View source for Radon-222

{{Short description|Most stable isotope of radon}}
{{Infobox isotope
|alternate_names = Radium emanation
|mass_number = 222
|symbol = Rn
|num_neutrons = 136
|num_protons = 86
|abundance = [[Trace radioisotope|Trace]]
|halflife = {{val|3.8215|(2)|u=days}}{{NUBASE2020|ref}}
|decay_product = Polonium-218
|decay_mass = 218
|decay_symbol = Po
|parent = Radium-226
|parent_mass = 226
|parent_symbol = Ra
|parent_decay =a
|mass = 222.0175760(21){{AME2020 II|ref}}
|spin = 0
|decay_mode1 =[[Alpha decay]]
|decay_energy1 =5.5904(3){{AME2020 II|name}}
}}

'''Radon-222''' (<sup>222</sup>Rn, Rn-222, historically '''radium emanation''' or radon) is the most stable [[isotope]] of [[radon]], with a [[half-life]] of approximately 3.8215(2) days{{NUBASE2020|ref}}. It is transient in the [[decay chain]] of [[primordial radionuclide|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 [[chemical element|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]].<ref name=EPA02 />

==History==
Following the 1898 discovery of [[radium]] through chemical analysis of [[radioactive]] ore, [[Marie Curie|Marie]] and [[Pierre Curie]] observed a new radioactive substance emanating from radium in 1899 that was strongly radioactive for several days.<ref name=RnIsotopes>{{cite journal |last1=Fry |first1=C. |last2=Thoennessen |first2=M. |title=Discovery of the astatine, radon, francium, and radium isotopes |date=2013 |journal=Atomic Data and Nuclear Data Tables |volume=99 |issue=5 |pages=497–519 |doi=10.1016/j.adt.2012.05.003 |arxiv=1205.5841|bibcode=2013ADNDT..99..497F |s2cid=12590893}}</ref> Around the same time, [[Ernest Rutherford]] and [[Robert B. Owens]] observed a similar (though shorter-lived) emission from [[thorium]] compounds.<ref name=isodiscovery>{{Thoennessen2016|pages=8}}</ref> 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''.<ref name=george>{{cite journal |last=George |first=A.C. |date=2008 |title=World History of Radon Research and Measurement from the Early 1900s to Today |journal=AIP Conference Proceedings |volume=1034 |issue=1 |pages=20–36 |doi=10.1063/1.2991210 |bibcode=2008AIPC.1034...20G |url=https://www.rtca.com/pdfs/historical%20evolution%20of%20radon%20measurements.pdf |citeseerx=10.1.1.618.9328 |access-date=2019-02-22 |archive-date=2016-08-24 |archive-url=https://web.archive.org/web/20160824161208/http://www.rtca.com/pdfs/HISTORICAL%20EVOLUTION%20OF%20RADON%20MEASUREMENTS.pdf |url-status=dead }}</ref>

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 <sup>222</sup>Rn, whereas the names ''actinon'' and ''thoron'' denoted <sup>219</sup>Rn and <sup>220</sup>Rn, respectively.<ref name=RnNatureChem /> In 1957, the [[International Union of Pure and Applied Chemistry]] (IUPAC) promoted the name ''radon'' to refer to the element rather than just <sup>222</sup>Rn; this was done under a new rule concerning isotope naming conventions.<ref name=RnNatureChem>{{cite journal |last1=Thornton |first1=B.F. |last2=Burdette |first2=S.C. |title=Recalling radon's recognition |date=2013 |journal=Nature Chemistry |volume=5 |issue=9 |page=804 |doi=10.1038/nchem.1731 |pmid=23965684 |bibcode=2013NatCh...5..804T |doi-access=free }}</ref> 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 <sup>222</sup>Rn was being discussed.<ref name=RnNatureChem />

==Decay properties==
[[File:Decay chain(4n+2, Uranium series).PNG|thumb|right|240px|The decay chain of uranium-238, known as the uranium series or radium series, of which radon-222 is a member.]]
Radon-222 is generated in the uranium series from the [[alpha decay]] 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.{{NUBASE2020|name}} Its final decay product is stable [[lead-206]].

