Editing Group 12 element (section)

==Chemistry==
Most of the chemistry has been observed only for the first three members of the group 12. The chemistry of copernicium is not well established and therefore the rest of the section deals only with zinc, cadmium and mercury.

===Periodic trends===
All elements in this group are [[metal]]s. The similarity of the metallic radii of cadmium and mercury is an effect of the [[lanthanide contraction]]. So, the trend in this group is unlike the trend in group 2, the [[alkaline earth]]s, where metallic radius increases smoothly from top to bottom of the group. All three metals have relatively low melting and boiling points, indicating that the [[metallic bond]] is relatively weak, with relatively little overlap between the [[valence band]] and the [[conduction band]].<ref name = "chemyst">{{cite web |url=http://www.alchemyst.co.uk/alchemystry/pdf/Inorganic/descriptive_pblock.pdf |title= Descriptive P-block Notes|author=Moss, Alex |year=2003 |publisher=Alchemyst Online|access-date=June 2, 2011}}</ref> Thus, zinc is close to the boundary between metallic and [[metalloid]] elements, which is usually placed between [[gallium]] and [[germanium]], though gallium participates in [[semi-conductor]]s such as [[gallium arsenide]].

Zinc and cadmium are [[Electropositivity|electropositive]] while mercury is not.<ref name = "chemyst"/> As a result, zinc and cadmium metal are good reducing agents. The elements of group 12 have an oxidation state of +2 in which the ions have the rather stable d<sup>10</sup> electronic configuration, with a full [[electron shell|sub-shell]]. However, mercury can easily be reduced to the +1 oxidation state; usually, as in the ion {{chem|Hg|2|2+}}, two mercury(I) ions come together to form a metal-metal bond and a [[diamagnetic]] species.<ref>{{cite journal |last1=Lindberg |first1=S. E. |last2=Stratton |first2=W. J. |year=1998 |title=Atmospheric Mercury Speciation: Concentrations and Behavior of Reactive Gaseous Mercury in Ambient Air|journal=Environmental Science and Technology |volume=32 |issue=1 |doi=10.1021/es970546u |pages=49–57 |bibcode = 1998EnST...32...49L }}</ref> Cadmium can also form species such as [Cd<sub>2</sub>Cl<sub>6</sub>]<sup>4−</sup> in which the metal's oxidation state is +1. Just as with mercury, the formation of a metal-metal bond results in a diamagnetic compound in which there are no unpaired electrons; thus, making the species very reactive. Zinc(I) is known mostly in the gas phase, in such compounds as linear Zn<sub>2</sub>Cl<sub>2</sub>, analogous to [[calomel]].  In the solid phase, the rather exotic compound [[decamethyldizincocene]] (Cp*Zn–ZnCp*) is known.

===Classification===
The elements in group 12 are usually considered to be [[d-block]] elements, but not [[transition element]]s as the d-shell is full. Some authors classify these elements as [[main-group element]]s because the [[valence electron]]s are in ns<sup>2</sup> orbitals. Nevertheless, they share many characteristics with the neighboring [[group 11 element]]s on the periodic table, which are almost universally considered to be transition elements. For example, zinc shares many characteristics with the neighboring transition metal, copper. Zinc complexes merit inclusion in the [[Irving-Williams series]] as zinc forms many complexes with the same [[stoichiometry]] as complexes of copper(II), albeit with smaller [[stability constants of complexes|stability constants]].<ref>{{cite journal |last1=Al-Niaimi |first1=N. S. |last2=Hamid|first2=H. A. |year=1976 |title=Stabilities of nickel(II), copper(II), zinc(II) and dioxouranium(II) complexes of some β-diketones  |journal= Journal of Inorganic and Nuclear Chemistry |volume=3 |issue=5|doi=10.1016/0022-1902(77)80167-X |pages=849–852 }}</ref> There is little similarity between cadmium and silver as compounds of silver(II) are rare and those that do exist are very strong oxidizing agents. Likewise the common oxidation state for gold is +3, which precludes there being much common chemistry between mercury and gold, though there are