Radon

Jai Ganesh · Today 16:16:48

Radon

Gist

Radon is a naturally occurring, radioactive, invisible, odorless, and tasteless gas formed from the decay of uranium in rocks and soil that can accumulate in buildings and is a leading cause of lung cancer. You can test your home for high radon levels with an inexpensive test kit and reduce levels by improving ventilation or installing a radon sump system.

Radon has limited but specific applications in controlled environments, primarily for radiotherapy where its gamma rays can treat cancer. It's also used in scientific research, such as atmospheric and geological studies, earthquake prediction, and as a tracer in various environmental applications like locating underground contamination and measuring groundwater flow. Historically, it was used in industrial radiography and X-ray production.

Summary

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 222Rn 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 222Rn, 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 238U and 232Th, 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. 222Rn occurs in significant quantities as a step in the normal radioactive decay chain of 238U, also known as the uranium series, which slowly decays into a variety of radioactive nuclides and eventually decays into stable 206Pb. 220Rn occurs in minute quantities as an intermediate step in the decay chain of 232Th, also known as the thorium series, which eventually decays into stable 208Pb.

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, 222Rn; thoron and actinon were also recognized by IUPAC as distinct isotopes of the element.

Under standard conditions, radon is gaseous and can be easily inhaled, posing a health hazard. However, the primary danger comes not from radon itself, but from its decay products, known as radon daughters. These decay products, often existing as single atoms or ions, can attach themselves to airborne dust particles. Although radon is a noble gas and does not adhere to lung tissue (meaning it is often exhaled before decaying), the radon daughters attached to dust are more likely to stick to the lungs. This increases the risk of harm, as the radon daughters can cause damage to lung tissue. Radon and its daughters are, taken together, 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 by location. A common source of environmental radon is uranium-containing minerals in the ground; it therefore accumulates in subterranean areas such as basements. 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. 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.

Details

Radon (Rn) is a chemical element, a heavy radioactive gas of Group 18 (noble gases) of the periodic table, generated by the radioactive decay of radium. (Radon was originally called radium emanation.) Radon is a colourless gas, 7.5 times heavier than air and more than 100 times heavier than hydrogen. The gas liquefies at −61.8 °C (−79.2 °F) and freezes at −71 °C (−96 °F). On further cooling, solid radon glows with a soft yellow light that becomes orange-red at the temperature of liquid air (−195 °C [−319 °F]).

Radon is rare in nature because its isotopes are all short-lived and because its source, radium, is a scarce element. The atmosphere contains traces of radon near the ground as a result of seepage from soil and rocks, both of which contain minute quantities of radium. (Radium occurs as a natural decay product of uranium present in various types of rocks.)

By the late 1980s, naturally occurring radon gas had come to be recognized as a potentially serious health hazard. Radioactive decay of uranium in minerals, especially granite, generates radon gas that can diffuse through soil and rock and enter buildings through basements (radon has a higher density than air) and through water supplies derived from wells (radon has a significant solubility in water). The gas can accumulate in the air of poorly ventilated houses. The decay of radon produces radioactive “daughters” (polonium, bismuth, and lead isotopes) that can be ingested from well water or can be absorbed in dust particles and then breathed into the lungs. Exposure to high concentrations of this radon and its daughters over the course of many years can greatly increase the risk of developing lung cancer. Indeed, radon is now thought to be the greatest cause of lung cancer among nonsmokers in the United States. Radon levels are highest in homes built over geological formations that contain uranium mineral deposits.

Concentrated samples of radon are prepared synthetically for medical and research purposes. Typically, a supply of radium is kept in a glass vessel in an aqueous solution or in the form of a porous solid from which the radon can readily flow. Every few days, the accumulated radon is pumped off, purified, and compressed into a small tube, which is then sealed and removed. The tube of gas is a source of penetrating gamma rays, which come mainly from one of radon’s decay products, bismuth-214. Such tubes of radon have been used for radiation therapy and radiography.

Natural radon consists of three isotopes, one from each of the three natural radioactive-disintegration series (the uranium, thorium, and actinium series). Discovered in 1900 by German chemist Friedrich E. Dorn, radon-222 (3.823-day half-life), the longest-lived isotope, arises in the uranium series. The name radon is sometimes reserved for this isotope to distinguish it from the other two natural isotopes, called thoron and actinon, because they originate in the thorium and the actinium series, respectively.

Radon-220 (thoron; 51.5-second half-life) was first observed in 1899 by American scientist Robert B. Owens and British scientist Ernest Rutherford, who noticed that some of the radioactivity of thorium compounds could be blown away by breezes in the laboratory. Radon-219 (actinon; 3.92-second half-life), which is associated with actinium, was found independently in 1904 by German chemist Friedrich O. Giesel and French physicist André-Louis Debierne. Radioactive isotopes having masses ranging from 204 through 224 have been identified, the longest-lived of these being radon-222, which has a half-life of 3.82 days. All the isotopes decay into stable end-products of helium and isotopes of heavy metals, usually lead.

Radon atoms possess a particularly stable electronic configuration of eight electrons in the outer shell, which accounts for the characteristic chemical inactivity of the element. Radon, however, is not chemically inert. For example, the existence of the compound radon difluoride, which is apparently more stable chemically than compounds of the other reactive noble gases, krypton and xenon, was established in 1962. Radon’s short lifetime and its high-energy radioactivity cause difficulties for the experimental investigation of radon compounds.

When a mixture of trace amounts of radon-222 and fluorine gas is heated to approximately 400 °C (752 °F), a nonvolatile radon fluoride is formed. The intense α-radiation of millicurie and curie amounts of radon provides sufficient energy to allow radon in such quantities to react spontaneously with gaseous fluorine at room temperature and with liquid fluorine at −196 °C (−321 °F). Radon is also oxidized by halogen fluorides such as ClF3, BrF3, BrF5, IF7, and [NiF6]2− in HF solutions to give stable solutions of radon fluoride. The products of these fluorination reactions have not been analyzed in detail because of their small masses and intense radioactivity. Nevertheless, by comparing reactions of radon with those of krypton and xenon it has been possible to deduce that radon forms a difluoride, RnF2, and derivatives of the difluoride. Studies show that ionic radon is present in many of these solutions and is believed to be Rn2+, RnF+, and RnF3−. The chemical behaviour of radon is similar to that of a metal fluoride and is consistent with its position in the periodic table as a metalloid element.

Element Properties

atomic number : 86
stablest isotope : (222)
melting point : −71 °C (−96 °F)
boiling point : −62 °C (−80 °F)
density : (1 atm, 0 °C [32 °F]) : 9.73 g/litre (0.13 ounce/gallon)
oxidation states : 0, +2.

Additional Information:

Appearance

Radon is a colourless and odourless gas. It is chemically inert, but radioactive.

Uses

Radon decays into radioactive polonium and alpha particles. This emitted radiation made radon useful in cancer therapy. Radon was used in some hospitals to treat tumours by sealing the gas in minute tubes, and implanting these into the tumour, treating the disease in situ. Other, safer treatments are now more commonly used.

In some places, high concentrations of radon can build up indoors, escaping from the ground or from granite buildings. Home testing kits are available which can be sent away for analysis.

Biological role

Radon has no known biological role. It is, however, thought that it may have had a significant role in evolution. This is because it is responsible for much of the Earth’s background radiation that can lead to genetic modifications.

Natural abundance

Radon is produced naturally from the decay of the isotope radium-226, which is found in rocks. It was first discovered as a radioactive gas produced from radium as it decayed. There is a detectable amount in the Earth’s atmosphere.

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Radon

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