Editing Radon (section)

=== Chemical properties ===
Radon is a member of the zero-[[Valence (chemistry)|valence]] elements that are called noble gases, and is chemically not very [[Reactivity (chemistry)|reactive]]. The [[inert pair effect]] stabilizes the 6s shell, making it unavailable for bonding—a consequence only understood within [[relativistic quantum chemistry]].<ref name="Thayer" />{{rp|66}} The 3.8-day half-life of {{sup|222}}Rn makes it useful in physical sciences as a natural [[Radioactive tracer|tracer]]. Because radon is a gas at standard conditions, unlike its decay-chain parents, it can readily be extracted from them for research.<ref name="Ullmann" />

It is [[Inert gas|inert]] to most common chemical reactions, such as [[combustion]], because the outer [[valence shell]] contains eight [[electron]]s. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.<ref>{{cite web |last=Bader |first=Richard F. W. |url=http://miranda.chemistry.mcmaster.ca/esam/ |title=An Introduction to the Electronic Structure of Atoms and Molecules |publisher=[[McMaster University]] |access-date=2008-06-26}}</ref> Its [[first ionization energy]]—the minimum energy required to extract one electron from it—is 1037&nbsp;kJ/mol.<ref>{{cite book |author=David R. Lide |title=CRC Handbook of Chemistry and Physics |edition=84th|publisher=CRC Press|location=Boca Raton, Florida|date=2003|chapter=Section 10, Atomic, Molecular, and Optical Physics; Ionization Potentials of Atoms and Atomic Ions}}</ref> In accordance with [[Periodic table|periodic trends]], radon has a lower [[electronegativity]] than the element one period before it, [[xenon]], and is therefore more reactive. Early studies concluded that the stability of radon [[hydrate]] should be of the same order as that of the hydrates of [[chlorine]] ({{chem|Cl|2}}) or [[sulfur dioxide]] ({{chem|SO|2}}), and significantly higher than the stability of the hydrate of [[hydrogen sulfide]] ({{chem|H|2|S}}).<ref>{{cite journal |doi=10.1070/RC1982v051n01ABEH002787 |title=The Chemistry of Radon |date=1982 |author=Avrorin, V. V. |journal=[[Russian Chemical Reviews]] |volume=51 |issue=1 |page=12 |last2=Krasikova |first2=R. N. |last3=Nefedov |first3=V. D. |last4=Toropova |first4=M. A. |bibcode = 1982RuCRv..51...12A|s2cid=250906059 }}</ref>

Because of its cost and radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all either [[fluoride]]s or [[oxide]]s. Radon can be [[Oxidation|oxidized]] by powerful oxidizing agents such as [[fluorine]], thus forming [[radon difluoride]] ({{chem|RnF|2}}).<ref>{{Unbulleted list citebundle|{{cite journal |author=Stein, L. |date=1970 |journal=[[Science (journal)|Science]] |volume=168 |doi=10.1126/science.168.3929.362 |title=Ionic Radon Solution |pmid=17809133 |issue=3929 |bibcode=1970Sci...168..362S |pages=362–4|s2cid=31959268 }}|{{cite journal |author=Pitzer, Kenneth S. |date=1975 |journal=[[Chemical Communications]] |volume=44 |pages=760–761 |title=Fluorides of radon and element 118 |doi=10.1039/C3975000760b |issue=18  |url=https://escholarship.org/uc/item/8xz4g1ff}}}}</ref> It decomposes back to its elements at a temperature of above {{Convert|523|K||abbr=}}, and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by [[hydrogen]] gas.<ref name="Stein" /> It has a low [[volatility (chemistry)|volatility]] and was thought to be {{chem|RnF|2}}. Because of the short half-life of radon and the radioactivity of its compounds, it has not been possible to study the compound in any detail. Theoretical studies on this molecule predict that it should have a Rn–F [[Bond length|bond distance]] of 2.08&nbsp;[[ångström]]s (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart [[xenon difluoride]] ({{chem|XeF|2}}).<ref>{{cite journal |doi=10.1021/jp9825516 |title=Chemical Bonding in XeF<sub>2</sub>, XeF<sub>4</sub>, KrF<sub>2</sub>, KrF<sub>4</sub>, RnF<sub>2</sub>, XeCl<sub>2</sub>, and XeBr<sub>2</sub>: From the Gas Phase to the Solid State |date=1998 |author=Meng-Sheng Liao |author2=Qian-Er Zhang |journal=[[The Journal of Physical Chemistry A]] |volume=102 |page=10647 |issue=52 |bibcode=1998JPCA..10210647L}}</ref> The [[Octahedral molecular geometry|octahedral molecule]] [[Radon hexafluoride|{{chem|RnF|6}}]] was predicted to have an even lower [[enthalpy of formation]] than the difluoride.<ref>{{cite journal |doi=10.1039/b212460m |title=Bonding in radon hexafluoride: An unusual relativistic problem? |date=2003 |author=Filatov, Michael |journal=[[Physical Chemistry Chemical Physics]] |volume=5 |page=1103 |last2=Cremer |first2=Dieter |issue=6 |bibcode=2003PCCP....5.1103F}}</ref> The [RnF]<sup>+</sup> [[ion]] is believed to form by the following reaction:<ref>{{cite journal |doi=10.1016/S0022-1139(00)85275-6 |title=Noble-gas fluorides |date=1986 |author=Holloway, J. |journal=Journal of Fluorine Chemistry |volume=33 |issue=1–4 |page=149|bibcode=1986JFluC..33..149H }}</ref>

