5g¹⁸6f¹⁴8s²8p²
_1/2 core. It can thus be considered a heavier congener of nobelium, which likewise has a pair of easily removed 7s² electrons over a stable [Rn]5f¹⁴ core, and is usually in the +2 state (strong oxidisers are required to obtain nobelium in the +3 state).^[19] Its first ionization energy should be about 400 kJ/mol and its metallic radius approximately 170 picometers. With a relative atomic mass of around 445 u,^[4] it should be a very heavy metal with a density of around 26 g/cm³.

Elements 157 to 166

The 7d transition metals in period 8 are expected to be elements 157 to 166. Although the 8s and 8p_1/2 electrons are bound so strongly in these elements that they should not be able to take part in any chemical reactions, the 9s and 9p_1/2 levels are expected to be readily available for hybridization.^[4][15] These 7d elements should be similar to the 4d elements yttrium through cadmium.^[19] In particular, element 164 with a 7d¹⁰9s⁰ electron configuration shows clear analogies with palladium with its 4d¹⁰5s⁰ electron configuration.^[16]

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.^[4][15][16]

Theoretical interest in the chemistry of unhexquadium is largely motivated by theoretical predictions that it, especially the isotopes ⁴⁷²164 and ⁴⁸²164 (with 164 protons and 308 or 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).^{[83][52][84][85]}

Calculations predict that the 7d electrons of element 164 (unhexquadium) should participate very readily in chemical reactions, so that it should be able to show stable +6 and +4 oxidation states in addition to the normal +2 state in

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).^[4][15] As with mercury but not copernicium, ionization of element 166 to 166²⁺ 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.^[13]

Elements 167 to 172

The next six elements on the periodic table are expected to be the last main-group elements in their period,

Because of some 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 period would be similar to the second and third period in having no transition metals.[16] That being said, the analogy is incomplete for elements 165 and 166; although they do start a new s-shell (9s), this is above a d-shell, making them chemically more similar to groups 11 and 12.^[17]

Beyond element 172

Beyond element 172, there is the potential to fill the 6g, 7f, 8d, 10s, 10p_1/2, and perhaps 6h_11/2 shells. 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. Thus, there will probably be another very long transition series, like the superactinides.^[16]

In element 173 (unsepttrium), the outermost electron might enter the 6g_7/2, 9p_3/2, or 10s subshells. Because spin–orbit interactions would create a very large energy gap between these and the 8p_3/2 subshell, this outermost electron is expected to be very loosely bound and very easily lost to form a 173⁺ cation. As a result, element 173 is expected to behave chemically like an alkali metal, and one that might be far more reactive than even caesium (francium and element 119 being less reactive than caesium due to relativistic effects):^[86][19] the calculated ionisation energy for element 173 is 3.070 eV,^[87] compared to the experimentally known 3.894 eV for caesium. Element 174 (unseptquadium) may add an 8d electron and form a closed-shell 174²⁺ cation; its calculated ionisation energy is 3.614 eV.^[87]

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 [172] 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.^[4][16][88]

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,^[14] which is expected to center on Z = 126, as the extension of the periodic and nuclide tables is restricted by the proton and the neutron drip lines and stability toward alpha decay and spontaneous fission.^[89] One calculation by Y. Gambhir et al., analyzing nuclear binding energy and stability in various decay channels, suggests a limit to the existence of bound nuclei at Z = 146.^[90] Other predictions of an end to the periodic table include Z = 128 (John Emsley) and Z = 155 (Albert Khazan).^[10]

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 atomic number 137, 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.^[91]

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

${\displaystyle v=Z\alpha c\approx {\frac {Zc}{137.04}}}$

where Z is the atomic number, and α is the fine-structure constant, a measure of the strength of electromagnetic interactions.^[92] 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 inaccurate when applied to such an element.

Relativistic Dirac equation

The relativistic Dirac equation gives the ground state energy as

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

where m is the rest mass of the electron.^[93] 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.^[94] 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 > Z_cr probably between 168 and 172.^[95] For Z > Z_cr, 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.^[96][97] This diving of the 1s subshell into the negative continuum has often been taken to constitute an "end" to the periodic table,^[13][91][98] but in fact it does not impose such a limit, as such resonances can be interpreted as Gamow states. The accurate description of such states in a multi-electron system, needed to extend calculations and the periodic table past Z_cr ≈ 172, are however still open problems.^[95]

Atoms with atomic numbers above Z_cr ≈ 172 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 2p_1/2 shell is expected to join around element 185 and the 2s shell around element 245. 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).^[99]

