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'''Astatine''' is a [[chemical element]]; it has [[Symbol (chemistry)|symbol]] '''At''' and [[atomic number]] 85. It is the [[abundance of elements in Earth's crust|rarest naturally occurring element]] in the [[Earth's crust]], occurring only as the [[Decay chain|decay product]] of various heavier elements. All of astatine's [[isotope]]s are short-lived; the most stable is astatine-210, with a [[half-life]] of 8.1&nbsp;hours. Consequently, a solid sample of the element has never been seen, because any macroscopic specimen would be immediately vaporized by the heat of its [[radioactivity]].

The bulk properties of astatine are not known with certainty. Many of them have been estimated from its position on the [[periodic table]] as a heavier analog of [[fluorine]], [[chlorine]], [[bromine]], and [[iodine]], the four stable [[halogen]]s. However, astatine also falls roughly along the [[dividing line between metals and nonmetals]], and some metallic behavior has also been observed and predicted for it. Astatine is likely to have a dark or lustrous appearance and may be a [[semiconductor]] or possibly a [[metal]]. Chemically, several [[anion]]ic species of astatine are known and most of its compounds resemble those of iodine, but it also sometimes displays metallic characteristics and shows some similarities to [[silver]].

The first synthesis of astatine was in 1940 by [[Dale R. Corson]], [[Kenneth Ross MacKenzie]], and [[Emilio G. Segrè]] at the [[University of California, Berkeley]]. They named it from the [[Ancient Greek]] {{transliteration|grc|ástatos}} ({{lang|grc|ἄστατος}}) 'unstable'. Four [[isotopes of astatine]] were subsequently found to be naturally occurring, although much less than one gram is present at any given time in the Earth's crust. Neither the most stable isotope, astatine-210, nor the medically useful astatine-211 occur naturally; they are usually produced by bombarding [[bismuth]]-209 with [[alpha particle]]s.

== Characteristics ==
Astatine is an extremely radioactive element; all its isotopes have [[half-lives]] of 8.1 hours or less, decaying into other astatine isotopes, [[bismuth]], [[polonium]], or [[radon]]. Most of its isotopes are very unstable, with half-lives of seconds or less. Of the first 101 elements in the periodic table, only [[francium]] is less stable, and all the astatine isotopes more stable than the longest-lived francium isotopes (<sup>205–211</sup>At) are synthetic and do not occur in nature.<ref name="Audi2003">{{NUBASE 2003|mode=cs1}}</ref>

The bulk properties of astatine are not known with any certainty.{{sfn|Greenwood|Earnshaw|2002|p=795}} Research is limited by its short half-life, which prevents the creation of weighable quantities.<ref name="MoreAtIC">{{cite book|title=Holleman-Wiberg: Inorganic Chemistry|publisher=Academic Press|page=423|editor-first1=N.|editor-last1=Wiberg|year=2001|isbn=978-0-12-352651-9|others=Translation of 101st German edition by M. Eagleson and W. D. Brewer, English language editor B. J. Aylett|url=https://books.google.com/books?id=Mtth5g59dEIC}}</ref> A visible piece of astatine would immediately vaporize itself because of the heat generated by its intense radioactivity.<ref name="Emsley" /> It remains to be seen if, with sufficient cooling, a macroscopic quantity of astatine could be deposited as a thin film.<ref name="Hermann" /> Astatine is usually classified as either a [[nonmetal]] or a [[metalloid]];<ref>{{cite book|title=Chemistry & Chemical Reactivity|year=2011|edition=8th|page=65|publisher=Cengage Learning|isbn=978-0-8400-4828-8|last1=Kotz|first1=J. C.|last2=Treichel|first2=P. M.|last3=Townsend|first3=J.}}</ref><ref name="Jahn2010">{{cite book|title=MIPS and Their Role in the Exchange of Metalloids|year=2010|last=Jahn|first=T. P.|page=41|volume=679|publisher=Springer|isbn=978-1-4419-6314-7|url=https://books.google.com/books?id=ayGhnf3A-L0C}}</ref> metal formation has also been predicted.<ref name="Hermann" /><ref>{{cite book|title=Concise Chemistry of the Elements|year=2002|pages=65, 122|publisher=Horwood|isbn=978-1-898563-71-6|last1=Siekierski|first1=S.|last2=Burgess|first2 =J.|url=https://books.google.com/books?id=1I8sByjBONoC&pg=PA3}}</ref>

=== Physical ===
Most of the physical properties of astatine have been estimated (by [[interpolation]] or [[extrapolation]]), using theoretically or empirically derived methods.<ref>{{cite book|title=Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry, Supplement II, Part 1, (F, Cl, Br, I, At)| pages=1064–1079|chapter=Astatine|year=1956|first=A. G.|last=Maddock |publisher = Longmans, Green & Co. (Ltd.)}}</ref> For example, halogens get darker with increasing atomic weight&nbsp;– [[fluorine]] is nearly colorless, [[chlorine]] is yellow-green, [[bromine]] is red-brown, and [[iodine]] is dark gray/violet. Astatine is sometimes described as probably being a black solid (assuming it follows this trend), or as having a metallic appearance (if it is a metalloid or a metal).<ref>{{cite book|url=https://books.google.com/books?id=NKUNAQAAIAAJ|title=Chemistry: A First Course in Modern Chemistry|year=1961|page=313|publisher=Ginn|last1=Garrett|first1=A. B.|last2=Richardson|first2=J. B.|last3=Kiefer|first3=A. S.}}</ref><ref>{{cite book|chapter-url=https://www.britannica.com/EBchecked/topic/603220/transuranium-element/81196/The-first-transactinoid-elements|chapter=Transuranium element|author-link=Glenn T. Seaborg|last=Seaborg|first=G. T.|title=Encyclopædia Britannica|year=2015|access-date=24 February 2015}}</ref><ref>{{cite book|title=Chemistry Expression: An Inquiry Approach|publisher=John Wiley and Sons|page=300|first=H. L.|last=Oon|year=2007|isbn=978-981-271-162-5|url=https://books.google.com/books?id=jd-BR-nfzIoC}}</ref>

Astatine sublimes less readily than iodine, having a lower [[vapor pressure]].<ref name="MoreAtIC" /> Even so, half of a given quantity of astatine will vaporize in approximately an hour if put on a clean glass surface at [[room temperature]].{{efn|This half-vaporization period grows to 16 hours if it is instead put on a gold or platinum surface; this may be caused by poorly understood interactions between astatine and these [[noble metal]]s.{{sfn|Lavrukhina|Pozdnyakov|1970|p=251}}}} The [[absorption spectrum]] of astatine in the [[middle ultraviolet]] region has lines at 224.401 and 216.225&nbsp;nm, suggestive of [[electron configuration|6p to 7s]] [[atomic electron transition|transitions]].<ref>{{cite journal |first1=R.|last1= McLaughlin |year=1964|title= Absorption Spectrum of Astatine |journal= Journal of the Optical Society of America
|volume=54 |issue=8|pages=965–967|doi=10.1364/JOSA.54.000965|url= http://www.escholarship.org/uc/item/3j83k4hx |bibcode= 1964JOSA...54..965M }}</ref>{{sfn|Lavrukhina|Pozdnyakov|1970|p=235}}

The structure of solid astatine is unknown.<ref>{{cite book | title = The Structures of the Elements | year = 1982 | page = 400 | publisher = Robert E. Krieger | isbn = 978-0-89874-230-5 | last = Donohue | first = J. | url=https://books.google.com/books?id=MshQAAAAYAAJ}}</ref> As an analog of iodine it may have an [[orthorhombic crystal system|orthorhombic crystalline structure]] composed of [[diatomic]] astatine molecules, and be a semiconductor (with a [[band gap]] of 0.7&nbsp;[[electronvolt|eV]]).<ref name="Vernon" /><ref>{{Cite journal |last=Batsanov |first=S. S. |date=1972 |title=Quantitative characteristics of bond metallicity in crystals |url= |journal=Journal of Structural Chemistry |language=en |volume=12 |issue=5 |pages=809–813 |doi=10.1007/BF00743349 |bibcode=1972JStCh..12..809B |s2cid=96816296 |issn=0022-4766}}</ref> Alternatively, if condensed astatine forms a metallic phase, as has been predicted, it may have a monatomic [[cubic close packed|face-centered cubic structure]]; in this structure, it may well be a [[superconductor]], like the similar high-pressure phase of iodine.<ref name="Hermann" /> Metallic astatine is expected to have a density of 8.91–8.95&nbsp;g/cm<sup>3</sup>.<ref name="Arblaster" />