: Rn (g) + 2 {{chem|[O|2|]|+|[SbF|6|]|-}} (s) → {{chem|[RnF]|+|[Sb|2|F|11|]|-}} (s) + 2 {{chem|O|2}} (g)

For this reason, [[antimony pentafluoride]] together with [[chlorine trifluoride]] and {{Chem|N|2|F|2|Sb|2|F|11}} have been considered for radon gas removal in [[Uranium mining|uranium mines]] due to the formation of radon–fluorine compounds.<ref name="Ullmann">{{Ullmann |first1=Cornelius |last1=Keller |first2=Walter |last2=Wolf |first3=Jashovam |last3=Shani |title=Radionuclides, 2. Radioactive Elements and Artificial Radionuclides |doi=10.1002/14356007.o22_o15}}</ref> Radon compounds can be formed by the decay of radium in radium halides, a reaction that has been used to reduce the amount of radon that escapes from targets during [[irradiation]].<ref name="Stein" /> Additionally, salts of the [RnF]<sup>+</sup> cation with the anions {{chem|SbF|6|-}}, {{chem|TaF|6|-}}, and {{chem|BiF|6|-}} are known.<ref name="Stein">{{cite journal |last1=Stein |first1=Lawrence |date=1983 |title=The Chemistry of Radon |journal=Radiochimica Acta |volume=32 |issue=1–3 |pages=163–171 |doi=10.1524/ract.1983.32.13.163|s2cid=100225806 }}</ref> Radon is also oxidised by [[dioxygen difluoride]] to {{chem|RnF|2}} at {{Convert|173|K||abbr=}}.<ref name="Stein" />

