Editing Extended periodic table (section)

====Superactinides====
The superactinides may 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.<ref name=nefedov/> In the superactinide series, the 7d{{sub|3/2}}, 8p{{sub|1/2}}, 6f{{sub|5/2}} and 5g{{sub|7/2}} shells should all fill simultaneously.<ref name="BFricke"/> This creates very complicated situations, so much so that complete and accurate CCSD calculations have been done only for elements 121 and 122.<ref name=Haire/> The first superactinide, [[unbiunium]] or eka-actinium (element 121), should be similar to [[lanthanum]] and [[actinium]]:<ref>{{Cite journal | last1 = Waber | first1 = J. T. | title = SCF Dirac–Slater Calculations of the Translawrencium Elements | doi = 10.1063/1.1672054 | journal = The Journal of Chemical Physics | volume = 51 | issue = 2 | page = 664| year = 1969 |bibcode = 1969JChPh..51..664W }}</ref> 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.<ref name=Haire/> Relativistic stabilization of the 8p subshell should result in a ground-state 8s{{sup|2}}8p{{sup|1}} valence electron configuration for element 121, in contrast to the ds{{sup|2}} configurations of lanthanum and actinium;<ref name=Haire/> nevertheless, this anomalous configuration does not appear to affect its calculated chemistry, which remains similar to that of actinium.<ref name=Amador>{{cite journal |last1=Amador |first1=Davi H. T. |last2=de Oliveira |first2=Heibbe C. B. |first3=Julio R. |last3=Sambrano |first4=Ricardo |last4=Gargano |first5=Luiz Guilherme M. |last5=de Macedo |date=12 September 2016 |title=4-Component correlated all-electron study on Eka-actinium Fluoride (E121F) including Gaunt interaction: Accurate analytical form, bonding and influence on rovibrational spectra |journal=Chemical Physics Letters |volume=662 |pages=169–175 |doi=10.1016/j.cplett.2016.09.025|bibcode=2016CPL...662..169A |hdl=11449/168956 |hdl-access=free }}</ref> Its first [[ionization energy]] is predicted to be 429.4&nbsp;kJ/mol, which would be lower than those of all known elements except for the [[alkali metals]] [[potassium]], [[rubidium]], [[caesium]], and [[francium]]: this value is even lower than that of the period 8 alkali metal ununennium (463.1&nbsp;kJ/mol). Similarly, the next superactinide, [[unbibium]] or eka-thorium (element 122), may be similar to [[cerium]] and [[thorium]], with a main oxidation state of +4, but would have a ground-state 7d{{sup|1}}8s{{sup|2}}8p{{sup|1}} or 8s{{sup|2}}8p{{sup|2}} valence electron configuration,<ref name="Umemoto">{{cite journal |last1=Umemoto |first1=Koichiro |last2=Saito |first2=Susumu |date=1996 |title=Electronic Configurations of Superheavy Elements |url=https://journals.jps.jp/doi/pdf/10.1143/JPSJ.65.3175 |journal=Journal of the Physical Society of Japan |volume=65 |issue=10 |pages=3175–9 |bibcode=1996JPSJ...65.3175U |doi=10.1143/JPSJ.65.3175 |access-date=31 January 2021|url-access=subscription }}</ref> unlike thorium's 6d{{sup|2}}7s{{sup|2}} configuration. Hence, its first [[ionization energy]] would be smaller than thorium's (Th: 6.3&nbsp;[[electronvolt|eV]]; element 122: 5.6&nbsp;eV) because of the greater ease of ionizing unbibium's 8p{{sub|1/2}} electron than thorium's 6d electron.<ref name=Haire/> The collapse of the 5g orbital itself is delayed until around element 125 ([[wikt:unbipentium|unbipentium]] or eka-neptunium); the electron configurations of the 119-electron isoelectronic series are expected to be [Og]8s{{sup|1}} for elements 119 through 122, [Og]6f{{sup|1}} for elements 123 and 124, and [Og]5g{{sup|1}} for element 125 onwards.<ref name=5gchem/>

