Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through which thorium and uranium slowly decay into various short-lived radioactive elements and eventually into stable lead. Radon itself is the immediate decay product of radium. Its most stable isotope, 222Rn, has a half-life of only 3. 8 days, making it one of the rarest elements. Since thorium and uranium are two of the most common radioactive elements on Earth, while also having three isotopes with half-lives on the order of several billion years, radon will be present on Earth long into the future despite its short half-life. The decay of radon produces many other short-lived nuclides, known as "radon daughters", ending at stable isotopes of lead. Unlike all other intermediate elements in the aforementioned decay chains, radon is, under standard conditions, gaseous and easily inhaled, and therefore a health hazard. It is often the single largest contributor to an individual's background radiation dose, but due to local differences in geology, the level of exposure to radon gas differs from place to place. A common source is uranium-containing minerals in the ground, and therefore it accumulates in subterranean areas such as basements. Radon can also occur in some ground water like spring waters and hot springs. Climate change may cause radon previously trapped underground to be released as permafrost thaws, particularly in areas like the Arctic, Alaska, Canada, Greenland and Russia. 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 link between breathing high concentrations of radon and incidence of lung cancer. Radon is a contaminant that affects indoor air quality worldwide. 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. Unlike the gaseous radon itself, radon daughters are solids and stick to surfaces, such as airborne dust particles, which can cause lung cancer if inhaled.

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Atomic properties

Atomic mass(222 u)

Atomic radii

Radius (calculated)120 pm
Covalent radius150 pm
Van der Waals radius220 pm

Atomic shell

Electron configurationXe 4f14 5d10 6s2 6p6
Ionization energy(1st) 10.7485 eV
(2nd) 21.4 eV
(3rd) 29.4 eV
(4th) 36.9 eV
(5th) 52.9 eV
Shell model

Physical properties

Density0.00973 g·cm−3
Molar volumesolid: 5.05·10-5 m3·mol−1


Melting point202 K
Boiling point211.5 K
Liquid range9.5 K
Critical point377 K @ 6.28 MPa


Melting enthalpy3 kJ·mol-1
Enthalpy of vaporization17 kJ·mol-1
Binding energy0 kJ·mol-1

Heat and conductivity

Thermal conductivity0.00364 W·m-1·K-1



Chemical properties

Oxidation state-


Mulliken scale2.59
Nagle scale2.27

Other properties

Natural occurrenceDecay product
Crystal structureFace-centered cubic
Goldschmidt Classificationsynthetic
Superconductorwithout transition tempperature
Radioactivitysignificantly radioactive

Natural abundances

5.999·10-13 ppb ≈ 822 mg
Flowing water
2·10-13 ppb ≈ 3.2 mg