similarities between mercury(I) and gold(I) such as the formation of linear dicyano complexes, [M(CN)<sub>2</sub>]<sup>−</sup>. According to [[IUPAC]]'s definition of transition metal as ''an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell'',<ref>{{GoldBookRef |title=transition element |file=T06456 }}</ref> zinc and cadmium are not transition metals, while mercury is. This is because only mercury is known to have a compound where its oxidation state is higher than +2, in [[mercury(IV) fluoride]] (though its existence is disputed, as later experiments trying to confirm its synthesis could not find evidence of HgF<sub>4</sub>).<ref>[http://www.speciation.net/News/Elusive-HgIV-species-has-been-synthesized-under-cryogenic-conditions-;~/2007/10/12/3303.html Elusive Hg(IV) species has been synthesized under cryogenic conditions]</ref><ref>{{cite journal|doi=10.1002/ange.200703710|title=Mercury is a Transition Metal: The First Experimental Evidence for HgF4|year=2007|last1=Wang|first1=Xuefang|last2=Andrews|first2=Lester|last3=Riedel|first3=Sebastian|last4=Kaupp|first4=Martin|journal=Angewandte Chemie|volume=119|issue=44|pages=8523–8527|bibcode=2007AngCh.119.8523W }}</ref> However, this classification is based on one highly atypical compound seen at non-equilibrium conditions and is at odds to mercury's more typical chemistry, and Jensen has suggested that it would be better to regard mercury as not being a transition metal.<ref name=Jensen/>

===Relationship with the alkaline earth metals===
Although group 12 lies in the [[d-block]] of the modern 18-column periodic table, the d electrons of zinc, cadmium, and (almost always) mercury behave as core electrons and do not take part in bonding. This behavior is similar to that of the [[main-group element]]s, but is in stark contrast to that of the neighboring [[group 11 element]]s ([[copper]], [[silver]], and [[gold]]), which also have filled d-subshells in their ground-state [[electron configuration]] but behave chemically as transition metals. For example, the bonding in [[chromium(II) sulfide]] (CrS) involves mainly the 3d electrons; that in [[iron(II) sulfide]] (FeS) involves both the 3d and 4s electrons; but that of [[zinc sulfide]] (ZnS) involves only the 4s electrons and the 3d electrons behave as core electrons. Indeed, useful comparison can be made between their properties and the first two members of [[alkaline earth metal|group 2]], [[beryllium]] and [[magnesium]], and in earlier short-form periodic table layouts, this relationship is illustrated more clearly. For instance, zinc and cadmium are similar to beryllium and magnesium in their [[atomic radius|atomic radii]], [[ionic radius|ionic radii]], [[electronegativity|electronegativities]], and also in the structure of their [[binary compound]]s and their ability to form complex ions with many [[nitrogen]] and [[oxygen]] [[ligand]]s, such as complex [[hydride]]s and [[amine]]s. However, beryllium and magnesium are small atoms, unlike the heavier [[alkaline earth metal]]s and like the group 12 elements (which have a greater nuclear charge but the same number of [[valence electron]]s), and the [[periodic trend]]s down group 2 from beryllium to [[radium]] (similar to that of the [[alkali metal]]s) are not as smooth when going down from beryllium to mercury (which is more similar to that of the p-block main groups) due to the [[d-block contraction|d-block]] and [[lanthanide contraction]]s. It is also the d-block and lanthanide contractions that give mercury many of its distinctive properties.<ref name="Jensen">{{cite journal|author1-link=William B. Jensen|last1=Jensen |first1=William B. |year=2003 |title=The Place of Zinc, Cadmium, and Mercury in the Periodic Table |journal=Journal of Chemical Education |volume=80 |issue=8 |pages=952–961 |doi=10.1021/ed080p952 |bibcode=2003JChEd..80..952J |url=http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/091.%20Zn-Cd-Hg.pdf |access-date=2012-05-06 |url-status=dead |archive-url=https://web.archive.org/web/20100611152417/http://www.che.uc.edu/jensen/W.%20B.%20Jensen/Reprints/091.%20Zn-Cd-Hg.pdf |archive-date=2010-06-11 }}</ref>