Evidence for (or against) the existence of diatomic astatine (At<sub>2</sub>) is sparse and inconclusive.<ref>{{cite journal | language = fr| title = Etude de la formation en phase gazeuse de composés interhalogénés d'astate par thermochromatographie | trans-title = Study of the gas-phase formation of interhalogen compounds of astatine by [[wikt:thermochromatography|thermochromatography]] | last1= Merinis | first1 = J. | last2= Legoux | first2 = G. | last3 = Bouissières | first3 = G. | journal = Radiochemical and Radioanalytical Letters | volume = 11 | pages = 59–64 | year = 1972 | issue = 1 }}</ref><ref>{{cite journal | title = The Mechanism of the Reaction of Elementary Astatine with Organic Solvents | last1= Takahashi | first1 = N. | last2= Otozai | first2 = K. | journal = Journal of Radioanalytical and Nuclear Chemistry | volume = 103 | pages = 1–9 | year = 1986 | issue= 1 | doi =10.1007/BF02165358 | bibcode= 1986JRNC..103....1T | s2cid= 93572282 }}</ref><ref>{{cite conference | title = Chemical Behavior of Astatine Molecules | last1= Takahashi | first1 = N. | last2= Yano | first2 = D. | last3= Baba | first3 = H. | book-title = Proceedings of the International Conference on Evolution in Beam Applications, Takasaki, Japan, 5–8 November 1991 | pages = 536–539| year = 1992 }}</ref>{{sfn|Zuckerman|Hagen|1989|p=21}}{{sfn|Kugler|Keller|1985|pp=110, 116, 210–211, 224}} Some sources state that it does not exist, or at least has never been observed,<ref name="Meyers2002">{{cite encyclopedia | encyclopedia = Encyclopedia of Physical Science and Technology | edition=3rd| title = Halogen Chemistry| year = 2001 | last = Meyers | first = R. A. | pages = 197–222 (202) | isbn = 978-0-12-227410-7 | publisher = Academic Press}}</ref><ref name="Ullmann">{{cite book | last1= Keller | first1 = C.| title = Ullmann's Encyclopedia of Industrial Chemistry| last2= Wolf | first2 = W. | last3 = Shani | first3 = J.| volume = 31 | pages = 89–117 (96) | year = 2011 | doi =10.1002/14356007.o22_o15 | isbn = 978-3-527-30673-2 | chapter= Radionuclides, 2. Radioactive Elements and Artificial Radionuclides}}</ref> while other sources assert or imply its existence.<ref name="boiling_point_chromatography">{{cite journal | title = Estimation Chemical Form Boiling Point Elementary Astatine by Radio Gas Chromatography | last1= Otozai | first1 = K. | last2= Takahashi | first2 = N. | journal = Radiochimica Acta | volume = 31 | pages = 201–203 | year = 1982 |url = http://www.mendeley.com/research/estimation-chemical-form-boiling-point-elementary-astatine-radio-gas-chromatography/ | issue = 3–4 | doi=10.1524/ract.1982.31.34.201| s2cid= 100363889 | url-access = subscription }}</ref><ref>{{cite book | title = Chemistry | edition=8th | year = 2008 | page = 56 | publisher = Cengage Learning | isbn = 978-0-547-12532-9 | last1 = Zumdahl | first1 = S. S. | last2 = Zumdahl | first2 = S. A. | url = https://books.google.com/books?id=LLWkH82PNbYC}}</ref><ref name="Housecroft">{{cite book | title = Inorganic chemistry | edition=3rd| year = 2008 | page = 533 | publisher = Pearson Education | isbn = 978-0-13-175553-6 | last1 = Housecroft | first1 = C. E. | last2 = Sharpe | first2 = A. G.}}</ref> Despite this controversy, many properties of diatomic astatine have been predicted;{{sfn|Kugler|Keller|1985|p=116}} for example, its bond length would be {{val|300|10|u=[[picometer|pm]]}}, dissociation energy <{{val|50|u=kJ/mol}},<ref>{{cite journal | last1=Burgers | first1=Peter C. | last2=Zeneyedpour | first2=Lona | last3=Luider | first3=Theo M. | last4=Holmes | first4=John L. | title=Estimation of thermodynamic and physicochemical properties of the alkali astatides: On the bond strength of molecular astatine (At<sub>2</sub>) and the hydration enthalpy of astatide (At<sup>−</sup>) | journal=Journal of Mass Spectrometry | volume=59 | issue=4 | date=2024 | article-number=e5010 | issn=1076-5174 | doi=10.1002/jms.5010| pmid=38488842 | doi-access=free }}</ref> and [[heat of vaporization]] (∆H<sub>vap</sub>) 54.39&nbsp;kJ/mol.<ref name="Glushko">{{cite book|first1=V. P.|last1=Glushko |first2=V. A.|last2=Medvedev|last3=Bergma|first3=G. A.|year=1966|title=Termicheskie Konstanty Veshchestv|language=ru|volume=1|publisher=Nakua|page=65}}</ref> Many values have been predicted for the [[melting point|melting]] and [[boiling point]]s of astatine, but only for At<sub>2</sub>.{{sfn|Kugler|Keller|1985|pp=116–117}}

=== Chemical ===
The chemistry of astatine is "clouded by the extremely low concentrations at which astatine experiments have been conducted, and the possibility of reactions with impurities, walls and filters, or radioactivity by-products, and other unwanted nano-scale interactions".<ref name="Vernon">{{cite journal|last1=Vernon|first1=R.|year=2013|title=Which Elements are Metalloids?|journal=Journal of Chemical Education|volume=90|issue=12|pages=1703–1707 (1704)|bibcode=2013JChEd..90.1703V|doi=10.1021/ed3008457}}</ref> Many of its apparent chemical properties have been observed using [[radioactive tracer|tracer]] studies on extremely dilute astatine solutions,<ref name="Housecroft" /><ref>{{cite book | title = College chemistry | year = 1960 | page = 457 | publisher = Appleton-Century-Crofts | last1 = Smith | first1 = A. | last2 = Ehret | first2 = W. F.}}</ref> typically less than 10<sup>−10</sup> mol·L<sup>−1</sup>.<ref name="Champion">{{cite journal|last1=Champion|first1=J.|last2=Seydou|first2=M.|last3=Sabatié-Gogova|first3=A.|last4=Renault|first4=E.|last5=Montavon|first5=G.|last6=Galland|first6=N.|year=2011|title=Assessment of an Effective Quasirelativistic Methodology Designed to Study Astatine Chemistry in Aqueous Solution|journal=[[Physical Chemistry Chemical Physics]]|volume=13|issue=33|pages=14984–14992 (14984)|bibcode=2011PCCP...1314984C|doi=10.1039/C1CP20512A|pmid=21769335|url=https://pubs.rsc.org/en/content/articlelanding/2011/CP/c1cp20512a|url-access=subscription}}</ref> Some properties, such as anion formation, align with other halogens.<ref name="MoreAtIC" /> Astatine has some metallic characteristics as well, such as [[electroplating|plating]] onto a [[cathode]],{{efn|It is also possible that this is [[sorption]] on a cathode.<ref>{{cite journal|doi=10.1007/BF02037143|journal=Journal of Radioanalytical and Nuclear Chemistry|volume=83|pages=291–299|title=Chemical Properties of Positive Singly Charged Astatine Ion in Aqueous Solution|year=1984|first1=M.|last1=Milanov|first2=V.|last2=Doberenz|first3=V. A.|last3=Khalkin|first4=A.|last4=Marinov|issue=2|bibcode=1984JRNC...83..291M |s2cid=97361684}}</ref>}} and [[Coprecipitation|coprecipitating]] with metal sulfides in [[hydrochloric acid]].{{sfn|Lavrukhina|Pozdnyakov|1970|p=234}} It forms complexes with [[EDTA]], a metal [[chelating agent]],<ref>{{cite journal |last1=Milesz|first1=S. |last2=Jovchev|first2=M. |last3=Schumann|first3=D. |last4=Khalkin|first4=V. A. |year=1988|title=The EDTA Complexes of Astatine |journal=Journal of Radioanalytical and Nuclear Chemistry |volume=127 |issue=3 |pages=193–198 |doi=10.1007/BF02164864|bibcode=1988JRNC..127..193M |s2cid=93032218 }}</ref> and is capable of acting as a metal in [[antibody]] [[radiolabeling]]; in some respects, astatine in the +1 state is akin to silver in the same state. Most of the organic chemistry of astatine is, however, analogous to that of iodine.<ref name="Guerard">{{cite journal |first1=F.|last1=Guérard|first2=J.-F.|last2=Gestin|first3=M. W.|last3=Brechbiel |year=2013 |title=Production of [<sup>211</sup>At]-Astatinated Radiopharmaceuticals and Applications in Targeted α-Particle Therapy |journal=Cancer Biotherapy and Radiopharmaceuticals |volume=28 |issue=1|pages=1–20 |doi=10.1089/cbr.2012.1292|pmc=3545490 |pmid=23075373}}</ref> It has been suggested that astatine can form a stable [[monatomic cation]] in aqueous solution.{{sfn|Lavrukhina|Pozdnyakov|1970|p=234}}<ref>{{cite journal |last1= Champion |first1= J.|last2=Alliot |first2=C. |last3=Renault |first3=E. |last4=Mokili |first4=B. M. |last5=Chérel |first5=M.| last6=Galland |first6=N. |last7=Montavon |first7=G. | year=2010 |title=Astatine Standard Redox Potentials and Speciation in Acidic Medium |journal=The Journal of Physical Chemistry A |volume=114 |issue=1 |pages=576–582 (581) |doi=10.1021/jp9077008|pmid= 20014840|url=http://hal.in2p3.fr/in2p3-00450771 |bibcode=2010JPCA..114..576C |s2cid= 15738065}}</ref>

