Radon

Radon is the chemical element with atomic number 86 and the symbol Rn. It is a colorless, odorless, radioactive noble gas that forms naturally from the decay of radium and uranium in rocks and soils. Radon has no stable isotopes, and the main concern about it today is that it can build up indoors and raise the risk of lung cancer.

History of radon

Radon was identified through its radioactivity. In 1899 Ernest Rutherford and Robert B. Owens detected a radioactive gas released by thorium. In the same year Pierre and Marie Curie detected a radioactive gas emanating from radium. In 1900 Friedrich Ernst Dorn at Halle, Germany, noted that a gas was accumulating inside ampoules of radium. All three groups were observing radon.

The gas from radium was the longer-lived isotope radon-222, which has a half-life of 3.8 days and was the isotope the Curies had observed. The radon that Rutherford detected was radon-220, with a half-life of just 56 seconds. PubChem lists 1900 as the year of discovery.

In 1900 Rutherford devoted himself to investigating the new gas and showed that it could be condensed to a liquid. In 1908 William Ramsay and Robert Whytlaw-Gray at University College London collected enough radon to determine its properties, and they reported that it was the heaviest gas known.

Properties of radon

PubChem lists radon with an atomic mass of 222.01758, which is the mass of its longest-lived isotope. Radon has no stable isotopes, so no standard atomic weight is given for it. Its electron configuration is [Xe] 6s2 4f14 5d10 6p6, a shorthand showing that the outermost shell is completely full. Like neon and the other noble gases, radon sits in group 18 of the periodic table, in period 6. A full outer shell makes radon chemically inert, although it is radioactive.

At room temperature radon is a gas with a density of about 0.00973 grams per cubic centimeter. It melts at 202 kelvin, about minus 71 degrees Celsius, and boils at 211.45 kelvin, about minus 62 degrees Celsius. The Royal Society of Chemistry describes it as a colorless and odorless gas.

The first ionization energy of radon, the energy needed to remove one electron from a neutral atom, is 10.745 electronvolts, or about 1037 kilojoules per mole. The Royal Society of Chemistry lists a non-bonded atomic radius of 220 picometers and a covalent radius of 146 picometers. Its electron affinity is listed as not stable, which means a neutral radon atom does not form a stable negative ion.

Occurrence and production of radon

Radon is produced naturally from the decay of radium-226, which is found in rocks. The World Health Organization adds that radon forms from the natural radioactive decay of uranium, which is found in all rocks and soils, and that radon can also be found in water. It was first discovered as a radioactive gas produced from radium as it decayed, and there is a detectable amount in the Earth’s atmosphere.

Radon escapes from the ground into the air, where it decays and produces further radioactive particles. Outdoors it dilutes quickly. The World Health Organization gives an average outdoor level of 5 to 15 becquerels per cubic meter, a unit that counts radioactive decays per second in each cubic meter of air. Indoors the levels can vary substantially, from 10 to more than 10,000 becquerels per cubic meter, with the highest levels in places such as mines, caves and water treatment facilities.

Uses of radon

Radon decays into radioactive polonium and alpha particles, and this emitted radiation made it useful in cancer therapy. Radon was used in some hospitals to treat tumors by sealing the gas in minute tubes and implanting them in the tumor, so that the radiation treated the disease in place. The Royal Society of Chemistry notes that other, safer treatments are now more commonly used.

The modern practical interest in radon is measurement. Home testing kits are available which can be sent away for analysis, and the World Health Organization states that radon concentration indoors can easily be measured with a small passive detector.

Isotopes of radon

All isotopes of radon are radioactive. An isotope is a form of an element whose atoms have the same number of protons but different numbers of neutrons. The Royal Society of Chemistry lists radon-222, which has a half-life of 3.823 days, radon-220, with a half-life of 55.6 seconds, and radon-211, with a half-life of 14.6 hours. Radon-222 and radon-220 decay by alpha emission, and radon-211 by positron emission, electron capture and alpha emission.

Because even the longest-lived isotope disappears within days, radon found today must be continually replenished by the decay of radium and uranium in the ground.

Health and environmental effects of radon

The Royal Society of Chemistry states that radon has no known biological role, although it may have played a significant part in evolution, because it is responsible for much of the Earth’s background radiation, which can lead to genetic modifications.

The World Health Organization states that radon is one of the leading causes of lung cancer and estimates that it causes between 3 and 14 percent of all lung cancers in a country, depending on the national average radon level and smoking prevalence. As people breathe, radioactive particles formed by radon decay are deposited on the cells lining the airways, where they can damage DNA. The risk of lung cancer is higher for smokers because of the combined effects of radon and cigarette smoking.

An increased rate of lung cancer was first seen in uranium miners exposed to very high concentrations of radon, and later studies in Europe, North America and China confirmed that low concentrations common in homes also pose a risk. The organization reports that the risk increases by about 16 percent for each 100 becquerels per cubic meter rise in long-term average radon concentration, and that well-tested, durable and cost-efficient methods exist for preventing radon entry into new buildings and reducing radon in existing buildings. This entry gives general background drawn from reference sources and is not advice.

This entry explains a medical term. It is not medical advice. If you are concerned about your health, speak with a qualified health professional.

Editorially reviewed by Specialty Digest Editorial TeamLast reviewed November 4, 2026Researched and drafted with AI assistanceReport an issue