Even though passing Z_cr does not mean elements can no longer exist, the increasing concentration of the 1s density close to the nucleus would likely make these electrons more vulnerable to K electron capture as Z_cr is approached. For such heavy elements, these 1s electrons would likely spend a significant fraction of time so close to the nucleus that they are actually inside it. This may pose another limit to the periodic table.^[100]

Because of the factor of m,

It has also been posited that in the region beyond A > 300, an entire "

Calculations published in 2020[102] suggest stability of up-down quark matter (udQM) nuggets against conventional nuclei beyond A ~ 266, and also show that udQM nuggets become supercritical earlier (Z_cr ~ 163, A ~ 609) than conventional nuclei (Z_cr ~ 177, A ~ 480).

Nuclear properties

Predicted half-lives (top) and decay modes (bottom) of superheavy nuclei. The line of synthesized proton-rich nuclei is expected to be broken soon after Z = 120, because of half-lives shorter than 1 microsecond from Z = 121, the increasing contribution of spontaneous fission instead of alpha decay from Z = 122 onward until it dominates from Z = 125, and the proton drip line around Z = 130. The white rings denote the expected location of the island of stability; the two squares outlined in white denote ²⁹¹Cn and ²⁹³Cn, predicted to be the longest-lived nuclides on the island with half-lives of centuries or millennia.^[60] The black square near the bottom of the second picture is uranium-238, the heaviest confirmed primordial nuclide (a nuclide stable enough to have survived from the Earth's formation to the present day).

Magic numbers and the island of stability

The stability of nuclei decreases greatly with the increase in atomic number after

The International Union of Pure and Applied Chemistry (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. However, a nuclide is generally considered to exist if its lifetime is longer than about 10⁻²² seconds, which is the time it takes for nuclear structure to form. Consequently, it is possible that some Z values can only be realised in nuclides and that the corresponding elements do not exist.^[100]

It is also possible that no further islands actually exist beyond 126, as the nuclear shell structure gets smeared out (as the electron shell structure already is expected to be around oganesson) and low-energy decay modes become readily available.^[108]

In some regions of the table of nuclides, there are expected to be additional regions of stability due to non-spherical nuclei that have different magic numbers than spherical nuclei do; the egg-shaped ²⁷⁰Hs (Z = 108, N = 162) is one such deformed doubly magic nucleus.^[109] In the superheavy region, the strong Coulomb repulsion of protons may cause some nuclei, including isotopes of oganesson, to assume a bubble shape in the ground state with a reduced central density of protons, unlike the roughly uniform distribution inside most smaller nuclei.^[110][111] Such a shape would have a very low fission barrier, however.^[112] Even heavier nuclei in some regions, such as ³⁴²136 and ⁴⁶⁶156, may instead become toroidal or red blood cell-like in shape, with their own magic numbers and islands of stability, but they would also fragment easily.^[113][114]

Predicted decay properties of undiscovered elements

As the main island of stability is thought to lie around ²⁹¹Cn and ²⁹³Cn, undiscovered elements beyond

Beyond element 123, no complete calculations are available and hence the data in this table must be taken as tentative.^{[16][86][117]} In the case of element 123, and perhaps also heavier elements, several possible electron configurations are predicted to have very similar energy levels, such that it is very difficult to predict the ground state. All configurations that have been proposed (since it was understood that the Madelung rule probably stops working here) are included.^{[117][79][118]}

The predicted block assignments up to 172 are Kulsha's,^[21] following the expected available valence orbitals. There is, however, not a consensus in the literature as to how the blocks should work after element 138.