Astatine has an [[electronegativity]] of 2.2 on the revised [[Pauling scale]]&nbsp;– lower than that of iodine (2.66) and the same as hydrogen. In [[hydrogen astatide]] (HAt), the negative charge is predicted to be on the hydrogen atom, implying that this compound could be referred to as astatine hydride according to certain nomenclatures.<ref>{{cite journal|last1=Dolg|first1=M.|last2=Kuchle|first2=W.|last3=Stoll|first3=H.|last4=Preuss|first4=H.|last5=Schwerdtfeger|first5=P.|title=''Ab Initio'' Pseudopotentials for Hg to Rn: II. Molecular Calculations on the Hydrides of Hg to At and the Fluorides of Rn|journal=Molecular Physics|volume=74|issue=6|pages=1265–1285 (1265, 1270, 1282)|year=1991|doi=10.1080/00268979100102951|bibcode=1991MolPh..74.1265D}}</ref><ref>{{cite journal|last1=Saue|first1=T.|last2=Faegri|first2=K.|last3=Gropen|first3=O.|title=Relativistic Effects on the Bonding of Heavy and Superheavy Hydrogen Halides|journal=Chemical Physics Letters|volume=263|pages=360–366 (361–362)|year=1996|doi=10.1016/S0009-2614(96)01250-X|issue=3–4|bibcode=1996CPL...263..360S}}</ref><ref name="H-">{{cite book|title=Relativistic Methods for Chemists|first=M.|last=Barysz|page=79|publisher=Springer|year=2010 | isbn = 978-1-4020-9974-8 | url=https://books.google.com/books?id=QbDEC3oL7uAC}}</ref><ref>{{cite journal|last=Thayer|first=J. S.|title=Relativistic Effects and the Chemistry of the Heaviest Main-group elements|journal=Journal of Chemical Education|volume=82|issue=11|pages=1721–1727 (1725)|year=2005|doi=10.1021/ed082p1721|bibcode=2005JChEd..82.1721T}}</ref> That would be consistent with the electronegativity of astatine on the [[Allred–Rochow scale]] (1.9) being less than that of hydrogen (2.2).<ref>{{cite book | title =Inorganic Chemistry| year = 2000 | page = 37 | publisher = University Science Books | isbn = 978-1-891389-01-6 | last1= Wulfsberg | first1=G. }}</ref>{{efn|The algorithm used to generate the Allred-Rochow scale fails in the case of hydrogen, providing a value that is close to that of oxygen (3.5). Hydrogen is instead assigned a value of 2.2. Despite this shortcoming, the Allred-Rochow scale has achieved a relatively high degree of acceptance.<ref>{{cite book | title = Inorganic Substances: A Prelude to the Study of Descriptive Inorganic Chemistry | year = 1990 | page = [https://archive.org/details/inorganicsubstan0000smit/page/135 135] | publisher = Cambridge University Press | last = Smith | first = D. W.| isbn = 978-0-521-33738-0 | url = https://archive.org/details/inorganicsubstan0000smit| url-access = registration }}</ref>}} However, official IUPAC stoichiometric nomenclature is based on an idealized convention of determining the relative electronegativities of the elements by the mere virtue of their position within the periodic table. According to this convention, astatine is handled as though it is more electronegative than hydrogen, irrespective of its true electronegativity. The [[electron affinity]] of astatine, at 233 kJ mol<sup>−1</sup>, is 21% less than that of iodine.<ref>{{cite journal|last1=Leimbach |first1=D.|last2=Sundberg |first2=J.|last3=Yangyang|first3=G.|display-authors=etal|date=February 2020|title=The electron affinity of astatine|journal=Nature Communications|volume=11|issue=1|page=3824|doi=10.1038/s41467-020-17599-2|pmid=32733029|pmc=7393155|arxiv=2002.11418|bibcode=2020NatCo..11.3824L}}</ref> In comparison, the value of Cl (349) is 6.4% higher than F (328); Br (325) is 6.9% less than Cl; and I (295) is 9.2% less than Br. The marked reduction for At was predicted as being due to [[spin–orbit interaction]]s.<ref name="Champion" /> The first [[ionization energy]] of astatine is about 899&nbsp;kJ mol<sup>−1</sup>, which continues the trend of decreasing first ionization energies down the halogen group (fluorine, 1681; chlorine, 1251; bromine, 1140; iodine, 1008).<ref name="1IP" />

== Compounds ==
{{Main|Astatine compounds}}
Less reactive than iodine, astatine is the least reactive of the halogens;<ref name="Anders">{{cite journal | journal = [[Annual Review of Nuclear Science]] | volume = 9 | pages = 203–220 | year = 1959 | doi = 10.1146/annurev.ns.09.120159.001223|doi-access=free | title = Technetium and astatine chemistry | first = E. | last = Anders| bibcode = 1959ARNPS...9..203A }} {{subscription required}}</ref> the chemical properties of tennessine, the next-heavier group 17 element, have not yet been investigated, however.<ref name="notgonnabeahalogen">{{Cite web|author = <!--no author; by design-->|title = Superheavy Element 117 Confirmed – On the Way to the "Island of Stability"|url = https://www.superheavies.de/english/research_program/highlights_element_117.htm#Is%20Element%20117%20a%20Metal|publisher = GSI Helmholtz Centre for Heavy Ion Research|access-date = 2015-07-26|archive-url = https://web.archive.org/web/20180803133710/https://www.superheavies.de/english/research_program/highlights_element_117.htm#Is%20Element%20117%20a%20Metal|archive-date = 2018-08-03}}</ref> Astatine [[chemical compound|compounds]] have been synthesized in nano-scale amounts and studied as intensively as possible before their radioactive disintegration. The reactions involved have been typically tested with dilute solutions of astatine mixed with larger amounts of iodine. Acting as a carrier, the iodine ensures there is sufficient material for laboratory techniques (such as filtration and [[precipitation (chemistry)|precipitation]]) to work.<ref name="Ru1968" /><ref>{{cite journal | journal =Analyst| year = 1952| volume = 77 | pages = 774–777| doi = 10.1039/AN9527700774 | title = Section 5: Radiochemical Methods. Analytical Chemistry of Astatine | first1 = A. H. W. Jr. | last1 = Aten| last2 =Doorgeest | first2 =T. | last3 =Hollstein | first3 =U.| last4 =Moeken | first4 =H. P. | issue =920| bibcode = 1952Ana....77..774A}} {{subscription required}}</ref>{{efn|Iodine can act as a carrier despite it reacting with astatine in water because these reactions require iodide (I<sup>−</sup>), not (only) I<sub>2</sub>.{{sfn|Zuckerman|Hagen|1989|p=31}}{{sfn|Zuckerman|Hagen|1989|p=38}}}} Like iodine, astatine has been shown to adopt odd-numbered oxidation states ranging from −1 to +7.<ref>{{Cite journal |last1=Chatterjee |first1=Sayandev |last2=Czerwinski |first2=Kenneth R. |last3=Fitzgerald |first3=Hilary A. |last4=Lakes |first4=Andrew L. |last5=Liao |first5=Zuolei |last6=Ludwig |first6=Russell C. |last7=McBride |first7=Katie M. |last8=Vlasenko |first8=Vladislav P. |date=2020 |title=Novel Platforms for Drug Delivery Applications |journal=Woodhead Publishing Series in Biomaterials |publisher=[[Woodhead Publishing]] |at=Subchapter 16.4.2: Redox behavior |doi=10.1016/B978-0-323-91376-8.00012-4}}</ref>

