Radon compounds

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Radon compounds are chemical compounds formed by the element radon (Rn). Radon is a noble gas, i.e. a zero-valence element, and is chemically not very reactive. The 3.8-day half-life of radon-222 makes it useful in physical sciences as a natural tracer. Because radon is a gas under normal circumstances, and its decay-chain parents are not, it can readily be extracted from them for research. [1]

It is inert to most common chemical reactions, such as combustion, because its outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. [2] Its first ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol. [3] In accordance with 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 (Cl
2) or sulfur dioxide (SO
2), and significantly higher than the stability of the hydrate of hydrogen sulfide (H
2S). [4]

Structure of radon difluoride Radon-difluoride-CPK.png
Structure of radon difluoride

Because of its cost[ citation needed ] and radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all being either fluorides or oxides. Radon can be oxidized by powerful oxidizing agents such as fluorine, thus forming radon difluoride (RnF
2). [5] [6] It decomposes back to its elements at a temperature of above 523 K (250 °C; 482 °F), and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by hydrogen gas. [7] It has a low volatility and was thought to be RnF
2.[ clarification needed ] 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 distance of 2.08  ångström (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart xenon difluoride (XeF
2). [8] The octahedral molecule RnF
6 was predicted to have an even lower enthalpy of formation than the difluoride. [9] The [RnF]+ ion is believed to form by the following reaction: [10]

Rn (g) + 2 [O
2]+
[SbF
6]−
 (s) → [RnF]+
[Sb
2F
11]−
 (s) + 2 O
2 (g)

For this reason, antimony pentafluoride together with chlorine trifluoride and N
2F
2Sb
2F
11 have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds. [1] 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. [7] Additionally, salts of the [RnF]+ cation with the anions SbF−
6, TaF−
6, and BiF−
6 are known. [7] Radon is also oxidised by dioxygen difluoride to RnF
2 at 173 K (−100 °C; −148 °F). [7]

Radon oxides are among the few other reported compounds of radon; [11] only the trioxide (RnO
3) has been confirmed. [12] The higher fluorides RnF
4 and RnF
6 have been claimed to exist [12] and are calculated to be stable, [13] but their identification is unclear. [12] They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been RnF
4, RnF
6, or both. [7] 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 hydrolysed to form RnO
3. While it has been suggested that these claims were really due to radon precipitating out as the solid complex [RnF]+
2[NiF6]2−, the fact that radon coprecipitates from aqueous solution with CsXeO
3F has been taken as confirmation that RnO
3 was formed, which has been supported by further studies of the hydrolysed solution. That [RnO3F]− 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 [HRnO3]+ and anionic [HRnO4]− forms of radon in weakly acidic aqueous solution (pH > 5), the procedure having previously been validated by examination of the homologous xenon trioxide. [12]

The decay technique has also been used. Avrorin et al. reported in 1982 that 212 Fr compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of RnO3, but this could not be confirmed. [14]

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 (RnF
2) and the high positive charge on radon in RnF+; spatial separation of RnF
2 molecules may be necessary to clearly identify higher fluorides of radon, of which RnF
4 is expected to be more stable than RnF
6 due to spin–orbit splitting of the 6p shell of radon (RnIV would have a closed-shell 6s2
6p2
1/2 configuration). Therefore, while RnF
4 should have a similar stability to xenon tetrafluoride (XeF
4), RnF
6 would likely be much less stable than xenon hexafluoride (XeF
6): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of XeF
6, because of the inert-pair effect. [15] [16] 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. [16] Extrapolation down the noble gas group would suggest also the possible existence of RnO, RnO2, and RnOF4, as well as the first chemically stable noble gas chlorides RnCl2 and RnCl4, but none of these have yet been found. [7]

Radon carbonyl (RnCO) has been predicted to be stable and to have a linear molecular geometry. [17] The molecules Rn
2 and RnXe were found to be significantly stabilized by spin-orbit coupling. [18] Radon caged inside a fullerene has been proposed as a drug for tumors. [19] [20] Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; RnF
8 should be highly unstable chemically (XeF8 is thermodynamically unstable). It is predicted that the most stable Rn(VIII) compound would be barium perradonate (Ba2RnO6), analogous to barium perxenate. [13] The instability of Rn(VIII) is due to the relativistic stabilization of the 6s shell, also known as the inert-pair effect. [13]