Radon oxides are among the few other reported [[Radon compounds|compounds of radon]];<ref>{{cite journal |title=The Chemistry of Radon |volume=51 |issue=1 |journal=[[Russian Chemical Reviews]] |date=1982 |page=12 |author=Avrorin, V. V. |author2=Krasikova, R. N. |author3=Nefedov, V. D. |author4=Toropova, M. A. |doi=10.1070/RC1982v051n01ABEH002787 |bibcode=1982RuCRv..51...12A|s2cid=250906059 }}</ref> only the trioxide ({{Chem|Rn|O|3}}) has been confirmed.<ref name="RnO3">{{cite book |last=Sykes |first=A. G. |title=Advances in Inorganic Chemistry |volume=46 |chapter=Recent Advances in Noble-Gas Chemistry |chapter-url=https://books.google.com/books?id=6iqXRtz6p3QC |access-date=2012-11-02 |date=1998 |publisher=Academic Press |isbn=978-0120236466 |pages=91–93}}</ref>> The higher fluorides {{chem|RnF|4}} and {{chem|RnF|6}} have been claimed, are calculated to be stable, but have not been confirmed.<ref name="Thayer">{{cite book |last1=Thayer |first1=John S. |title=Relativistic Methods for Chemists |volume=10 |year=2010 |page=80 |doi=10.1007/978-1-4020-9975-5_2|chapter=Relativistic Effects and the Chemistry of the Heavier Main Group Elements |isbn=978-1-4020-9974-8 |series=Challenges and Advances in Computational Chemistry and Physics }}</ref> They may have been observed in experiments where unknown radon-containing products distilled together with [[xenon hexafluoride]]: these may have been {{chem|RnF|4}}, {{chem|RnF|6}}, or both.<ref name="Stein" /> Trace-scale heating of radon with xenon, fluorine, [[bromine pentafluoride]], and either [[sodium fluoride]] or [[nickel fluoride]] was claimed to produce a higher fluoride as well which [[Hydrolysis|hydrolysed]] to form {{chem|RnO|3}}. While it has been suggested that these claims were really due to radon precipitating out as the solid complex [RnF]{{su|p=+|b=2}}[NiF<sub>6</sub>]<sup>2−</sup>, the fact that radon [[Coprecipitation|coprecipitates]] from [[aqueous solution]] with {{Chem|CsXeO|3|F}} has been taken as confirmation that {{chem|RnO|3}} was formed, which has been supported by further studies of the hydrolysed solution. That [RnO<sub>3</sub>F]<sup>−</sup> did not form in other experiments may have been due to the high concentration of fluoride used. [[Electromigration]] studies also suggest the presence of cationic [HRnO<sub>3</sub>]<sup>+</sup> and anionic [HRnO<sub>4</sub>]<sup>−</sup> forms of radon in [[Weak Acid|weakly acidic]] aqueous solution (pH &gt; 5), the procedure having previously been validated by examination of the homologous xenon trioxide.<ref name="RnO3" />

The [[decay technique]] has also been used. Avrorin et al. reported in 1982 that <sup>212</sup>[[francium|Fr]] compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of RnO<sub>3</sub>, but this could not be confirmed.<ref>{{cite journal |last1=Avrorin |first1=V. V. |last2=Krasikova |first2=R. N. |last3=Nefedov |first3=V. D. |last4=Toropova |first4=M. A. |date=1982 |title=The Chemistry of Radon |url= |journal=Russian Chemical Reviews |volume=51 |issue=1 |pages=12–20 |doi=10.1070/RC1982v051n01ABEH002787 |bibcode=1982RuCRv..51...12A |s2cid=250906059 |access-date=}}</ref>

It is likely that the difficulty in identifying higher fluorides of radon stems from radon being kinetically hindered from being oxidised beyond the divalent state because of the strong ionicity of [[radon difluoride]] ({{chem|RnF|2}}) and the high positive charge on radon in RnF<sup>+</sup>; spatial separation of {{chem|RnF|2}} molecules may be necessary to clearly identify higher fluorides of radon, of which {{chem|RnF|4}} is expected to be more stable than {{chem|RnF|6}} due to [[Spin–orbit interaction|spin–orbit]] splitting of the 6p shell of radon (Rn<sup>IV</sup> would have a closed-shell 6s{{su|p=2}}6p{{su|b=1/2|p=2}} configuration). Therefore, while {{chem|RnF|4}} should have a similar stability to [[xenon tetrafluoride]] ({{chem|XeF|4}}), {{chem|RnF|6}} would likely be much less stable than [[xenon hexafluoride]] ({{chem|XeF|6}}): [[radon hexafluoride]] would also probably be a [[octahedral molecular geometry|regular octahedral]] molecule, unlike the distorted octahedral structure of {{chem|XeF|6}}, because of the [[inert pair effect]].<ref>{{cite journal |last1=Liebman |first1=Joel F. |date=1975 |title=Conceptual Problems in Noble Gas and Fluorine Chemistry, II: The Nonexistence of Radon Tetrafluoride |journal=Inorg. Nucl. Chem. Lett. |volume=11 |issue=10 |pages=683–685 |doi=10.1016/0020-1650(75)80185-1}}</ref><ref name="Seppelt">{{cite journal |last1=Seppelt |first1=Konrad |date=2015 |title=Molecular Hexafluorides |journal=Chemical Reviews |volume=115 |issue=2 |pages=1296–1306 |doi=10.1021/cr5001783|pmid=25418862 }}</ref> Because radon is quite electropositive for a noble gas, it is possible that radon fluorides actually take on highly fluorine-bridged structures and are not volatile.<ref name="Seppelt"/> Extrapolation down the noble gas group would suggest also the possible existence of RnO, RnO<sub>2</sub>, and RnOF<sub>4</sub>, as well as the first chemically stable noble gas chlorides RnCl<sub>2</sub> and RnCl<sub>4</sub>, but none of these have yet been found.<ref name="Stein" />

