Editing Extended periodic table (section)

====Elements 157 to 166====
The 7d transition metals in period 8 are expected to be elements 157 to 166. Although the 8s and 8p<sub>1/2</sub> 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<sub>1/2</sub> levels are expected to be readily available for hybridization.<ref name="Fricke"/><ref name=Haire/> These 7d elements should be similar to the 4d elements [[yttrium]] through [[cadmium]].<ref name=primefan/> In particular, element 164 with a 7d<sup>10</sup>9s<sup>0</sup> electron configuration shows clear analogies with [[palladium]] with its 4d<sup>10</sup>5s<sup>0</sup> electron configuration.<ref name=BFricke/>

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.<ref name="Fricke"/><ref name=Haire/><ref name="BFricke"/>

Theoretical interest in the chemistry of unhexquadium is largely motivated by theoretical predictions that it, especially the isotopes <sup>472</sup>164 and <sup>482</sup>164 (with 164 [[proton]]s and 308 or 318 [[neutron]]s), would be at the center of a hypothetical second [[island of stability]] (the first being centered on [[copernicium]], particularly the isotopes <sup>291</sup>Cn, <sup>293</sup>Cn, and <sup>296</sup>Cn which are expected to have half-lives of centuries or millennia).<ref name=magickoura/><ref name="Kratz">
{{cite conference
|last1=Kratz |first1=J. V.
|date=5 September 2011
|title=The Impact of Superheavy Elements on the Chemical and Physical Sciences
|url=http://tan11.jinr.ru/pdf/06_Sep/S_1/02_Kratz.pdf
|conference=4th International Conference on the Chemistry and Physics of the Transactinide Elements
|access-date=27 August 2013
}}</ref><ref name="eurekalert.org">{{cite web|url=http://www.eurekalert.org/pub_releases/2008-04/acs-nse031108.php|title=Nuclear scientists eye future landfall on a second 'island of stability'|date=6 April 2008|website=EurekAlert!|access-date=2015-12-17}}</ref><ref name="link.springer.com">{{cite journal | doi = 10.1007/BF01406719 | volume = 228 | issue = 5 | title = Investigation of the stability of superheavy nuclei aroundZ=114 andZ=164 | journal = Zeitschrift für Physik | pages = 371–386 | bibcode = 1969ZPhy..228..371G | year = 1969 | last1 = Grumann | first1 = Jens | last2 = Mosel | first2 = Ulrich | last3 = Fink | first3 = Bernd | last4 = Greiner | first4 = Walter | s2cid = 120251297 }}</ref>

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 [[aqueous solution]]s with strong [[ligand]]s. Element 164 should thus be able to form compounds like 164([[carbonyl|CO]])<sub>4</sub>, 164([[phosphorus trifluoride|PF<sub>3</sub>]])<sub>4</sub> (both [[tetrahedral molecular geometry|tetrahedral]] like the corresponding palladium compounds), and {{chem|164([[cyanide|CN]])|2|2-}} ([[linear molecular geometry|linear]]), which is very different behavior from that of [[lead]], which element 164 would be a heavier [[Homologous series|homologue]] of if not for relativistic effects. Nevertheless, the divalent state would be the main one in aqueous solution (although the +4 and +6 states would be possible with stronger ligands), and unhexquadium(II) should behave more similarly to lead than unhexquadium(IV) and unhexquadium(VI).<ref name=Haire/><ref name="BFricke">{{Cite journal |last1=Fricke |first1=Burkhard |year=1975 |title=Superheavy elements: a prediction of their chemical and physical properties |journal=Recent Impact of Physics on Inorganic Chemistry |volume=21 |pages=[https://archive.org/details/recentimpactofph0000unse/page/89 89–144] |doi=10.1007/BFb0116498 |url=https://archive.org/details/recentimpactofph0000unse/page/89 |access-date=4 October 2013 |series=Structure and Bonding |isbn=978-3-540-07109-9 }}</ref>

