Atomic number 86

Radon

Rn
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86 electrons in 6 shells: 2, 8, 18, 32, 18, 8.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Noble gas
Period: 6
Group: 18
Block: p
Atomic mass: 222.01758 Da (longest-lived isotope)
Electron configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶
Phase at 25 °C: gas
Melting point: −71 °C
Boiling point: −61.7 °C
Density: 9.73 g/L
Discovered: 1900, Friedrich Ernst Dorn

Neighbours in the table

Radon entered chemistry as a gas that radium gave off. Friedrich Ernst Dorn noticed it in 1900 in work on the decay of radium, and Los Alamos records that he called it radium emanation. IAEA’s nuclide chart dates the isotope radon-222 a year earlier, to 1899. In 1908 Ramsay and Gray isolated the gas, named it niton after the Latin nitens, shining, and found it, in Los Alamos’s account, the heaviest gas then known. It has been radon since 1923.

Radon needs six shells for its 86 electrons: 2, 8, 18, 32, 18 and 8, or [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶. That outer 6s² 6p⁶ octet is complete, so an extra electron would have to go into a higher shell; OpenStax gives this as the reason noble gases resist taking one. PubChem lists radon’s electron affinity as 0 eV. Radon has no stable isotope, and PubChem gives the mass of its longest-lived isotope, radon-222, as 222.018 Da.

At room temperature radon is a colourless, odourless gas with a density of 9.73 g per litre. It freezes at −71 °C, and the solid shines. Los Alamos describes a brilliant phosphorescence that shifts towards yellow with further cooling and reaches orange-red once the solid is chilled to the temperature of liquid air. Jefferson Lab agrees that the solid glows yellow, then orange-red.

Where it is found

Radon forms underground by decay. HSDB, the Hazardous Substances Data Bank quoted in PubChem’s safety record, traces three branches: radon-222 from the uranium-238 series via radium-226, radon-220 from thorium-232’s and radon-219 from uranium-235’s.

Jefferson Lab estimates 4 × 10⁻¹³ milligrams of radon per kilogram of crust and 6 × 10⁻¹⁶ milligrams per litre of seawater. Los Alamos puts about 1 g of radium in the top six inches of soil under each square mile. Some spring water carries radon too, as at Hot Springs in Arkansas. HSDB adds underground mines, tunnels and caves.

EPA’s US averages are 0.4 pCi/L (picocuries per litre) outdoors and 1.3 pCi/L inside homes; OpenStax puts the indoor figure at 1.25 pCi/L. Among the ways in EPA lists are cracked floors, construction joints, gaps around pipes, the water supply and building materials. OpenStax explains the pooling by weight: about eight times denser than air, radon settles in basements and lower floors. HSDB notes that indoor air normally sits at lower pressure than the soil gas beneath, a difference its most effective basement remedy reverses.

Why it matters

Alpha particles are stopped by the outermost layer of skin cells, so OpenStax counts alpha sources as mostly safe outside the body and hazardous once inhaled. Radon slips past that barrier because it is a gas and is breathed in easily, Jefferson Lab notes. Its solid decay products, lead-214 and bismuth-214 among them along with two polonium isotopes, cling to dust that is inhaled as well.

EPA’s best estimate is about 21,000 US lung-cancer deaths a year from radon, within a range of 8,000 to 45,000. CDC gives more than 21,000. By EPA’s estimates, radon causes more lung cancer among non-smokers than anything else. The agency advises fixing any home at 4 pCi/L or above, a level it converts to 150 becquerels per cubic metre (Bq/m³) on one page and 148 on another. Because no exposure is known to be safe, EPA also suggests action between 2 and 4 pCi/L.

Radon’s quick loss from moving water makes it a tracer. IUPAC’s notes in PubChem’s element record explain that radon-222 is soon lost from streams and river channels, so a river stretch rich in radon signals groundwater entering nearby. Radon-222 also helps date groundwater, alongside helium/radon and xenon/radon ratios. The US Agency for Toxic Substances and Disease Registry adds earthquake prediction and prospecting for petroleum and uranium.

Properties

Properties of radon Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	86
Atomic mass	222.01758 Da	Da	PubChem, longest-lived isotope
Melting point	−71 °C	°C	Los Alamos: −71 °C
Boiling point	−61.7 °C	°C	Los Alamos: −61.7 °C
Density	9.73 g/L	g/L
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶
Phase at 25 °C	gas

This element has no standard atomic weight; the atomic mass shown is that of its longest-lived isotope, as PubChem gives it.

Safety

Radon is a radioactive gas with no smell or taste, and PubChem’s safety record files it under carcinogens. The International Chemical Safety Cards call it carcinogenic to humans, and the International Agency for Research on Cancer (IARC) places it in Group 1. Haz-Map’s summary gives its decay mode as alpha, its critical organ as the lung and its internal toxicity as high. The PubChem Laboratory Chemical Safety Summary and CDC’s Radon and Your Health page are the references for homes, mines and caves where radon builds up.

Questions

What is the half-life of radon?

It depends on the isotope. For radon-222, IAEA's evaluation gives 3.8222 days and the IAEA Atomic Mass Data Center table 3.8215 days, while Los Alamos and OpenStax round to 3.823 days. IAEA and the AMDC table give alpha emission as all of its decays, and Jefferson Lab names the product, polonium-218. Radon-220 lasts 55.6 seconds in the AMDC table. Radon-219 is gone faster still: 3.96 seconds by AMDC, 4.0 by HSDB.

Does radon form any compounds?

Reports differ on which compounds exist. Los Alamos mentions a fluoride formed with fluorine, plus radon clathrates, cage structures that trap the atom. OpenStax says radon apparently forms RnF₂, judging from radiochemical tracer experiments, whereas Jefferson Lab names radon fluoride, RnF, as the only confirmed compound. Los Alamos lists oxidation states of 6, 2 and 0; Jefferson Lab gives 0 alone.

Is radon still used to treat cancer?

Hospitals once did, and accounts of today's practice differ. Los Alamos says a few hospitals still pump radon off a radium source and seal it in tiny tubes called seeds or needles, though most now buy the seeds ready-made from suppliers. Jefferson Lab says small amounts are sometimes used against some cancers. ATSDR, the US Agency for Toxic Substances and Disease Registry, says radon is no longer used to treat cancer or other diseases.

Why is radon more dangerous for smokers and children?

For smokers, EPA calls the lung-cancer risk significant because radon and tobacco smoke act synergistically, meaning the combined risk is greater than the two separate risks added together. Children are a different case. CDC says they may take in higher doses than adults, since they breathe faster and their lungs differ in shape and size, but it adds that the data cannot yet show whether their risk is higher.

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