Only a few compounds with metals have been reported, in the form of astatides of sodium,<ref name="Emsley" /> [[palladium]], silver, [[thallium]], and lead.{{sfn|Kugler|Keller|1985|pp=213–214}} Some characteristic properties of silver and sodium astatide, and the other hypothetical alkali and alkaline earth astatides, have been estimated by extrapolation from other metal halides.{{sfn|Kugler|Keller|1985|pp=214–218}}

[[File:Hydrogen-astatide-calculated-3D-sf.svg|thumb|left|upright=0.6|[[Hydrogen astatide]]<!--Image captions don't count for overlinking&nbsp;– see [[WP:OVERLINK]] --> [[space-filling model]]]]

The formation of an astatine compound with hydrogen&nbsp;– usually referred to as [[hydrogen astatide]]&nbsp;– was noted by the pioneers of astatine chemistry.{{sfn|Kugler|Keller|1985|p=211}} As mentioned, there are grounds for instead referring to this compound as astatine hydride. It is easily [[oxidized]]; acidification by dilute [[nitric acid]] gives the At<sup>0</sup> or At<sup>+</sup> forms, and the subsequent addition of silver(I) may only partially, at best, precipitate astatine as silver(I) astatide (AgAt). Iodine, in contrast, is not oxidized, and precipitates readily as [[silver(I) iodide]].<ref name="MoreAtIC" />{{sfn|Kugler|Keller|1985|pp=109–110, 129, 213}}

Astatine is known to bind to [[boron]],<ref>{{cite book|title=Contemporary boron chemistry|first=M.|last=Davidson|page=146|year=2000 | publisher = Royal Society of Chemistry| isbn = 978-0-85404-835-9 |url = https://books.google.com/books?id=XWgD4uO1VCMC}}</ref> carbon, and [[nitrogen]].{{sfn|Zuckerman|Hagen|1989|p=276}} Various boron cage compounds have been prepared with At–B bonds, these being more stable than At–C bonds.<ref>{{cite book |last1=Elgqvist|first1=J. |last2=Hultborn|first2=R.|last3=Lindegren|first3=S.|last4=Palm|first4=S.|editor-last=Speer |editor-first=S. |title=Targeted Radionuclide Therapy|publisher=Lippincott Williams & Wilkins|date=2011 |pages=380–396 (383)|chapter=Ovarian cancer: background and clinical perspectives
|isbn=978-0-7817-9693-4}}</ref> Astatine can replace a hydrogen atom in [[benzene]] to form astatobenzene C<sub>6</sub>H<sub>5</sub>At; this may be oxidized to C<sub>6</sub>H<sub>5</sub>AtCl<sub>2</sub> by chlorine. By treating this compound with an [[base (chemistry)|alkaline]] solution of hypochlorite, C<sub>6</sub>H<sub>5</sub>AtO<sub>2</sub> can be produced.{{sfn|Zuckerman|Hagen|1989|pp=190–191}} The dipyridine-astatine(I) cation, [At(C<sub>5</sub>H<sub>5</sub>N)<sub>2</sub>]<sup>+</sup>, forms [[ionic compound]]s with [[perchlorate]]{{sfn|Zuckerman|Hagen|1989|p=276}} (a [[non-coordinating anion]]<ref>{{Cite journal|first1 = M.|last1 = Brookhart|author-link1 = Maurice Brookhart|first2 = B.|last2 = Grant|first3 = A. F.|last3 = Volpe|title = [(3,5-(CF<sub>3</sub>)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>4</sub>B]-[H(OEt<sub>2</sub>)<sub>2</sub>]<sup>+</sup>: a convenient reagent for generation and stabilization of cationic, highly electrophilic organometallic complexes|journal = [[Organometallics]]|year = 1992|volume = 11|issue = 11|pages = 3920–3922|doi = 10.1021/om00059a071}}</ref>) and with [[nitrate]], [At(C<sub>5</sub>H<sub>5</sub>N)<sub>2</sub>]NO<sub>3</sub>.{{sfn|Zuckerman|Hagen|1989|p=276}} This cation exists as a [[coordination complex]] in which two [[dative covalent bond]]s separately link the astatine(I) centre with each of the [[pyridine]] rings via their nitrogen atoms.{{sfn|Zuckerman|Hagen|1989|p=276}}

With oxygen, there is evidence of the species AtO<sup>−</sup> and AtO<sup>+</sup> in aqueous solution, formed by the reaction of astatine with an oxidant such as elemental bromine or (in the last case) by [[sodium persulfate]] in a solution of [[perchloric acid]].<ref name="MoreAtIC" />{{sfn|Kugler|Keller|1985|p=111}} The species previously thought to be {{chem2|AtO2-}} has since been determined to be {{chem2|AtO(OH)2-}}, a hydrolysis product of AtO<sup>+</sup> (another such hydrolysis product being AtOOH).<ref>{{cite journal |last1=Sergentu |first1=Dumitru-Claudiu |last2=Teze |first2=David |first3=Andréa |last3=Sabatié-Gogova |first4=Cyrille |last4=Alliot |first5=Ning |last5=Guo |first6=Fadel |last6=Bassel |first7=Isidro |last7=Da Silva |first8=David |last8=Deniaud |first9=Rémi |last9=Maurice |first10=Julie |last10=Champion |first11=Nicolas |last11=Galland |first12=Gilles |last12=Montavon |title=Advances on the Determination of the Astatine Pourbaix Diagram: Predomination of AtO(OH)<sub>2</sub><sup>−</sup> over At<sup>−</sup> in Basic Conditions |journal=Chem. Eur. J. |year=2016 |volume=22 |issue=9 |pages=2964–71 |doi=10.1002/chem.201504403|pmid=26773333 }}</ref> The well characterized {{chem2|AtO3-}} anion can be obtained by, for example, the oxidation of astatine with potassium hypochlorite in a solution of [[potassium hydroxide]].{{sfn|Zuckerman|Hagen|1989|pp = 190–191}}{{sfn|Kugler|Keller|1985|p=222}} Preparation of [[lanthanum]] triastatate La(AtO<sub>3</sub>)<sub>3</sub>, following the oxidation of astatine by a hot Na<sub>2</sub>S<sub>2</sub>O<sub>8</sub> solution, has been reported.{{sfn|Lavrukhina|Pozdnyakov|1970|p=238}} Further oxidation of {{chem2|AtO3-}}, such as by [[xenon difluoride]] (in a hot alkaline solution) or [[periodate]] (in a [[neutral solution|neutral]] or alkaline solution), yields the perastatate ion {{chem2|AtO4-}}; this is only stable in neutral or alkaline solutions.{{sfn|Kugler|Keller|1985|pp=112, 192–193}} Astatine is also thought to be capable of forming cations in salts with oxyanions such as [[iodate]] or [[dichromate]]; this is based on the observation that, in acidic solutions, monovalent or intermediate positive states of astatine coprecipitate with the insoluble salts of metal cations such as silver(I) iodate or thallium(I) dichromate.{{sfn|Zuckerman|Hagen|1989|pp=190–191}}{{sfn|Kugler|Keller|1985|p=219}}

