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 Rn has a sufficiently long half-life (3.825 days) 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 Rn, 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 chains of U and Th, both of which are abundant radioactive nuclides with half-lives of at least several billion years. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Rn occurs in significant quantities as a step in the normal radioactive decay chain of U, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into stable Pb. Rn occurs in minute quantities as an intermediate step in the decay chain of Th, also known as the thorium series, which eventually decays into stable Pb.
Radon was discovered in 1899 by Ernest Rutherford and Robert B. Owens at McGill University in Montreal, and was the fifth radioactive element to be discovered. First known as "emanation", the radioactive gas was identified during experiments with radium, thorium oxide, and actinium by Friedrich Ernst Dorn, Rutherford and Owens, and André-Louis Debierne, respectively, and each element's emanation was considered to be a separate substance: radon, thoron, and actinon. Sir William Ramsay and Robert Whytlaw-Gray considered that the radioactive emanations may contain a new element of the noble gas family, and isolated "radium emanation" in 1909 to determine its properties. In 1911, the element Ramsay and Whytlaw-Gray isolated was accepted by the International Commission for Atomic Weights, and in 1923, the International Committee for Chemical Elements and the International Union of Pure and Applied Chemistry (IUPAC) chose radon as the accepted name for the element's most stable isotope, Rn; thoron and actinon were also recognized by IUPAC as distinct isotopes of the element.
A common source of environmental radon is uranium-containing minerals in the ground. Radon can also occur in ground water, such as spring waters and hot springs. Radon trapped in permafrost may be released by climate-change-induced thawing of permafrosts, and radon may also be released into groundwater and the atmosphere following seismic events leading to earthquakes, which has led to its investigation in the field of earthquake prediction. It is possible to test for radon in buildings, and to use techniques such as sub-slab depressurization for mitigation.
Epidemiological studies have shown a clear association between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. Because radon is denser than air it accumulates in basements and crawlspaces under dwellings. According to the United States Environmental Protection Agency (EPA), radon is the second most frequent cause of lung cancer, after cigarette smoking, causing 21,000 lung cancer deaths per year in the United States. About 2,900 of these deaths occur among people who have never smoked. While radon is the second most frequent cause of lung cancer, it is the number one cause among non-smokers, according to EPA policy-oriented estimates. Significant uncertainties exist for the health effects of low-dose exposures. Due to local differences in geology, the level of exposure to radon gas differs by location.
Characteristics
Physical properties
Radon is a colorless, odorless, and tasteless gas and therefore is not detectable by human senses alone. At standard temperature and pressure, it forms a monatomic gas with a density of 9.73 kg/m3, about 8 times the density of the Earth's atmosphere at sea level, 1.217 kg/m3. It is one of the densest gases at room temperature (a few are denser, e.g. and ) and is the densest of the noble gases. Radon is colorless at standard temperature and pressure. When cooled below its boiling point of , concentrated liquid radon emits radioluminescence of varying color; solidified radon emits a blue to yellow to red light when cooled further beyond its freezing point of . Due to the hazards associated with high concentrations of radon, liquid and solid radon is almost never seen. Measurements of the solubility of radon-222 are unusual in that they take advantage of radon's radioactivity to compare the amount in gas and in solution.
Chemical properties
Radon is a member of the zero-valence elements that are called noble gases, and is chemically not very reactive. The inert pair effect stabilizes the 6s shell, making it unavailable for bonding—a consequence only understood within relativistic quantum chemistry. The 3.8-day half-life of Rn makes it useful in physical sciences as a natural tracer. Because radon is a gas at standard conditions, unlike its decay-chain parents, it can readily be extracted from them for research.
It is inert to most common chemical reactions, such as combustion, because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. Its first ionization energy—the minimum energy required to extract one electron from it—is 1037 kJ/mol. 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 () or sulfur dioxide (), and significantly higher than the stability of the hydrate of hydrogen sulfide ().
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 fluorides or oxides. Radon can be oxidized by powerful oxidizing agents such as fluorine, thus forming radon difluoride (). It decomposes back to its elements at a temperature of above , and is reduced by water to radon gas and hydrogen fluoride: it may also be reduced back to its elements by hydrogen gas. It has a low volatility and was thought to be . 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 ångströms (Å), and that the compound is thermodynamically more stable and less volatile than its lighter counterpart xenon difluoride (). The octahedral molecule was predicted to have an even lower enthalpy of formation than the difluoride. The ion is believed to form by the following reaction:
For this reason, antimony pentafluoride together with chlorine trifluoride and have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds. 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. Additionally, salts of the cation with the anions , , and are known. Radon is also oxidised by dioxygen difluoride to at .
Radon oxides are among the few other reported compounds of radon; only the trioxide () has been confirmed. The higher fluorides and have been claimed, are calculated to be stable, but have not been confirmed. They may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride: these may have been , , or both. 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 . While it has been suggested that these claims were really due to radon precipitating out as the solid complex , the fact that radon coprecipitates from aqueous solution with has been taken as confirmation that was formed, which has been supported by further studies of the hydrolysed solution. That 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 and anionic forms of radon in weakly acidic aqueous solution (pH > 5), the procedure having previously been validated by examination of the homologous xenon trioxide.
The decay technique has also been used. Avrorin et al. reported in 1982 that Fr compounds cocrystallised with their caesium analogues appeared to retain chemically bound radon after electron capture; analogies with xenon suggested the formation of , but this could not be confirmed.
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 () and the high positive charge on radon in ; spatial separation of molecules may be necessary to clearly identify higher fluorides of radon, of which is expected to be more stable than due to spin–orbit splitting of the 6p shell of radon (RnIV would have a closed-shell 6s6p configuration). Therefore, while should have a similar stability to xenon tetrafluoride (), would likely be much less stable than xenon hexafluoride (): radon hexafluoride would also probably be a regular octahedral molecule, unlike the distorted octahedral structure of , because of the inert pair effect. 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. Extrapolation down the noble gas group would suggest also the possible existence of RnO, , and , as well as the first chemically stable noble gas chlorides and , but none of these have yet been found.
Radon carbonyl (RnCO) has been predicted to be stable and to have a linear molecular geometry. The molecules and RnXe were found to be significantly stabilized by spin-orbit coupling. Radon caged inside a fullerene has been proposed as a drug for tumors. Despite the existence of Xe(VIII), no Rn(VIII) compounds have been claimed to exist; should be highly unstable chemically ( is thermodynamically unstable).
Radon reacts with the liquid halogen fluorides ClF, , , , , and to form . In halogen fluoride solution, radon is nonvolatile and exists as the and cations; addition of fluoride anions results in the formation of the complexes and , paralleling the chemistry of beryllium(II) and aluminium(III). The standard electrode potential of the /Rn couple has been estimated as +2.0 V, although there is no evidence for the formation of stable radon ions or compounds in aqueous solution.
Isotopes
Radon has no stable isotopes and no standard atomic weight. Thirty-nine radioactive isotopes have been characterized, with mass numbers ranging from 193 to 231. Six of them, from 217 to 222 inclusive, occur naturally. The most stable isotope is Rn (half-life 3.82 days), which is a decay product of Ra, the latter being itself a decay product of U. A trace amount of the (highly unstable) isotope Rn (half-life about 35 milliseconds) is also among the daughters of Rn. The isotope Rn would be produced by the double beta decay of natural Po; while energetically possible, this process has however never been seen.