{| class="wikitable centered" style="text-align:center;"
|+Comparison of the properties of the alkaline earth metals and the group 12 elements (predictions for copernicium)<ref name="Jensen"/>
! Name
! Beryllium
! Magnesium
! Calcium
! Strontium
! Barium
! Radium
|-
| style="background:lightgrey; text-align:left;"|[[Valence electron]] configuration
| 2s<sup>2</sup> || 3s<sup>2</sup> || 4s<sup>2</sup> || 5s<sup>2</sup> || 6s<sup>2</sup> || 7s<sup>2</sup>
|-
| style="background:lightgrey; text-align:left;"|Core electron configuration
| &#91;[[Helium|He]]&#93; || &#91;[[Neon|Ne]]&#93; || &#91;[[Argon|Ar]]&#93; || &#91;[[Krypton|Kr]]&#93; || &#91;[[Xenon|Xe]]&#93; || &#91;[[Radon|Rn]]&#93;
|-
| style="background:lightgrey; text-align:left;"|[[Oxidation state]]s<ref group="note" name="oxidation-states">See [[list of oxidation states of the elements]]. Oxidation states in '''bold''' are common.</ref>
| '''+2''', +1 || '''+2''', +1 || '''+2''', +1 || '''+2''', +1 || '''+2''' || '''+2'''
|-
| style="background:lightgrey; text-align:left;"|[[Melting point]]
| 1560 [[Kelvin|K]] (1287 [[Celsius|°C]]) || 923 K (650&nbsp;°C) || 1115 K (842&nbsp;°C) || 1050 K (777&nbsp;°C) || 1000 K (727&nbsp;°C) || 973 K (700&nbsp;°C)
|-
| style="background:lightgrey; text-align:left;"|[[Boiling point]]
| 2742 K (2469&nbsp;°C) || 1363 K (1090&nbsp;°C) || 1757 K (1484&nbsp;°C) || 1655 K (1382&nbsp;°C) || 2170 K (1897&nbsp;°C) || 2010 K (1737&nbsp;°C)
|-
| style="background:lightgrey; text-align:left;"|Appearance
| white-gray metallic || shiny gray metallic || dull silver-gray || silvery white metallic || silvery gray || silvery white metallic
|-
| style="background:lightgrey; text-align:left;"|[[Density]]
| 1.85&nbsp;g·cm<sup>−3</sup> || 1.738&nbsp;g·cm<sup>−3</sup> || 1.55&nbsp;g·cm<sup>−3</sup> || 2.64&nbsp;g·cm<sup>−3</sup> || 3.51&nbsp;g·cm<sup>−3</sup> || 5.5&nbsp;g·cm<sup>−3</sup>
|-
| style="background:lightgrey; text-align:left;"|Pauling [[electronegativity]]
| 1.57 || 1.31 || 1.00 || 0.95 || 0.89 || 0.9
|-
| style="background:lightgrey; text-align:left;"|[[Atomic radius]]
| 105 pm || 150 pm || 180 pm || 200 pm || 215 pm || 215 pm
|-
| style="background:lightgrey; text-align:left;"|Crystal [[ionic radius]]
| 59 pm || 86 pm || 114 pm || 132 pm || 149 pm || 162 pm
|-
| style="background:lightgrey; text-align:left;"|[[Flame test]] color
| white<ref name="Jensen"/> || brilliant white<ref name="rsc">{{cite web|url=http://www.rsc.org/chemsoc/visualelements/PAGES/data/intro_groupii_data.html |title=Visual Elements: Group 2–The Alkaline Earth Metals |author=Royal Society of Chemistry |work=Visual Elements |publisher=Royal Society of Chemistry|access-date=13 January 2012|author-link=Royal Society of Chemistry }}</ref> || brick-red<ref name="rsc"/> || crimson<ref name="rsc"/> || apple green<ref name="rsc"/> || crimson red{{refn|The color of the flame test of pure radium has never been observed; the crimson red color is an extrapolation from the flame test color of its compounds.<ref name="RaFlameTest">{{cite book | url = https://books.google.com/books?id=3cgQLgEACAAJ | title = The Radiochemistry of Radium | last1 = Kirby | first1 = H. W. | last2 = Salutsky | first2 = Murrell L. | year = 1964 | publisher = National Academies Press }}{{Dead link|date=May 2023 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>|group=note}}
|-
| style="background:lightgrey; text-align:left;"|[[Organometallic chemistry]]
| good || good || poor || very poor || very poor || extremely poor
|-
| style="background:lightgrey; text-align:left;"|[[Hydroxide]]
| [[amphoteric]] || [[base (chemistry)|basic]] || basic || strongly basic || strongly basic || strongly basic
|-
| style="background:lightgrey; text-align:left;"|[[Oxide]]
| amphoteric || strongly basic || strongly basic || strongly basic || strongly basic || strongly basic
|-
! Name
! Beryllium
! Magnesium
! Zinc
! Cadmium
! Mercury
! Copernicium
|-
| style="background:lightgrey; text-align:left;"|Valence electron configuration