Astatine may form bonds to the other [[chalcogen]]s; these include S<sub>7</sub>At<sup>+</sup> and {{chem2|At(CSN)2-}} with [[sulfur]], a coordination [[selenourea]] compound with [[selenium]], and an astatine–[[tellurium]] [[colloid]] with tellurium.{{sfn|Zuckerman|Hagen|1989|pp=192–193}}

[[File:Astatine-iodide-3D-vdW.svg|thumb|upright=0.9|Structure of [[astatine iodide]], one of the astatine [[interhalogen]]s and the heaviest known diatomic interhalogen]]

Astatine is known to react with its lighter homologs iodine, [[bromine]], and [[chlorine]] in the vapor state; these reactions produce diatomic [[interhalogen compound]]s with formulas AtI, AtBr, and AtCl.{{sfn|Zuckerman|Hagen|1989|p=31}} The first two compounds may also be produced in water&nbsp;– astatine reacts with iodine/[[iodide]] solution to form AtI, whereas AtBr requires (aside from astatine) an iodine/[[iodine monobromide]]/[[bromide]] solution. The excess of iodides or bromides may lead to {{chem2|AtBr2-}} and {{chem2|AtI2-}} ions,{{sfn|Zuckerman|Hagen|1989|p=31}} or in a chloride solution, they may produce species like {{chem2|AtCl2-}} or {{chem2|AtBrCl-}} via equilibrium reactions with the chlorides.{{sfn|Zuckerman|Hagen|1989|p=38}} Oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule likely to be either AtCl or AtOCl. Similarly, {{chem2|AtOCl2-}} or {{chem2|AtCl2-}} may be produced.{{sfn|Zuckerman|Hagen|1989|p=31}} The polyhalides PdAtI<sub>2</sub>, CsAtI<sub>2</sub>, TlAtI<sub>2</sub>,{{sfn|Zuckerman|Hagen|1990|p=212}}<ref>{{cite journal |first1=G. A.|last1=Brinkman |first2=H. W.|last2=Aten|year=1963 |title=Decomposition of Caesium Diiodo Astatate (I), (CsAtI<sub>2</sub>) |journal=Radiochimica Acta |volume=2 |issue=1|page=48 |doi=10.1524/ract.1963.2.1.48|s2cid=99398848 }}</ref>{{sfn|Zuckerman|Hagen|1990|p=60}} and PbAtI{{sfn|Zuckerman|Hagen|1989|p=426}} are known or presumed to have been precipitated. In a plasma ion source [[mass spectrometer]], the ions [AtI]<sup>+</sup>, [AtBr]<sup>+</sup>, and [AtCl]<sup>+</sup> have been formed by introducing lighter halogen vapors into a [[helium]]-filled cell containing astatine, supporting the existence of stable neutral molecules in the plasma ion state.{{sfn|Zuckerman|Hagen|1989|p=31}} No astatine fluorides<!--AtF, AtF5, or whatever else, "no" means "no" (not AtF, not AtF5, not At45F387, or whatever)--> have been discovered yet. Their absence has been speculatively attributed to the extreme reactivity of such compounds, including the reaction of an initially formed fluoride with the walls of the glass container to form a non-volatile product.{{efn|An initial attempt to fluoridate astatine using chlorine trifluoride resulted in formation of a product which became stuck to the glass. Chlorine monofluoride, chlorine, and tetrafluorosilane were formed. The authors called the effect "puzzling", admitting they had expected formation of a volatile fluoride.<ref>{{cite journal |first1=E. H.|last1=Appelman|first2=E. N.|last2=Sloth|first3=M. H. |last3=Studier |year=1966 |title=Observation of Astatine Compounds by Time-of-Flight Mass Spectrometry |journal=Inorganic Chemistry |volume=5 |issue = 5|pages=766–769 |doi=10.1021/ic50039a016}}</ref> Ten years later, the compound was predicted to be non-volatile, out of line with the lighter halogens but similar to [[radon fluoride]];<ref>{{cite journal |first1=K. S.|last1=Pitzer|year=1975 |title=Fluorides of Radon and Element 118 |journal=Journal of the Chemical Society, Chemical Communications |volume=5 |issue = 18|pages=760b–761 |doi=10.1039/C3975000760B|url=https://escholarship.org/uc/item/8xz4g1ff}}</ref> by this time, the latter had been shown to be ionic.<ref>{{cite encyclopedia | last1 = Bartlett | first1 = N. | last2 = Sladky | first2 = F. O. | editor1-first = J. C. | editor1-last = Bailar | editor2-first= H. J. | editor2-last = Emeléus | editor3-first = R. | editor3-last = Nyholm | editor4-first = A. F. | editor4-last = Trotman-Dickenson | display-editors=3| encyclopedia =Comprehensive Inorganic Chemistry | title = The Chemistry of Krypton, Xenon and Radon | year = 1973 | publisher = Pergamon | volume = 1 | isbn = 978-0-08-017275-0 | pages =213–330}}</ref>}} Thus, although the synthesis of an astatine fluoride is thought to be possible, it may require a liquid halogen fluoride solvent, as has already been used for the characterization of radon fluoride.{{sfn|Zuckerman|Hagen|1989|p=31}}{{sfn|Kugler|Keller|1985|pp=112, 192–193}}

== History ==
{{image frame|width=300|align=left|content={{Superimpose
  | base = Periodic table by Mendeleev, 1871.svg
  | base_class = skin-invert-image
  | base_width = 300px
  | base_alt = Periodic table by Mendeleev (1871), with astatine missing below chlorine, bromine and iodine ("J")
  | base_caption = Mendeleev's table of 1871, with an empty space at the eka-iodine position
  | base_link = Periodic_table#History
  | float = Cercle rouge 100%.svg
  | float_alt =
  | float_width = 22px
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  | x = 226 | y =104}}
|caption={{longitem|[[Dmitri Mendeleev]]'s table of 1871, with an empty space at the eka-iodine position}}
}}
In 1869, when [[Dmitri Mendeleev]] published his [[Periodic table#Mendeleev|periodic table]], the space under iodine was empty; after [[Niels Bohr]] established the physical basis of the classification of chemical elements, it was suggested that the fifth halogen belonged there. Before its officially recognized discovery, it was called "eka-iodine" (from [[Sanskrit]] {{tlit|sa|eka}} 'one') to imply it was one space under iodine (in the same manner as [[Mendeleev's predicted elements|eka-silicon, eka-boron, and others]]).<ref>{{cite book|last=Ball|first=P.|title=The Ingredients: A Guided Tour of the Elements|publisher=Oxford University Press|year=2002|pages=100–102 | isbn = 978-0-19-284100-1 }}</ref> Scientists tried to find it in nature; given its extreme rarity, these attempts resulted in several false discoveries.{{sfn|Lavrukhina|Pozdnyakov|1970|pp=227–228}}