Element 164 is expected to be a soft [[Lewis acid]] and have Ahrlands softness parameter close to 4&nbsp;[[electronvolt|eV]]. It should be at most moderately reactive, having a first ionization energy that should be around 685&nbsp;kJ/mol, comparable to that of [[molybdenum]].<ref name="Fricke"/><ref name="BFricke"/> Due to the [[lanthanide contraction|lanthanide, actinide, and superactinide contractions]], element 164 should have a metallic radius of only 158&nbsp;[[picometer|pm]], very close to that of the much lighter [[magnesium]], despite its expected atomic weight of around 474&nbsp;[[atomic mass unit|u]] which is about 19.5&nbsp;times the atomic weight of magnesium.<ref name="Fricke"/> This small radius and high weight cause it to be expected to have an extremely high density of around 46&nbsp;g·cm<sup>−3</sup>, over twice that of [[osmium]], currently the most dense element known, at 22.61&nbsp;g·cm<sup>−3</sup>; element 164 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&nbsp;g·cm<sup>−3</sup>).<ref name="Fricke"/> Metallic element 164 should have a very large cohesive energy ([[enthalpy]] of crystallization) due to its [[Covalent bond|covalent]] bonds, most probably resulting in a high melting point. In the metallic state, element 164 should be quite noble and analogous to palladium and [[platinum]]. Fricke et al. suggested some formal similarities to [[oganesson]], as both elements have closed-shell configurations and similar ionisation energies, although they note that while oganesson would be a very bad noble gas, element 164 would be a good noble metal.<ref name="BFricke"/>

Elements 165 (unhexpentium) and 166 (unhexhexium), the last two 7d metals, should behave similarly to [[alkali metal|alkali]] and [[alkaline earth metal]]s 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 (element)|mercury]]).<ref name="Fricke"/><ref name=Haire/> As with mercury but not copernicium, ionization of element 166 to 166<sup>2+</sup> is expected to result in a 7d<sup>10</sup> 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.<ref name=PT172/>

<div style="float: center; margin: 1px; font-size:85%;">
:{| class="wikitable"
|+ Some predicted properties of elements 156–166<br/>The metallic radii and densities are first approximations.<ref name="Fricke"/><ref name="PT172"/><ref name=Haire/><br/>Most analogous group is given first, followed by other similar groups.<ref name="BFricke"/>
! Property
! 156
! 157
! 158
! 159
! 160
! 161
! 162
! 163
! 164
! 165
! 166
|-
! [[Standard atomic weight]]
| [445]
| [448]
| [452]
| [456]
| [459]
| [463]
| [466]
| [470]
| [474]
| [477]
| [481]
|-
! [[Periodic table group|Group]]
| [[ytterbium|Yb]] group
| [[group 3 element|3]]
| [[group 4 element|4]]
| [[group 5 element|5]]
| [[group 6 element|6]]
| [[group 7 element|7]]
| [[group 8 element|8]]
| [[group 9 element|9]]
| [[group 10 element|10]]
| [[group 11 element|11]]<br/>(1)
| [[group 12 element|12]]<br/>(2)
|-
! Valence [[electron configuration]]
| 7d<sup>2</sup>
| 7d<sup>3</sup>
| 7d<sup>4</sup>
| 7d<sup>5</sup>
| 7d<sup>6</sup>
| 7d<sup>7</sup>
| 7d<sup>8</sup>
| 7d<sup>9</sup>
| 7d<sup>10</sup>
| 7d<sup>10</sup> 9s<sup>1</sup>
| 7d<sup>10</sup> 9s<sup>2</sup>
|-
! Stable [[oxidation state]]s
| '''2'''
| '''3'''
| '''4'''
| '''1''', '''5'''
| '''2''', '''6'''
| '''3''', '''7'''
| '''4''', '''8'''
| '''5'''
| '''0''', '''2''', '''4''', '''6'''
| '''1''', '''3'''
| '''2'''
|-
! First [[ionization energy]]
| 400 kJ/mol
| 450 kJ/mol
| 520 kJ/mol
| 340 kJ/mol
| 420 kJ/mol
| 470 kJ/mol
| 560 kJ/mol
| 620 kJ/mol
| 690 kJ/mol
| 520 kJ/mol
| 630 kJ/mol
|-
! [[Metallic radius]]
| 170 pm
| 163 pm
| 157 pm
| 152 pm
| 148 pm
| 148 pm
| 149 pm
| 152 pm
| 158 pm
| 250 pm
| 200 pm
|-
! [[Density]]
| 26 g/cm<sup>3</sup>
| 28 g/cm<sup>3</sup>
| 30 g/cm<sup>3</sup>
| 33 g/cm<sup>3</sup>
| 36 g/cm<sup>3</sup>
| 40 g/cm<sup>3</sup>
| 45 g/cm<sup>3</sup>
| 47 g/cm<sup>3</sup>
| 46 g/cm<sup>3</sup>
| 7 g/cm<sup>3</sup>
| 11 g/cm<sup>3</sup>
|}
</div>