Radon [[carbonyl]] (RnCO) has been predicted to be stable and to have a [[linear molecular geometry]].<ref>{{cite journal |doi=10.1002/qua.963 |title=Prediction of the existence of radon carbonyl: RnCO |date=2002 |author=Malli, Gulzari L. |journal=[[International Journal of Quantum Chemistry]] |volume=90 |page=611 |issue=2}}</ref> The molecules {{chem|Rn|2}} and RnXe were found to be significantly stabilized by [[Angular momentum coupling|spin-orbit coupling]].<ref>{{cite journal |doi=10.1002/(SICI)1097-461X(1998)66:2<131::AID-QUA4>3.0.CO;2-W |title=Relativistic pseudopotential calculations on Xe<sub>2</sub>, RnXe, and Rn<sub>2</sub>: The van der Waals properties of radon |date=1998 |author=Runeberg, Nino |journal=[[International Journal of Quantum Chemistry]] |volume=66 |page=131 |last2=Pyykkö |first2=Pekka |issue=2}}</ref> Radon caged inside a [[fullerene]] has been proposed as a drug for [[tumors]].<ref>{{Unbulleted list citebundle|{{cite news |last=Browne |first=Malcolm W. |url=https://query.nytimes.com/gst/fullpage.html?res=9F0CE2DE1E3CF936A35750C0A965958260&sec=&spon=&pagewanted=all |title=Chemists Find Way to Make An 'Impossible' Compound |work=The New York Times |date=1993-03-05 |access-date=2009-01-30}}|{{Cite journal |last1=Dolg |first1=M. |last2=Küchle |first2=W. |last3=Stoll |first3=H. |last4=Preuss |first4=H. |last5=Schwerdtfeger |first5=P. |date=1991-12-20 |title=Ab initio pseudopotentials for Hg to Rn: II. Molecular calculations on the hydrides of Hg to At and the fluorides of Rn |journal=Molecular Physics |language=en |volume=74 |issue=6 |pages=1265–1285 |doi=10.1080/00268979100102951 |issn=0026-8976 |bibcode=1991MolPh..74.1265D}}}}</ref> Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; {{chem|RnF|8}} should be highly unstable chemically<ref name="Thayer" /> (XeF<sub>8</sub> is thermodynamically unstable).

Radon reacts with the liquid [[interhalogen|halogen fluorides]] ClF, {{chem|ClF|3}}, {{chem|ClF|5}}, {{chem|BrF|3}}, {{chem|BrF|5}}, and {{chem|IF|7}} to form {{chem|RnF|2}}. In halogen fluoride solution, radon is nonvolatile and exists as the RnF<sup>+</sup> and Rn<sup>2+</sup> cations; addition of fluoride anions results in the formation of the complexes {{chem|RnF|3|-}} and {{chem|RnF|4|2-}}, paralleling the chemistry of [[beryllium]](II) and [[aluminium]](III).<ref name="Stein" /> The [[standard electrode potential]] of the Rn<sup>2+</sup>/Rn couple has been estimated as +2.0&nbsp;V,<ref>{{cite journal |title=Standard Electrode Potentials and Temperature Coefficients in Water at 298.15 K |last=Bratsch |first=Steven G. |date=29 July 1988 |journal=Journal of Physical and Chemical Reference Data |volume=18 |issue=1 |pages=1–21 |bibcode=1989JPCRD..18....1B |doi=10.1063/1.555839 |s2cid=97185915 }}</ref> although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.<ref name="Stein" />