The first claimed discovery of eka-iodine was made by [[Fred Allison]] and his associates at the Alabama Polytechnic Institute (now [[Auburn University]]) in 1931. The discoverers named element 85 "alabamine", and assigned it the symbol Ab, designations that were used for a few years.<ref>{{cite journal | title = Evidence of the Detection of Element 85 in Certain Substances| first1 = F. | last1 =Allison | author-link1=Fred Allison| first2 = E. J. | last2 =Murphy| first3 = E. R. | last3 =Bishop| first4 = A. L. | last4 =Sommer | journal = Physical Review | volume = 37 | pages = 1178–1180 | year = 1931 | doi = 10.1103/PhysRev.37.1178 | issue = 9|bibcode = 1931PhRv...37.1178A }} {{subscription required}}</ref><ref>{{cite magazine | url = http://www.time.com/time/magazine/article/0,9171,743159,00.html | archive-url = https://web.archive.org/web/20070930015028/http://www.time.com/time/magazine/article/0,9171,743159,00.html | archive-date = 30 September 2007 | title = Alabamine & Virginium |magazine=[[Time (magazine)|Time]] | date=15 February 1932}}</ref><ref>{{cite journal | title = What Happened to Alabamine, Virginium, and Illinium? | last= Trimble | first = R. F. | journal = Journal of Chemical Education | volume = 52 | page = 585 | year = 1975 | doi =10.1021/ed052p585 | issue = 9|bibcode = 1975JChEd..52..585T }} {{subscription required}}</ref> In 1934, [[Herbert G. MacPherson|H. G. MacPherson]] of [[University of California, Berkeley]] disproved Allison's method and the validity of his discovery.<ref>{{cite journal| last = MacPherson| first = H. G.| title = An Investigation of the Magneto-optic Method of Chemical Analysis| journal = Physical Review| volume = 47| issue = 4| pages = 310–315|year=1934|doi = 10.1103/PhysRev.47.310|bibcode = 1935PhRv...47..310M }}</ref> There was another claim in 1937, by the chemist Rajendralal De. Working in Dacca in [[British India]] (now [[Dhaka]] in [[Bangladesh]]), he chose the name "dakin" for element 85, which he claimed to have isolated as the [[thorium series]] equivalent of [[Radium#History|radium F]] (polonium-210) in the [[radium series]].<ref name="Mellor" /> The properties he reported for dakin do not correspond to those of astatine,<ref name="Mellor" /> and astatine's radioactivity would have prevented him from handling it in the quantities he claimed.<ref name="findingekaiodine" /> Moreover, astatine is not found in the thorium series, and the true identity of dakin is not known.<ref name=Mellor>{{cite book | first=J. W. | last=Mellor | author-link=Joseph William Mellor | title=A Comprehensive Treatise on Inorganic and Theoretical Chemistry | publisher=Longmans, Green | year=1965 | oclc=13842122 | page=[https://archive.org/details/comprehensivetre00mellrich/page/n1081 1066] | url = https://archive.org/details/comprehensivetre00mellrich }}</ref>

In 1936, the team of Romanian physicist [[Horia Hulubei]] and French physicist [[Yvette Cauchois]] claimed to have discovered element 85 by observing its X-ray emission lines. In 1939, they published another paper which supported and extended previous data. In 1944, Hulubei published a summary of data he had obtained up to that time, claiming it was supported by the work of other researchers. He chose the name "dor", presumably from the Romanian for "longing" [for peace], as [[World War II]] had started five years earlier. As Hulubei was writing in French, a language which does not accommodate the "-ine" suffix, dor would likely have been rendered in English as "dorine", had it been adopted. In 1947, Hulubei's claim was effectively rejected by the Austrian chemist [[Friedrich Paneth]], who would later chair the [[IUPAC]] committee responsible for recognition of new elements. Even though Hulubei's samples did contain astatine-218, his means to detect it were too weak, by current standards, to enable correct identification; moreover, he could not perform chemical tests on the element.<ref name=findingekaiodine>{{cite journal|title= Finding Eka-Iodine: Discovery Priority in Modern Times|first1=S. C.|last1= Burdette|first2= B. F.|last2= Thornton|journal=Bulletin for the History of Chemistry |year=2010 |volume=35 |issue=2 |pages=86–96|doi=10.70359/bhc2010v035p086 |url=http://www.scs.illinois.edu/~mainzv/HIST/bulletin_open_access/v35-2/v35-2%20p86-96.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.scs.illinois.edu/~mainzv/HIST/bulletin_open_access/v35-2/v35-2%20p86-96.pdf |archive-date=2022-10-09 |url-status=live}}</ref> He had also been involved in an earlier [[Francium#Erroneous and incomplete discoveries|false claim]] as to the discovery of element 87 (francium) and this is thought to have caused other researchers to downplay his work.<ref>{{cite book| title = A Tale of 7 Elements| year = 2013| edition = Google Play| pages = [https://archive.org/details/taleofseveneleme0000scer/page/188 188–190, 206]| first1 = E.| last1 = Scerri| publisher = Oxford University Press| isbn = 978-0-19-539131-2| url = https://archive.org/details/taleofseveneleme0000scer/page/188}}</ref>
[[File:Segre.jpg|thumb|upright|alt=A greyscale photo of the upper body of a man|[[Emilio Segrè]], one of the discoverers of the [[main-group element]] astatine]]

In 1940, the Swiss chemist [[Walter Minder]] announced the discovery of element 85 as the beta [[decay product]] of radium A (polonium-218), choosing the name "helvetium" (from {{lang|la|[[Helvetia#Name of Switzerland|Helvetia]]}}, the Latin name of Switzerland). [[Berta Karlik]] and Traude Bernert were unsuccessful in reproducing his experiments, and subsequently attributed Minder's results to contamination of his radon stream ([[radon-222]] is the parent isotope of polonium-218).<ref>{{cite journal | doi = 10.1007/BF01487965 | language= de | title = Über Eine Vermutete β-Strahlung des Radium A und die Natürliche Existenz des Elementes 85 | trans-title = About a Suspected β-radiation of Radium A, and the Natural Existence of the Element 85 | year = 1942 | last1 = Karlik | first1 = B.| author-link=Berta Karlik | journal = Naturwissenschaften | volume = 30 | pages = 685–686 | last2 = Bernert | first2 = T.| issue = 44–45|bibcode = 1942NW.....30..685K | s2cid= 6667655 }} {{subscription required}}</ref>{{efn|In other words, some other substance was undergoing beta decay (to a different end element), not polonium-218.}} In 1942, Minder, in collaboration with the English scientist [[Alice Leigh-Smith]], announced the discovery of another isotope of element 85, presumed to be the product of [[Thorium#History|thorium A]] (polonium-216) beta decay. They named this substance "anglo-helvetium",<ref>{{cite journal | title = Experimental Evidence of the Existence of Element 85 in the Thorium Family | author-link1 = Alice Leigh-Smith | first1 = A.| last1 = Leigh-Smith | author-link2 = Walter Minder | first2 = W.| last2 = Minder | journal = Nature | volume = 150 | pages = 767–768 | year = 1942 | doi = 10.1038/150767a0 | issue = 3817|bibcode = 1942Natur.150..767L | s2cid = 4121704 }} {{subscription required}}</ref> but Karlik and Bernert were again unable to reproduce these results.<ref name="Ru1968">{{cite journal | doi = 10.1070/RC1968v037n02ABEH001603 | title = Astatine | year = 1968 | last1= Nefedov | first1 = V. D. | journal = Russian Chemical Reviews | volume = 37 | pages = 87–98 | last2 = Norseev | first2 = Yu. V. | last3 = Toropova | first3 = M. A. | last4 = Khalkin | first4 = Vladimir A. | issue = 2|bibcode = 1968RuCRv..37...87N | s2cid = 250775410 }} {{subscription required}}</ref>