Radon reacts with the liquid halogen fluorides ClF, ClF
3, ClF
5, BrF
3, BrF
5, and IF
7 to form RnF
2. In halogen fluoride solution, radon is nonvolatile and exists as the RnF+ and Rn2+ cations; addition of fluoride anions results in the formation of the complexes RnF−
3 and RnF2−
4, paralleling the chemistry of beryllium(II) and aluminium(III). [7] The standard electrode potential of the Rn2+/Rn couple has been estimated as +2.0 V, [21] although there is no evidence for the formation of stable radon ions or compounds in aqueous solution. [7]

See also

Related Research Articles

<span class="mw-page-title-main">Noble gas</span> Group of low-reactive, gaseous chemical elements

The noble gases are the naturally occurring members of group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). Under standard conditions, these elements are odorless, colorless, monatomic gases with very low chemical reactivity and cryogenic boiling points.

<span class="mw-page-title-main">Radon</span> Chemical element, symbol Rn and atomic number 86

Radon is a chemical element; it has symbol Rn and atomic number 86. It is a radioactive noble gas and is colorless and odorless. Of the three naturally occurring radon isotopes, only radon-222, has a sufficiently long half-life for it to be released from the soil and rock where it is generated. Radon isotopes are the immediate decay products of radium isotopes. The instability of radon-222, its most stable isotope, makes radon one of the rarest elements. Radon will be present on Earth for several billion more years, despite its short half-life, because it is constantly being produced as a step in the decay chain of uranium-238, and that of thorium-232, each of which is an extremely abundant radioactive nuclide with a half-life of several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Radon-222 occurs in significant quantities as a step in the normal radioactive decay chain of uranium-238, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into lead-206, which is stable. Radon-220 occurs in minute quantities as an intermediate step in the decay chain of thorium-232, also known as the thorium series, which eventually decays into lead-208, which is stable.

<span class="mw-page-title-main">Xenon</span> Chemical element, symbol Xe and atomic number 54

Xenon is a chemical element; it has symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized.

<span class="mw-page-title-main">Xenon hexafluoroplatinate</span> Chemical compound

Xenon hexafluoroplatinate is the product of the reaction of platinum hexafluoride with xenon, in an experiment that proved the chemical reactivity of the noble gases. This experiment was performed by Neil Bartlett at the University of British Columbia, who formulated the product as "Xe+[PtF6]−", although subsequent work suggests that Bartlett's product was probably a salt mixture and did not in fact contain this specific salt.

In chemistry, noble gas compounds are chemical compounds that include an element from the noble gases, group 18 of the periodic table. Although the noble gases are generally unreactive elements, many such compounds have been observed, particularly involving the element xenon.

In chemistry, an interhalogen compound is a molecule which contains two or more different halogen atoms and no atoms of elements from any other group.

<span class="mw-page-title-main">Xenon tetrafluoride</span> Chemical compound

Xenon tetrafluoride is a chemical compound with chemical formula XeF
4. It was the first discovered binary compound of a noble gas. It is produced by the chemical reaction of xenon with fluorine:

<span class="mw-page-title-main">Xenon hexafluoride</span> Chemical compound

Xenon hexafluoride is a noble gas compound with the formula XeF6. It is one of the three binary fluorides of xenon that have been studied experimentally, the other two being XeF2 and XeF4. All known are exergonic and stable at normal temperatures. XeF6 is the strongest fluorinating agent of the series. It is a colorless solid that readily sublimes into intensely yellow vapors.

<span class="mw-page-title-main">Xenon difluoride</span> Chemical compound

Xenon difluoride is a powerful fluorinating agent with the chemical formula XeF
2, and one of the most stable xenon compounds. Like most covalent inorganic fluorides it is moisture-sensitive. It decomposes on contact with water vapor, but is otherwise stable in storage. Xenon difluoride is a dense, colourless crystalline solid.

Xenon compounds are compounds containing the element xenon (Xe). After Neil Bartlett's discovery in 1962 that xenon can form chemical compounds, a large number of xenon compounds have been discovered and described. Almost all known xenon compounds contain the electronegative atoms fluorine or oxygen. The chemistry of xenon in each oxidation state is analogous to that of the neighboring element iodine in the immediately lower oxidation state.

<span class="mw-page-title-main">Krypton difluoride</span> Chemical compound

Krypton difluoride, KrF2 is a chemical compound of krypton and fluorine. It was the first compound of krypton discovered. It is a volatile, colourless solid at room temperature. The structure of the KrF2 molecule is linear, with Kr−F distances of 188.9 pm. It reacts with strong Lewis acids to form salts of the KrF+ and Kr
2F+
3 cations.

