of radon, photographed by Ernest Rutherford
in 1908. Numbers at the side of the spectrum are wavelengths. The middle spectrum is of radon, while the outer two are of helium
(added to calibrate the wavelengths).
Radon is a colorless, odorless, and tasteless gas and therefore is not detectable by human senses alone. At standard temperature and pressure, radon 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. Radon is one of the densest gases at room temperature and is the densest of the noble gases. Although colorless at standard temperature and pressure, when cooled below its freezing point of 202 K (−71 °C; −96 °F), radon emits a brilliant radioluminescence that turns from yellow to orange-red as the temperature lowers. Upon condensation, radon glows because of the intense radiation it produces. Radon is sparingly soluble in water, but more soluble than lighter noble gases. Radon is appreciably more soluble in organic liquids than in water.
Being a noble gas, radon is chemically not very reactive. However, the 3.8-day half-life of radon-222 makes it useful in physical sciences as a natural tracer. Because radon is a gas at standard conditions, unlike its parents, it can readily be extracted from them for research.
Radon is a member of the zero-valence elements that are called noble gases. 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. 1037 kJ/mol is required to extract one electron from its shells (also known as the first ionization energy). 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
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 250 °C, 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 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 distance of 2.08 Å, and that the compound is thermodynamically more stable and less volatile than its lighter counterpart 2. The octahedral molecule RnF
6 was predicted to have an even lower enthalpy of formation than the difluoride. The higher fluorides RnF4 and RnF6 have been claimed, and are calculated to be stable, but it is doubtful whether they have yet been synthesized. The [RnF]+ ion is believed to form by the following reaction:
- Rn (g) + 2 [O
(s) → [RnF]+
(s) + 2 O
For this reason, antimony pentafluoride together with chlorine trifluoride and N2F2Sb2F11 have been considered for radon gas removal in uranium mines due to the formation of radon–fluorine compounds. The existence of RnF2 allows for safer handling of radon's parent radium as the fluoride, as the alpha radiation from 226Ra is not strong enough to cause radiolysis of the strong Ra–F bond; thus 226RaF2 decays to form involatile 222RnF2. Additionally, salts of the [RnF]+ cation with the anions SbF−
6, and BiF−
6 are known. Radon is also oxidised by dioxygen difluoride to RnF2 at −100 °C.
Radon oxides are among the few other reported compounds of radon; only the trioxide (RnO3) has been confirmed. Higher fluorides may have been observed in experiments where unknown radon-containing products distilled together with xenon hexafluoride, and perhaps in the production of radon trioxide: these may have been RnF4, RnF6, or both. 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. Radon carbonyl RnCO has been predicted to be stable and to have a linear molecular geometry. The molecules Rn
2 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; RnF8 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. The instability of Rn(VIII) is due to the relativistic stabilization of the 6s shell, also known as the inert pair effect.
Radon reacts with the liquid halogen fluorides ClF, ClF3, ClF5, BrF3, BrF5, and IF7 to form RnF2. In halogen fluoride solution, radon is involatile 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). The standard electrode potential of the Rn2+/Rn couple has been estimated as +2.0 V, though there is no evidence for the formation of stable radon ions or compounds in aqueous solution.
The radium or uranium series.
Radon has no stable isotopes. Thirty-seven radioactive isotopes have been characterized, with atomic masses ranging from 193 to 229. The most stable isotope is 222Rn, which is a decay product of 226Ra, a decay product of 238U. A trace amount of the (highly unstable) isotope 218Rn is also among the daughters of 222Rn.
Three other radon isotopes have a half-life of over an hour: 211Rn, 210Rn and 224Rn. The 220Rn isotope is a natural decay product of the most stable thorium isotope (232Th), and is commonly referred to as thoron. It has a half-life of 55.6 seconds and also emits alpha radiation. Similarly, 219Rn is derived from the most stable isotope of actinium (227Ac)—named "actinon"—and is an alpha emitter with a half-life of 3.96 seconds. No radon isotopes occur significantly in the neptunium (237Np) decay series, though a trace amount of the (extremely unstable) isotope 217Rn is produced.
222Rn belongs to the radium and uranium-238 decay chain, and has a half-life of 3.8235 days. Its four first products (excluding marginal decay schemes) are very short-lived, meaning that the corresponding disintegrations are indicative of the initial radon distribution. Its decay goes through the following sequence:
- 222Rn, 3.82 days, alpha decaying to...
- 218Po, 3.10 minutes, alpha decaying to...
- 214Pb, 26.8 minutes, beta decaying to...
- 214Bi, 19.9 minutes, beta decaying to...
- 214Po, 0.1643 ms, alpha decaying to...
- 210Pb, which has a much longer half-life of 22.3 years, beta decaying to...
- 210Bi, 5.013 days, beta decaying to...
- 210Po, 138.376 days, alpha decaying to...
- 206Pb, stable.
The radon equilibrium factor is the ratio between the activity of all short-period radon progenies (which are responsible for most of radon's biological effects), and the activity that would be at equilibrium with the radon parent.
If a closed volume is constantly supplied with radon, the concentration of short-lived isotopes will increase until an equilibrium is reached where the rate of decay of each decay product will equal that of the radon itself. The equilibrium factor is 1 when both activities are equal, meaning that the decay products have stayed close to the radon parent long enough for the equilibrium to be reached, within a couple of hours. Under these conditions each additional pCi/L of radon will increase exposure, by 0.01 WL (Working Level -a measure of radioactivity commonly used in mining. A detailed explanation of WL is given in Concentration Units). These conditions are not always met; in many homes, the equilibrium fraction is typically 40%; that is, there will be 0.004 WL of daughters for each pCi/L of radon in air. 210Pb takes much longer (decades) to come in equilibrium with radon, but, if the environment permits accumulation of dust over extended periods of time, 210Pb and its decay products may contribute to overall radiation levels as well.
Because of their electrostatic charge, radon progenies adhere to surfaces or dust particles, whereas gaseous radon does not. Attachment removes them from the air, usually causing the equilibrium factor in the atmosphere to be less than one. The equilibrium factor is also lowered by air circulation or air filtration devices, and is increased by airborne dust particles, including cigarette smoke. In high concentrations, airborne radon isotopes contribute significantly to human health risk. The equilibrium factor found in epidemiological studies is 0.4.