Later in 1940, [[Dale R. Corson]], [[Kenneth Ross MacKenzie]], and [[Emilio Segrè]] isolated the element at the University of California, Berkeley. Instead of searching for the element in nature, the scientists created it by bombarding [[bismuth-209]] with [[alpha particle]]s in a [[cyclotron]] (particle accelerator) to produce, after emission of two neutrons, astatine-211.{{sfn|Corson|MacKenzie|Segrè|1940}} The discoverers, however, did not immediately suggest a name for the element. The reason for this was that at the time, an element created synthetically in "invisible quantities" that had not yet been discovered in nature was not seen as a completely valid one; in addition, chemists were reluctant to recognize radioactive isotopes as legitimately as stable ones.<ref name="Davis-1959">{{Cite book|url=http://nova.wpunj.edu/pardir/Book%20Reviews/The_Chemical_Elements.pdf|title=The Chemical Elements|last=Davis|first=Helen Miles|publisher=[[Science Service]], [[Ballantine Books]]|year=1959|edition=2nd|page=29|access-date=14 August 2016|archive-url=https://web.archive.org/web/20170823203118/http://nova.wpunj.edu/pardir/Book%20Reviews/The_Chemical_Elements.pdf|archive-date=23 August 2017}}</ref> In 1943, astatine was found as a product of two naturally occurring [[decay chain]]s by [[Berta Karlik]] and Traude Bernert, first in the so-called [[uranium series]], and then in the [[actinium series]].<ref>{{cite journal | language = de | title = Eine Neue Natürliche α-Strahlung | trans-title = A New Natural α-radiation | last1 = Karlik | first1 = B.| pages = 298–299 | volume = 31 | issue = 25–26 | year = 1943 | journal = Naturwissenschaften | doi = 10.1007/BF01475613 | last2 = Bernert | first2 = T.|bibcode = 1943NW.....31..298K | s2cid = 38193384 }} {{subscription required}}</ref><ref>{{cite journal | language = de | title = Das Element 85 in den Natürlichen Zerfallsreihen | trans-title = The Element 85 in the Natural Decay Chains |journal = Zeitschrift für Physik | volume = 123 | issue = 1–2 | year = 1943 | doi = 10.1007/BF01375144 | pages = 51–72 | last1 = Karlik | first1 = B.| last2 = Bernert | first2 = T.|bibcode = 1944ZPhy..123...51K | s2cid = 123906708 }} {{subscription required}}</ref> (Since then, astatine was also found in a third decay chain, the [[neptunium series]].<ref>{{cite book |first1=C. M.|last1=Lederer|first2=J. M.|last2=Hollander|first3=I.|last3=Perlman |year=1967 |title=Table of Isotopes |edition=6th |pages=1–657|publisher=[[John Wiley & Sons]]}}</ref>) Friedrich Paneth in 1946 called to finally recognize synthetic elements, quoting, among other reasons, recent confirmation of their natural occurrence, and proposed that the discoverers of the newly discovered unnamed elements name these elements. In early 1947, ''[[Nature (journal)|Nature]]'' published the discoverers' suggestions; a letter from Corson, MacKenzie, and Segrè suggested the name "astatine"<ref name="Davis-1959" /> coming from the [[Ancient Greek]] {{Transliteration|grc|ástatos}} ({{lang|grc|ἄστατος}}) meaning {{gloss|unstable}}, because of its propensity for [[radioactive decay]], with the ending "-ine", found in the names of the four previously discovered halogens. The name was also chosen to continue the tradition of the four stable halogens, where the name referred to a property of the element.<ref name="Corson2003" />

Corson and his colleagues classified astatine as a metal on the basis of its [[analytical chemistry]].{{sfn|Corson|MacKenzie|Segrè|1940|pp=672, 677}} Subsequent investigators reported iodine-like,<ref>{{cite journal |last1=Hamilton |first1=J. G. |last2=Soley|first2=M. H. |year=1940 |title=A Comparison of the Metabolism of Iodine and of Element 85 (Eka-Iodine)|journal= Proceedings of the National Academy of Sciences|volume=26 |issue=8 |pages=483–489|bibcode = 1940PNAS...26..483H |doi = 10.1073/pnas.26.8.483 |pmc=1078214 |pmid=16588388|doi-access=free }}</ref><ref>{{cite journal |last1=Neumann|first1=H. M.| year=1957| title=Solvent Distribution Studies of the Chemistry of Astatine|journal= Journal of Inorganic and Nuclear Chemistry|volume=4 |issue=5–6 |pages=349–353 |doi=10.1016/0022-1902(57)80018-9 }}</ref> cationic,<ref>{{cite journal |last1=Johnson|first1=G. L. |last2=Leininger |first2=R. F. |last3=Segrè |first3=E. |s2cid=95324453 |year=1949 |title=Chemical Properties of Astatine. I |journal=Journal of Chemical Physics |volume=17 |issue=1 |pages=1–10|doi=10.1063/1.1747034 |bibcode = 1949JChPh..17....1J |hdl=2027/mdp.39015086446914 |hdl-access=free }}</ref><ref>{{cite journal |last1=Dreyer|first1=I. |last2=Dreyer|first2=R. |last3=Chalkin|first3=V. A. |year=1979 |title=Cations of Astatine in Aqueous Solutions; Production and some Characteristics |journal=Radiochemical and Radioanalytical Letters |language=de|volume=36 |issue=6|pages=389–398}}</ref> or [[amphoteric]] behavior.<ref name="Aten">{{cite book |last1=Aten| first1=A. H. W. Jr. |year=1964 |title=The Chemistry of Astatine |volume=6 |pages=207–223 | doi=10.1016/S0065-2792(08)60227-7|series=Advances in Inorganic Chemistry and Radiochemistry|isbn=978-0-12-023606-0}}</ref><ref name="Ru1968"/> In a 2003 retrospective, Corson wrote that "some of the properties [of astatine] are similar to iodine&nbsp;... it also exhibits metallic properties, more like its metallic neighbors Po and Bi."<ref name="Corson2003">{{cite journal |last1=Corson |first1=D. R. |year=2003 |title=Astatine |journal=Chemical & Engineering News |volume=81 |issue=36 |page=158 |url=http://pubs.acs.org/cen/80th/print/astatine.html|doi=10.1021/cen-v081n036.p158 |url-access=subscription }}</ref>

== Isotopes ==
{{Main|Isotopes of astatine}}
<div style="float:left; margin:0 1.4em 0 0">
{| class="wikitable sortable" style="font-size:85%; text-align:right"
|+ Alpha decay characteristics for sample astatine isotopes{{efn|In the table, "alpha decay half-life" refers to the half-life if decay modes other than alpha are omitted.}}
|-
! Mass<br />number
! Half-life<ref name="Audi2003" />
! Probability<br />of alpha<br />decay<ref name="Audi2003" />
! Alpha-<br />decay<br />half-life
|-
! style="background:lightyellow;"| 207
| {{val|1.80|u=h}}
| {{nts|8.6}}%
| style="background:lightyellow;"| {{val|20.9|u=h}}
|-
! style="background:lightyellow;"| 208
| {{val|1.63|u=h}}
| {{nts|0.55}}%
| style="background:lightyellow;"| {{val|12.3|u=d}}
|-
! style="background:lightyellow;"| 209
| {{val|5.41|u=h}}
| {{nts|4.1}}%
| style="background:lightyellow;"| {{val|5.5|u=d}}
|-
! style="background:lightyellow;"| 210
| {{val|8.1|u=h}}
| {{nts|0.175}}%
| style="background:lightyellow;"| {{val|193|u=d}}
|-
! style="background:lightyellow;"| 211
| {{val|7.21|u=h}}
| {{nts|41.8}}%
| style="background:lightyellow;"| {{val|17.2|u=h}}
|-
! style="background:lightyellow;"| 212
| {{val|0.31|u=s}}
| {{ntsh|99.99}}≈100%
| style="background:lightyellow;"| {{val|0.31|u=s}}
|-
! style="background:lightyellow;"| 213
| {{val|125|u=ns}}
| {{nts|100}}%<!-- {{efn|The alpha probability peak at astatine-213 is because alpha decay produces an isotope that has an unusually stable number of neutrons, 126.}} -->
| style="background:lightyellow;"| {{val|125|u=ns}}
|-
! style="background:lightyellow;"| 214
| {{val|558|u=ns}}
| {{nts|100}}%
| style="background:lightyellow;"| {{val|558|u=ns}}
|-
! style="background:lightyellow;"| 219
| {{val|56|u=s}}
| {{nts|97}}%
| style="background:lightyellow;"| {{val|58|u=s}}
|-
! style="background:lightyellow;"| 220
| {{val|3.71|u=min}}
| {{nts|8}}%
| style="background:lightyellow;"| {{val|46.4|u=min}}
|-
! style="background:lightyellow;"| 221
| {{val|2.3|u=min}}
| {{ntsh|0}}experimentally<br />alpha-stable
| style="background:lightyellow;"| {{ntsh|1e900}}∞
|}</div>

There are 41 known [[isotope]]s of astatine, with [[mass number]]s of 188 and 190–229.<ref name="Fry" /><ref name=At188190>{{cite web |last1=Kokkonen |first1=Henna |title=Decay properties of the new isotopes 188At and 190At |url=https://jyx.jyu.fi/bitstream/handle/123456789/87126/URN%3ANBN%3Afi%3Ajyu-202305243198.pdf?sequence=1 |publisher=University of Jyväskylä |access-date=8 June 2023}}</ref> Theoretical modeling suggests that about 37 more isotopes could exist.<ref name="Fry">{{cite journal |first1=C.|last1=Fry|first2=M.|last2=Thoennessen|year=2013 |title=Discovery of the astatine, radon, francium, and radium isotopes |journal=[[Atomic Data and Nuclear Data Tables]] |volume=09 |issue=5|pages=497–519 |doi=10.1016/j.adt.2012.05.003|bibcode = 2013ADNDT..99..497F |arxiv=1205.5841|s2cid=12590893}}</ref> No stable or long-lived astatine isotope has been observed, nor is one expected to exist.{{sfn|Lavrukhina|Pozdnyakov|1970|p=229}}