Chemical element			Block	Predicted electron configurations[15][16][86][18]
119	Uue	Ununennium	s-block	[Og] 8s1
120	Ubn	Unbinilium	s-block	[Og] 8s2
121	Ubu	Unbiunium	g-block	[Og] 8s2 8p1 1/2[79]
122	Ubb	Unbibium	g-block	[Og] 8s2 8p2 1/2[79] [Og] 7d1 8s2 8p1 1/2
123	Ubt	Unbitrium	g-block	[Og] 6f1 8s2 8p2 1/2[119] [Og] 6f1 7d1 8s2 8p1 1/2[117][79] [Og] 6f2 8s2 8p1 1/2 [Og] 8s2 8p2 1/2 8p1 3/2[117]
124	Ubq	Unbiquadium	g-block	[Og] 6f2 8s2 8p2 1/2[79][119] [Og] 6f3 8s2 8p1 1/2
125	Ubp	Unbipentium	g-block	[Og] 6f4 8s2 8p1 1/2[79] [Og] 5g1 6f2 8s2 8p2 1/2[119] [Og] 5g1 6f3 8s2 8p1 1/2 [Og] 8s2 0.81(5g1 6f2 8p2 1/2) + 0.17(5g1 6f1 7d2 8p1 1/2) + 0.02(6f3 7d1 8p1 1/2)
126	Ubh	Unbihexium	g-block	[Og] 5g1 6f4 8s2 8p1 1/2[79] [Og] 5g2 6f2 8s2 8p2 1/2[119] [Og] 5g2 6f3 8s2 8p1 1/2 [Og] 8s2 0.998(5g2 6f3 8p1 1/2) + 0.002(5g2 6f2 8p2 1/2)
127	Ubs	Unbiseptium	g-block	[Og] 5g2 6f3 8s2 8p2 1/2[79] [Og] 5g3 6f2 8s2 8p2 1/2[119] [Og] 8s2 0.88(5g3 6f2 8p2 1/2) + 0.12(5g3 6f1 7d2 8p1 1/2)
128	Ubo	Unbioctium	g-block	[Og] 5g3 6f3 8s2 8p2 1/2[79] [Og] 5g4 6f2 8s2 8p2 1/2[119] [Og] 8s2 0.88(5g4 6f2 8p2 1/2) + 0.12(5g4 6f1 7d2 8p1 1/2)
129	Ube	Unbiennium	g-block	[Og] 5g4 6f3 7d1 8s2 8p1 1/2 [Og] 5g4 6f3 8s2 8p2 1/2[79][119] [Og] 5g5 6f2 8s2 8p2 1/2 [Og] 5g4 6f3 7d1 8s2 8p1 1/2
130	Utn	Untrinilium	g-block	[Og] 5g5 6f3 7d1 8s2 8p1 1/2 [Og] 5g5 6f3 8s2 8p2 1/2[79][119] [Og] 5g6 6f2 8s2 8p2 1/2 [Og] 5g5 6f3 7d1 8s2 8p1 1/2
131	Utu	Untriunium	g-block	[Og] 5g6 6f3 8s2 8p2 1/2[79][119] [Og] 5g7 6f2 8s2 8p2 1/2 [Og] 8s2 0.86(5g6 6f3 8p2 1/2) + 0.14(5g6 6f2 7d2 8p1 1/2)
132	Utb	Untribium	g-block	[Og] 5g7 6f3 8s2 8p2 1/2[119] [Og] 5g8 6f2 8s2 8p2 1/2
133	Utt	Untritrium	g-block	[Og] 5g8 6f3 8s2 8p2 1/2[119]
134	Utq	Untriquadium	g-block	[Og] 5g8 6f4 8s2 8p2 1/2[119]
135	Utp	Untripentium	g-block	[Og] 5g9 6f4 8s2 8p2 1/2[119]
136	Uth	Untrihexium	g-block	[Og] 5g10 6f4 8s2 8p2 1/2[119]
137	Uts	Untriseptium	g-block	[Og] 5g11 6f4 8s2 8p2 1/2[119]
138	Uto	Untrioctium	g-block	[Og] 5g12 6f4 8s2 8p2 1/2[119] [Og] 5g12 6f3 7d1 8s2 8p2 1/2
139	Ute	Untriennium	g-block	[Og] 5g13 6f3 7d1 8s2 8p2 1/2[119] [Og] 5g13 6f2 7d2 8s2 8p2 1/2
140	Uqn	Unquadnilium	g-block	[Og] 5g14 6f3 7d1 8s2 8p2 1/2[119] [Og] 5g15 6f1 8s2 8p2 1/2 8p2 3/2
141	Uqu	Unquadunium	g-block	[Og] 5g15 6f2 7d2 8s2 8p2 1/2[119]
142	Uqb	Unquadbium	g-block	[Og] 5g16 6f2 7d2 8s2 8p2 1/2[119]
143	Uqt	Unquadtrium	f-block	[Og] 5g17 6f2 7d2 8s2 8p2 1/2[119]
144	Uqq	Unquadquadium	f-block	[Og] 5g18 6f2 7d2 8s2 8p2 1/2[119] [Og] 5g18 6f1 7d3 8s2 8p2 1/2 [Og] 5g17 6f2 7d3 8s2 8p2 1/2 [Og] 8s2 0.95(5g17 6f2 7d3 8p2 1/2) + 0.05(5g17 6f4 7d1 8p2 1/2)
145	Uqp	Unquadpentium	f-block	[Og] 5g18 6f3 7d2 8s2 8p2 1/2[119]
146	Uqh	Unquadhexium	f-block	[Og] 5g18 6f4 7d2 8s2 8p2 1/2[119]
147	Uqs	Unquadseptium	f-block	[Og] 5g18 6f5 7d2 8s2 8p2 1/2[119]