<span class="mw-page-title-main">Dioxygenyl</span> Chemical compound

The dioxygenyl ion, O+
2, is a rarely-encountered oxycation in which both oxygen atoms have a formal oxidation state of +1/2. It is formally derived from oxygen by the removal of an electron:

Dioxygenyl hexafluoroplatinate is a compound with formula O2PtF6. It is a hexafluoroplatinate of the unusual dioxygenyl cation, O2+, and is the first known compound containing this cation. It can be produced by the reaction of dioxygen with platinum hexafluoride. The fact that PtF
6 is strong enough to oxidise O
2, whose first ionization potential is 12.2 eV, led Neil Bartlett to correctly surmise that it might be able to oxidise xenon (first ionization potential 12.13 eV). This led to the discovery of xenon hexafluoroplatinate, which proved that the noble gases, previously thought to be inert, are able to form chemical compounds.

<span class="mw-page-title-main">Mercury(IV) fluoride</span> Chemical compound

Mercury(IV) fluoride, HgF4, is the first mercury compound to be reported with mercury in the +4 oxidation state. Mercury, like the other group 12 elements (cadmium and zinc), has an s2d10 electron configuration and generally only forms bonds involving its 6s orbital. This means that the highest oxidation state mercury normally attains is +2, and for this reason it is sometimes considered a post-transition metal instead of a transition metal. HgF4 was first reported from experiments in 2007, but its existence remains disputed; experiments conducted in 2008 could not replicate the compound.

<span class="mw-page-title-main">Plutonium hexafluoride</span> Chemical compound

Plutonium hexafluoride is the highest fluoride of plutonium, and is of interest for laser enrichment of plutonium, in particular for the production of pure plutonium-239 from irradiated uranium. This isotope of plutonium is needed to avoid premature ignition of low-mass nuclear weapon designs by neutrons produced by spontaneous fission of plutonium-240.

A hexafluoride is a chemical compound with the general formula QXnF6, QXnF6m−, or QXnF6m+. Many molecules fit this formula. An important hexafluoride is hexafluorosilicic acid (H2SiF6), which is a byproduct of the mining of phosphate rock. In the nuclear industry, uranium hexafluoride (UF6) is an important intermediate in the purification of this element.

<span class="mw-page-title-main">Xenon hexafluororhodate</span> Chemical compound

Xenon hexafluororhodate (XeRhF6) is a deep-red noble gas compound first synthesised in 1963 by Neil Bartlett. It is analogous to xenon hexafluoroplatinate.

Fluorine forms a great variety of chemical compounds, within which it always adopts an oxidation state of −1. With other atoms, fluorine forms either polar covalent bonds or ionic bonds. Most frequently, covalent bonds involving fluorine atoms are single bonds, although at least two examples of a higher order bond exist. Fluoride may act as a bridging ligand between two metals in some complex molecules. Molecules containing fluorine may also exhibit hydrogen bonding. Fluorine's chemistry includes inorganic compounds formed with hydrogen, metals, nonmetals, and even noble gases; as well as a diverse set of organic compounds. For many elements the highest known oxidation state can be achieved in a fluoride. For some elements this is achieved exclusively in a fluoride, for others exclusively in an oxide; and for still others the highest oxidation states of oxides and fluorides are always equal.

<span class="mw-page-title-main">Xenon fluoride nitrate</span> Chemical compound

Xenon fluoride nitrate, also known as fluoroxenonium nitrate, is the chemical compound with formula FXeONO2.

<span class="mw-page-title-main">Radon hexafluoride</span> Chemical compound

Radon hexafluoride is a binary chemical compound of radon and fluorine with the chemical formula RnF
6. This is still a hypothetical compound that has not been synthesized so far.

References

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  13. 1 2 3 Thayer, John S. (2010). "Relativistic Effects and the Chemistry of the Heavier Main Group Elements". Relativistic Methods for Chemists. Challenges and Advances in Computational Chemistry and Physics. Vol. 10. p. 80. doi:10.1007/978-1-4020-9975-5_2. ISBN   978-1-4020-9974-8.
  14. ↑ Avrorin, V. V.; Krasikova, R. N.; Nefedov, V. D.; Toropova, M. A. (1982). "The Chemistry of Radon". Russian Chemical Reviews. 51 (1): 12–20. Bibcode:1982RuCRv..51...12A. doi:10.1070/RC1982v051n01ABEH002787. S2CID   250906059.
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