In theory, <sup>222</sup>Rn is capable of [[double beta decay]] to <sup>222</sup>Ra, and depending on the mass measurement, single [[beta decay]] to <sup>222</sup>Fr may also be allowed.<ref name=BelliBaF2>{{cite journal |last1=Belli |first1=P. |last2=Bernabei |first2=R. |last3=Cappella |first3=F. |last4=Caracciolo |first4=V. |last5=Cerulli |first5=R. |last6=Danevich |first6=F.A. |last7=Di Marco |first7=A. |last8=Incicchitti |first8=A. |last9=Poda |first9=D.V. |last10=Polischuk |first10=O.G. |last11=Tretyak |first11=V.I. |title=Investigation of rare nuclear decays with BaF<sub>2</sub> crystal scintillator contaminated by radium |date=2014 |journal=European Physical Journal A |volume=50 |issue=9 |pages=134–143 |doi=10.1140/epja/i2014-14134-6 |arxiv=1407.5844|bibcode=2014EPJA...50..134B |s2cid=118513731 }}</ref>{{efn|AME2020 gives <sup>222</sup>Rn a lower mass than <sup>222</sup>Fr (the available energy is {{nobr|(−6 ± 8) keV}}),{{NUBASE2020|name}}{{AME2020 II|name}} which would forbid single beta decay, though it is possible within the given error margin and is explicitly predicted by Belli et al.}} These decay modes have been searched for, yielding lower [[partial half-life]] limits of 8 years for both transitions. If the beta decay of <sup>222</sup>Rn is possible, it is predicted to have a very low decay energy (24 ± 21 [[electronvolt|keV]]) and thus a half-life on the order of 10<sup>5</sup> years, also resulting in a very low branching probability relative to alpha decay.<ref name=BelliBaF2 /> The last edition of Atomic Mass Evaluation{{AME2020 II|name}} gives even lower value of the decay energy {{nobr|(−6 ± 8) keV}}; thus the single beta decay is, probably, forbidden kinematically. 

==Occurrence and hazards==
{{main|Health effects of radon}}

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 mining#Health risks|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.<ref name=EPA01>{{cite report |date=2003 |title=EPA assessment of risks from radon in homes |publisher=Office of Radiation and Indoor Air, United States Environmental Protection Agency |url=https://www.epa.gov/sites/production/files/2015-05/documents/402-r-03-003.pdf}}</ref> At higher concentrations, gaseous <sup>222</sup>Rn may be inhaled and decay before exhalation, which leads to a buildup of its daughters <sup>218</sup>Po and <sup>214</sup>Po in the lungs, whose high-energy alpha and [[gamma radiation|gamma]] radiation damages cells. Extended periods of exposure to <sup>222</sup>Rn and its progeny ultimately induce lung cancer.<ref name=EPA01 /> Alternatively, radon may enter the body through contaminated drinking water or through the decay of ingested radium<ref name=EPA02>{{cite report |publisher=United States Environmental Protection Agency |title=EPA Facts about Radon |pages=1–3 |url=https://semspub.epa.gov/work/11/176336.pdf |access-date=22 February 2019}}</ref> – making radon diffusion one of the greatest dangers of radium.<ref name=EPA03>{{cite web |title=Radiation protection: Radium |url=http://www.epa.gov/radiation/radionuclides/radium.html |publisher=United States Environmental Protection Agency |url-status=dead |archive-url=https://web.archive.org/web/20150211154556/http://www.epa.gov/radiation/radionuclides/radium.html |archive-date=11 February 2015 |access-date=22 February 2019}}</ref> Thus, <sup>222</sup>Rn is a [[carcinogen]]; in fact, it is the second leading cause of lung cancer in the [[United States]] after [[cigarette smoking]],<ref name=EPA02 /> with over 20,000 deaths per year attributed to radon-induced lung cancer.<ref name=EPA01 /><ref name=factsheet>{{cite web |title= Radon Fact Sheet: What it is, how it affects us, why it matters |url=https://www.radon.com/radon_facts/ |publisher=Air Chek, Inc. |access-date=22 February 2019}}</ref>

==See also==
* [[Isotopes of radon]]

==Notes==
{{notelist}}

==References==
{{reflist}}

[[Category:Isotopes of radon]]
[[Category:IARC Group 1 carcinogens]]