148	Uqo	Unquadoctium	f-block	[Og] 5g18 6f6 7d2 8s2 8p2 1/2[119]
149	Uqe	Unquadennium	f-block	[Og] 5g18 6f6 7d3 8s2 8p2 1/2[119]
150	Upn	Unpentnilium	f-block	[Og] 5g18 6f6 7d4 8s2 8p2 1/2 [Og] 5g18 6f7 7d3 8s2 8p2 1/2[119]
151	Upu	Unpentunium	f-block	[Og] 5g18 6f8 7d3 8s2 8p2 1/2[119]
152	Upb	Unpentbium	f-block	[Og] 5g18 6f9 7d3 8s2 8p2 1/2[119]
153	Upt	Unpenttrium	f-block	[Og] 5g18 6f10 7d3 8s2 8p2 1/2 [Og] 5g18 6f11 7d2 8s2 8p2 1/2[119]
154	Upq	Unpentquadium	f-block	[Og] 5g18 6f11 7d3 8s2 8p2 1/2 [Og] 5g18 6f12 7d2 8s2 8p2 1/2[119]
155	Upp	Unpentpentium	f-block	[Og] 5g18 6f12 7d3 8s2 8p2 1/2 [Og] 5g18 6f13 7d2 8s2 8p2 1/2[119]
156	Uph	Unpenthexium	f-block	[Og] 5g18 6f13 7d3 8s2 8p2 1/2 [Og] 5g18 6f14 7d2 8s2 8p2 1/2[119]
157	Ups	Unpentseptium	d-block	[Og] 5g18 6f14 7d3 8s2 8p2 1/2[119]
158	Upo	Unpentoctium	d-block	[Og] 5g18 6f14 7d4 8s2 8p2 1/2[119]
159	Upe	Unpentennium	d-block	[Og] 5g18 6f14 7d5 8s2 8p2 1/2 [Og] 5g18 6f14 7d4 8s2 8p2 1/2 9s1[119]
160	Uhn	Unhexnilium	d-block	[Og] 5g18 6f14 7d6 8s2 8p2 1/2 [Og] 5g18 6f14 7d5 8s2 8p2 1/2 9s1[119]
161	Uhu	Unhexunium	d-block	[Og] 5g18 6f14 7d7 8s2 8p2 1/2 [Og] 5g18 6f14 7d6 8s2 8p2 1/2 9s1[119]
162	Uhb	Unhexbium	d-block	[Og] 5g18 6f14 7d8 8s2 8p2 1/2 [Og] 5g18 6f14 7d7 8s2 8p2 1/2 9s1[119]
163	Uht	Unhextrium	d-block	[Og] 5g18 6f14 7d9 8s2 8p2 1/2 [Og] 5g18 6f14 7d8 8s2 8p2 1/2 9s1[119]
164	Uhq	Unhexquadium	d-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2[119]
165	Uhp	Unhexpentium	d-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 9s1[119]
166	Uhh	Unhexhexium	d-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 9s2[119]
167	Uhs	Unhexseptium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 9s2 9p1 1/2 [Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p1 3/2 9s2[119]
168	Uho	Unhexoctium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 9s2 9p2 1/2 [Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p2 3/2 9s2[119]
169	Uhe	Unhexennium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p1 3/2 9s2 9p2 1/2 [Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p3 3/2 9s2[119]
170	Usn	Unseptnilium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p2 3/2 9s2 9p2 1/2 [Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p4 3/2 9s2[119]
171	Usu	Unseptunium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p3 3/2 9s2 9p2 1/2 [Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p4 3/2 9s2 9p1 1/2[119]
172	Usb	Unseptbium	p-block	[Og] 5g18 6f14 7d10 8s2 8p2 1/2 8p4 3/2 9s2 9p2 1/2[119]
173	Ust	Unsepttrium	?	[172] 6g1 [172] 9p1 3/2 [172] 10s1[87]
174	Usq	Unseptquadium	?	[172] 8d1 10s1[87]
...	...	...	...	...
184	Uoq	Unoctquadium	?	[172] 6g5 7f4 8d3

See also

• Table of nuclides
• Hypernucleus
• Neutronium

References