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. Element 126 is also predicted to display a variety of other [[oxidation state]]s: recent calculations have suggested a stable [[Fluoride|monofluoride]] 126F may be possible, resulting from a bonding interaction between the 5g&nbsp;[[Atomic orbital|orbital]] on element 126 and the 2[[p-orbital|p]]&nbsp;orbital on [[fluorine]].<ref name=Jacoby>{{Cite journal|last=Jacoby|first=Mitch|title=As-yet-unsynthesized superheavy atom should form a stable diatomic molecule with fluorine|journal=Chemical & Engineering News|year=2006|volume=84|issue=10|pages=19|doi=10.1021/cen-v084n010.p019a}}</ref> Other predicted oxidation states include +2, +4, and +6; +4 is expected to be the most usual oxidation state of unbihexium.<ref name="BFricke"/> The superactinides from unbipentium (element 125) to unbiennium (element 129) are predicted to exhibit a +6 oxidation state and form [[hexafluorides]], though 125F{{sub|6}} and 126F{{sub|6}} are predicted to be relatively weakly bound.<ref name="5gchem">{{cite journal|last1=Dongon|first1=J.P.|last2=Pyykkö|first2=P.|date=2017|title=Chemistry of the 5g elements. Relativistic calculations on hexafluorides|journal= Angewandte Chemie International Edition|volume=56|issue=34|pages=10132–10134|doi=10.1002/anie.201701609|pmid=28444891|s2cid=205400592 |url=https://hal-cea.archives-ouvertes.fr/cea-01515489/document}}</ref> The [[bond dissociation energies]] are expected to greatly increase at element 127 and even more so at element 129. This suggests a shift from strong ionic character in fluorides of element 125 to more covalent character, involving the 8p orbital, in fluorides of element 129. The bonding in these superactinide hexafluorides is mostly between the highest 8p subshell of the superactinide and the 2p subshell of fluorine, unlike how uranium uses its 5f and 6d orbitals for bonding in [[uranium hexafluoride]].<ref name=5gchem/>

