Radon-222 (²²²Rn, Rn-222, historically radium emanation or radon) is the most stable

isotope Isotopes are distinct nuclear species (or ''nuclides'') of the same chemical element. They have the same atomic number (number of protons in their Atomic nucleus, nuclei) and position in the periodic table (and hence belong to the same chemica ...

radon 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 Rn has a sufficiently long half-life (3.825 days) for it to b ...

, with a

half-life Half-life is a mathematical and scientific description of exponential or gradual decay. Half-life, half life or halflife may also refer to: Film * Half-Life (film), ''Half-Life'' (film), a 2008 independent film by Jennifer Phang * ''Half Life: ...

of approximately 3.8215(2) days. It is transient in the

decay chain In nuclear science a decay chain refers to the predictable series of radioactive disintegrations undergone by the nuclei of certain unstable chemical elements. Radioactive isotopes do not usually decay directly to stable isotopes, but rather ...

of primordial uranium-238 and is the immediate decay product of radium-226. Radon-222 was first observed in 1899, and was identified as an isotope of a new element several years later. In 1957, the name ''radon'', formerly the name of only radon-222, became the name of the element. Owing to its gaseous nature and high radioactivity, radon-222 is one of the leading causes of lung cancer.

History

Following the 1898 discovery of

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

through chemical analysis of

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...

ore, Marie and

Pierre Curie Pierre Curie ( ; ; 15 May 1859 – 19 April 1906) was a French physicist, Radiochemistry, radiochemist, and a pioneer in crystallography, magnetism, piezoelectricity, and radioactivity. He shared the 1903 Nobel Prize in Physics with his wife, ...

observed a new radioactive substance emanating from radium in 1899 that was strongly radioactive for several days. Around the same time,

Ernest Rutherford Ernest Rutherford, 1st Baron Rutherford of Nelson (30 August 1871 – 19 October 1937) was a New Zealand physicist who was a pioneering researcher in both Atomic physics, atomic and nuclear physics. He has been described as "the father of nu ...

and Robert B. Owens observed a similar (though shorter-lived) emission from

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

compounds. German physicist Friedrich Ernst Dorn extensively studied these emanations in the early 1900s and attributed them to a new gaseous element, radon. In particular, he studied the product in the uranium series, radon-222, which he called ''radium emanation''. In the early 20th century, the element radon was known by several different names. Chemist William Ramsay, who extensively studied the element's chemical properties, suggested the name ''niton'', and Rutherford originally suggested ''emanation''. At that time, ''radon'' only referred to the isotope ²²²Rn, whereas the names ''actinon'' and ''thoron'' denoted ²¹⁹Rn and ²²⁰Rn, respectively. In 1957, the

International Union of Pure and Applied Chemistry The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...

(IUPAC) promoted the name ''radon'' to refer to the element rather than just ²²²Rn; this was done under a new rule concerning isotope naming conventions. This decision was controversial because it was believed to give undue credit to Dorn's identification of radon-222 over Rutherford's identification of radon-220, and the historical use of the name radon created confusion as to whether the element or the isotope ²²²Rn was being discussed.

Decay properties

Radon-222 is generated in the uranium series from the

alpha decay Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an a ...

of radium-226, which has a half-life of 1600 years. Radon-222 itself alpha decays to polonium-218 with a half-life of approximately 3.82 days, making it the most stable isotope of radon. Its final decay product is stable lead-206. In theory, ²²²Rn is capable of double beta decay to ²²²Ra, and depending on the mass measurement, single

beta decay In nuclear physics, beta decay (β-decay) is a type of radioactive decay in which an atomic nucleus emits a beta particle (fast energetic electron or positron), transforming into an isobar of that nuclide. For example, beta decay of a neutron ...

to ²²²Fr may also be allowed. These decay modes have been searched for, yielding lower partial half-life limits of 8 years for both transitions. If the beta decay of ²²²Rn is possible, it is predicted to have a very low decay energy (24 ± 21 keV) and thus a half-life on the order of 10⁵ years, also resulting in a very low branching probability relative to alpha decay. The last edition of Atomic Mass Evaluation gives even lower value of the decay energy ; thus the single beta decay is, probably, forbidden kinematically.

Occurrence and hazards

All radon isotopes are hazardous owing to their radioactivity, gaseous nature, chemical inertness, and radioactivity of their decay products (progeny). Radon-222 is especially dangerous because its longer half-life allows it to permeate soil and rocks, where it is produced in trace quantities from decays of uranium-238, and concentrate in buildings and uranium mines. This contrasts with the other natural isotopes that decay far more quickly (half-lives less than 1 minute) and thus do not contribute significantly to radiation exposure. At higher concentrations, gaseous ²²²Rn may be inhaled and decay before exhalation, which leads to a buildup of its daughters ²¹⁸Po and ²¹⁴Po in the lungs, whose high-energy alpha and

gamma Gamma (; uppercase , lowercase ; ) is the third letter of the Greek alphabet. In the system of Greek numerals it has a value of 3. In Ancient Greek, the letter gamma represented a voiced velar stop . In Modern Greek, this letter normally repr ...

radiation damages cells. Extended periods of exposure to ²²²Rn and its progeny ultimately induce lung cancer. Alternatively, radon may enter the body through contaminated drinking water or through the decay of ingested radium – making radon diffusion one of the greatest dangers of radium. Thus, ²²²Rn is a

carcinogen A carcinogen () is any agent that promotes the development of cancer. Carcinogens can include synthetic chemicals, naturally occurring substances, physical agents such as ionizing and non-ionizing radiation, and biologic agents such as viruse ...

; in fact, it is the second leading cause of lung cancer in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

after cigarette smoking, with over 20,000 deaths per year attributed to radon-induced lung cancer.

Notes

References

{{reflist Isotopes of radon IARC Group 1 carcinogens

History

Decay properties

Occurrence and hazards

See also

Notes

References