| 2s<sup>2</sup> || 3s<sup>2</sup> || 4s<sup>2</sup> || 5s<sup>2</sup> || 6s<sup>2</sup> || ? 7s<sup>2</sup>
|-
| style="background:lightgrey; text-align:left;"|Core electron configuration
| [He] || [Ne] || [Ar]3d<sup>10</sup> || [Kr]4d<sup>10</sup> || [Xe]4f<sup>14</sup>5d<sup>10</sup> || ? [Rn]5f<sup>14</sup>6d<sup>10</sup>
|-
| style="background:lightgrey; text-align:left;"|Oxidation states<ref group="note" name="oxidation-states"/>
| '''+2''', +1 || '''+2''', +1 || '''+2''', +1 || '''+2''', +1 || '''+2''', '''+1''' || ? '''+4''', '''+2''', '''+1''', '''0'''<ref name="superheavy chemistry">{{Cite web|author=H. W. Gäggeler |title=Gas Phase Chemistry of Superheavy Elements |year=2007 |pages=26–28 |publisher=[[Paul Scherrer Institute]] |url=http://lch.web.psi.ch/files/lectures/TexasA&M/TexasA&M.pdf |url-status=dead |archive-url=https://web.archive.org/web/20120220090755/http://lch.web.psi.ch/files/lectures/TexasA%26M/TexasA%26M.pdf |archive-date=2012-02-20 }}</ref><ref name="Haire">{{cite book| title = The Chemistry of the Actinide and Transactinide Elements| editor1-last = Morss|editor2-first = Norman M.| editor2-last = Edelstein| editor3-last = Fuger|editor3-first = Jean| last = Haire|first = Richard G.| chapter = Transactinides and the future elements| publisher = [[Springer Science+Business Media]]| year = 2006| page = 1675| isbn = 978-1-4020-3555-5| location = Dordrecht, The Netherlands| edition = 3rd}}</ref><ref name=BFricke>{{cite book |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>
|-
| style="background:lightgrey; text-align:left;"|Melting point
| 1560 K (1287&nbsp;°C) || 923 K (650&nbsp;°C) || 693 K (420&nbsp;°C) || 594 K (321&nbsp;°C) || 234 K (−39&nbsp;°C) || 283±11 K (10&nbsp;°C)
|-
| style="background:lightgrey; text-align:left;"|Boiling point
| 2742 K (2469&nbsp;°C) || 1363 K (1090&nbsp;°C) || 1180 K (907&nbsp;°C) || 1040 K (767&nbsp;°C) || 630 K (357&nbsp;°C) || 340±10 K (60&nbsp;°C)
|-
| style="background:lightgrey; text-align:left;"|Appearance
| white-gray metallic || shiny gray metallic || silvery bluish-gray metallic || silver-gray || silvery || ?
|-
| style="background:lightgrey; text-align:left;"|Density
| 1.85&nbsp;g·cm<sup>−3</sup> || 1.738&nbsp;g·cm<sup>−3</sup> || 7.14&nbsp;g·cm<sup>−3</sup> || 8.65&nbsp;g·cm<sup>−3</sup> || 13.534&nbsp;g·cm<sup>−3</sup> || 14.0&nbsp;g·cm<sup>−3</sup>
|-
| style="background:lightgrey; text-align:left;"|Pauling electronegativity
| 1.57 || 1.31 || 1.65 || 1.69 || 2.00 || ?
|-
| style="background:lightgrey; text-align:left;"|Atomic radius
| 105 pm || 150 pm || 135 pm || 155 pm || 150 pm || ? 147 pm<ref name="Haire"/>
|-
| style="background:lightgrey; text-align:left;"|Crystal ionic radius
| 59 pm || 86 pm || 88 pm || 109 pm || 116 pm || ? 75 pm<ref name="Haire"/>
|-
| style="background:lightgrey; text-align:left;"|Flame test color
| white || brilliant white || bluish-green{{refn|Sometimes reported as white.<ref name="Jensen"/>|group="note"}} || ? || ? || ?
|-
| style="background:lightgrey; text-align:left;"|Organometallic chemistry
| good || good || good || good || good || ?
|-
| style="background:lightgrey; text-align:left;"|Hydroxide
| amphoteric || basic || amphoteric || weakly basic || ? || ?
|-
| style="background:lightgrey; text-align:left;"|Oxide
| amphoteric || strongly basic || amphoteric || mildly basic || mildly basic || ?
|}

===Compounds===
{{see also|Compounds of zinc|Organozinc compounds|Organocadmium compound|Organomercury}}
All three metal ions form many [[Tetrahedral molecular geometry|tetrahedral]] species, such as {{chem|MCl|4|2-}}. Both zinc and cadmium can also form octahedral complexes such as the [[Metal ions in aqueous solution|aqua ions]] [M(H<sub>2</sub>O)<sub>6</sub>]<sup>2+</sup> which are present in aqueous solutions of salts of these metals.<ref>{{cite book |last= Richens |first= David T. |title=The Chemistry of Aqua Ions |publisher=J. Wiley  |date=September 1997 |isbn= 978-0-471-97058-3}}</ref> Covalent character is achieved by using the s and p orbitals. Mercury, however, rarely exceeds a [[coordination number]] of four. Coordination numbers of 2, 3, 5, 7 and 8 are also known.