Despite the ability of early superactinides to reach high oxidation states, it has been calculated that the 5g electrons will be most difficult to ionize; the 125{{sup|6+}} and 126{{sup|7+}} ions are expected to bear a 5g{{sup|1}} configuration, similar to the 5f{{sup|1}} configuration of the Np{{sup|6+}} ion.<ref name="PT172"/><ref name=5gchem/> Similar behavior is observed in the low chemical activity of the 4f electrons in [[lanthanide]]s; this is a consequence of the 5g orbitals being small and deeply buried in the electron cloud.<ref name="PT172"/> 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 [[orbital hybridization|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.<ref name="Fricke"/>
<div style="margin:0 auto; font-size:85%;">
:{| class="wikitable"
|+ Some predicted compounds of the superactinides (X = a [[halogen]])<ref name="PT172"/><ref name=5gchem/><ref>{{cite journal |last=Makhyoun |first=M. A. |date=October 1988 |title=On the electronic structure of 5g<sup>1</sup> complexes of element 125: a quasi-relativistic MS-Xα study |journal=Journal de Chimie Physique et de Physico-Chimie Biologique |volume=85 |issue=10 |pages=917–24 |doi=10.1051/jcp/1988850917 |bibcode=1988JCP....85..917M }}</ref>
!
! 121
! 122
! 123
! 124
! 125
! 126
! 127
! 128
! 129
! 132
! 142
! 143
! 144
! 145
! 146
! 148
! 153
! 154
! 155
! 156
! 157
|-
! Compound
| 121X<sub>3</sub>
| 122X<sub>4</sub>
| 123X<sub>5</sub>
| 124X<sub>6</sub>
| 125F<br/>125F<sub>6</sub><br/>{{chem|125O|2|2+}}
| 126F<br/>126F<sub>6</sub><br/>126O<sub>4</sub>
| 127F<sub>6</sub>
| 128F<sub>6</sub>
| 129F<br/>129F<sub>6</sub>
|
| 142X<sub>4</sub><br/>142X<sub>6</sub>
| 143F<sub>6</sub>
| 144X<sub>6</sub><br/>{{chem|144O|2|2+}}<br/>144F<sub>8</sub><br/>144O<sub>4</sub>
| 145F<sub>6</sub>
|
| 148O<sub>6</sub>
|
|
|
|
|
|-
! Analogs
| [[lanthanum|La]]X<sub>3</sub><br/>[[actinium|Ac]]X<sub>3</sub>
| [[cerium|Ce]]X<sub>4</sub><br/>[[thorium|Th]]X<sub>4</sub>
|
|
| {{chem|[[neptunium|Np]]O|2|2+}}
|
|
|
|
|
| [[thorium tetrafluoride|ThF<sub>4</sub>]]
|
| [[uranium hexafluoride|UF<sub>6</sub>]]<br/>[[uranyl|{{chem|UO|2|2+}}]]<br/>[[plutonium|Pu]]F<sub>8</sub><br/>PuO<sub>4</sub>
|
|
| [[uranium hexoxide|UO<sub>6</sub>]]
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|-
! Oxidation states
| 3
| 4
| 5
| 6
| 1, 6, 7
| 1, 2, 4, 6, 8
| 6
| 6
| 1, 6
| 6
| 4, 6
| 6, 8
| 3, 4, 5, 6, 8
| 6
| 8
| 12
| 3
| 0, 2
| 3, 5
| 2
| 3
|}
</div>{{clear}}
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{{sub|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{{sub|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 function]]s outside the chemically inactive 8s and 8p<sub>1/2</sub> shells. This may cause element 154 to be rather [[reactivity (chemistry)|unreactive]] with [[noble gas]]-like properties.<ref name="Fricke"/><ref name=Haire/> Calculations by Pyykkö nonetheless expect that at element 155, the 6f shell is still chemically ionizable: 155{{sup|3+}} should have a full 6f shell, and the fourth ionization potential should be between those of [[terbium]] and [[dysprosium]], both of which are known in the +4 state.<ref name=PT172/>

Similarly to the [[lanthanide contraction|lanthanide and actinide contractions]], there should be a superactinide contraction in the superactinide series where the [[ionic radius|ionic radii]] of the superactinides are smaller than expected. In the [[lanthanide]]s, the contraction is about 4.4&nbsp;pm per element; in the [[actinide]]s, it is about 3&nbsp;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&nbsp;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.<ref name="Fricke"/>

[[Pekka Pyykkö]] divides these superactinides into three series: a 5g series (elements 121 to 138), an 8p<sub>1/2</sub> 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<sub>1/2</sub> 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.<ref name="PT172"/>

Andrey Kulsha has called the 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{{sub|1/2}} electrons would stop being chemically active. Because the two rows are separated by the addition of a complete 5g{{sup|18}} subshell, they could be considered analogues of each other as well.<ref name=primefan/><ref name=sicius/>

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 7d{{sup|2}} electrons over a stable [Og]5g{{sup|18}}6f{{sup|14}}8s{{sup|2}}8p{{su|p=2|b=1/2}} core. It can thus be considered a heavier congener of [[nobelium]], which likewise has a pair of easily removed 7s{{sup|2}} electrons over a stable [Rn]5f{{sup|14}} core, and is usually in the +2 state (strong oxidisers are required to obtain nobelium in the +3 state).<ref name=primefan/> Its first ionization energy should be about 400&nbsp;kJ/mol and its metallic radius approximately 170&nbsp;picometers. With a relative atomic mass of around 445&nbsp;u,<ref name=Fricke/> it should be a very heavy metal with a density of around 26&nbsp;g/cm<sup>3</sup>.