Astatine's [[alpha decay]] energies follow the same trend as for other heavy elements.{{sfn|Lavrukhina|Pozdnyakov|1970|p=229}} Lighter astatine isotopes have quite [[Alpha particle#Energy and absorption|high energies]] of alpha decay, which become lower as the nuclei become heavier. Astatine-211 has a significantly higher energy than the previous isotope, because it has a nucleus with 126 neutrons, and 126 is a [[magic number (physics)|magic number]] corresponding to a filled neutron shell. Despite having a similar half-life to the previous isotope (8.1&nbsp;hours for astatine-210 and 7.2&nbsp;hours for astatine-211), the alpha decay probability is much higher for the latter: 41.81% against only 0.18%.<ref name="Audi2003" />{{efn|This means that, if decay modes other than alpha are omitted, then astatine-210 has an alpha decay half-life of 4,628.6&nbsp;hours (128.9&nbsp;days) and astatine-211 has one of only 17.2 hours (0.7&nbsp;days). Therefore, astatine-211 is very much less stable toward alpha decay than astatine-210.}} The two following isotopes release even more energy, with astatine-213 releasing the most energy. For this reason, it is the shortest-lived astatine isotope.{{sfn|Lavrukhina|Pozdnyakov|1970|p=229}} Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists, because of the increasing role of [[beta decay]] (electron emission).{{sfn|Lavrukhina|Pozdnyakov|1970|p=229}} This decay mode is especially important for astatine; as early as 1950 it was postulated that all isotopes of the element undergo beta decay,<ref>{{cite book|title=Isotope Geology|edition=2nd|publisher=Pergamon Press|page=403|year=1956 | isbn = 978-0-470-70800-2 |first=K.|last=Rankama}}</ref> though nuclear mass measurements indicate that <sup>215</sup>At is in fact [[beta-stable]], as it has the lowest mass of all [[isobar (nuclide)|isobars]] with ''A''&nbsp;=&nbsp;215.<ref name="Audi2003" /> Astatine-210 and most of the lighter isotopes exhibit beta plus decay ([[positron emission]]), astatine-217 and heavier isotopes except astatine-218 exhibit beta minus decay, while astatine-211 undergoes [[electron capture]].{{NUBASE2020|ref}}

The most stable isotope is astatine-210, which has a half-life of 8.1&nbsp;hours. The primary decay mode is beta plus, to the relatively long-lived (in comparison to astatine isotopes) alpha emitter [[polonium-210]]. In total, only five isotopes have half-lives exceeding one hour (astatine-207 to -211). The least stable ground state isotope is astatine-213, with a half-life of 125 nanoseconds. It undergoes alpha decay to the extremely long-lived [[bismuth-209]].<ref name="Audi2003" />

Astatine has 24 known [[nuclear isomer]]s, which are nuclei with one or more [[nucleon]]s ([[proton]]s or [[neutron]]s) in an [[excited state]]. A nuclear isomer may also be called a "[[metastability|meta]]-state", meaning the system has more [[internal energy]] than the "[[ground state]]" (the state with the lowest possible internal energy), making the former likely to decay into the latter. There may be more than one isomer for each isotope. The most stable of these nuclear isomers is astatine-202m1,{{efn|"m1" means that this state of the isotope is the next possible one above&nbsp;– with an energy greater than&nbsp;– the ground state. "m2" and similar designations refer to further higher energy states. The number may be dropped if there is only one well-established meta state, such as astatine-216m. Other designation techniques are sometimes used.}} which has a half-life of about 3 minutes, longer than those of all the ground states bar those of isotopes 203–211 and 220. The least stable is astatine-213m1; its half-life of 110 [[nanosecond]]s is shorter than 125 nanoseconds for astatine-213, the shortest-lived ground state.{{NUBASE2020|ref}}

== Natural occurrence ==
[[File:Decay Chain(4n+1, Neptunium Series).svg|thumb|upright=0.7|alt=a sequence of differently colored balls, each containing a two-letter symbol and some numbers|Neptunium series, showing the decay products, including astatine-217, formed from neptunium-237]]

Astatine is the rarest naturally occurring element.{{efn|Emsley<ref name="Emsley">{{cite book|last=Emsley|first=J.|title=Nature's Building Blocks: An A-Z Guide to the Elements|edition=New|year=2011|publisher=Oxford University Press | isbn = 978-0-19-960563-7 |pages=57–58}}</ref> states that this title has been lost to [[berkelium]], "a few atoms of which can be produced in very-highly concentrated uranium-bearing deposits"; however, his assertion is not corroborated by any primary source.}} The total amount of astatine in the Earth's crust (quoted mass 2.36 × 10<sup>25</sup> grams)<ref>{{cite book | title = CRC Handbook of Chemistry and Physics | edition = 85th | year = 2004 | pages = [https://archive.org/details/crchandbookofche81lide/page/14 14–10] | publisher = CRC Press | isbn = 978-0-8493-0485-9 | editor-first = D. R. | editor-last = Lide | url = https://archive.org/details/crchandbookofche81lide/page/14 }}</ref> is estimated by some to be less than one gram at any given time.<ref name="MoreAtIC" /> Other sources estimate the amount of ephemeral astatine, present on earth at any given moment, to be up to one ounce<ref name="Stwertka" /> (about 28 grams).

Any astatine present at the formation of the Earth has long since disappeared; the four naturally occurring isotopes (astatine-215, -217, -218 and -219){{sfn|Lavrukhina|Pozdnyakov|1970|p=228–229}} are instead continuously produced as a result of the [[decay chain|decay]] of radioactive [[thorium]] and [[uranium]] ores, and trace quantities of [[neptunium-237]]. The landmass of North and South America combined, to a depth of 16 kilometers (10 miles), contains only about one trillion astatine-215 atoms at any given time (around 3.5 × 10<sup>−10</sup> grams).<ref>{{cite book| url = https://books.google.com/books?id=1RMLAAAAMAAJ | title = Only a Trillion| first = I.| last = Asimov | author-link = Isaac Asimov | publisher = Abelard-Schuman | year = 1957 | page = 24}}</ref> Astatine-217 is produced via the radioactive decay of neptunium-237. Primordial remnants of the latter isotope—due to its relatively short half-life of 2.14 million years—are no longer present on Earth. However, trace amounts occur naturally as a product of transmutation reactions in [[uranium ore]]s.<ref>{{cite encyclopedia |editor-last1=Kolthoff |editor-first1= I. M.|editor-last2=Elving |editor-first2=P. J. |title=Treatise on Analytical Chemistry |date=1964 |publisher=Interscience Encyclopedia |series=Part II: Analytical Chemistry of the Elements|volume=4 |location=New York |page=487}}</ref> Astatine-218 was the first astatine isotope discovered in nature.{{sfn|Kugler|Keller|1985|p=4}} Astatine-219, with a half-life of 56 seconds, is the longest lived of the naturally occurring isotopes.<ref name="Audi2003" />

Isotopes of astatine are sometimes not listed as naturally occurring because of misconceptions<ref name="Aten" /> that there are no such isotopes,<ref>{{cite journal|last1=Maiti|first1=M.|last2=Lahiri|first2=S.|year=2011|title=Production cross section of At radionuclides from <sup>7</sup>Li+<sup>nat</sup>Pb and <sup>9</sup>Be+<sup>nat</sup>Tl reactions|journal=Physical Review C|volume=84|issue=6|pages=07601–07604 (07601)|arxiv=1109.6413|bibcode=2011PhRvC..84f7601M|doi=10.1103/PhysRevC.84.067601|s2cid=115321713}}</ref> or discrepancies in the literature. Astatine-216 has been counted as a naturally occurring isotope but reports of its observation{{sfn|Greenwood|Earnshaw|2002|p=796}} (which were described as doubtful) have not been confirmed.{{sfn|Kugler|Keller|1985|p=5}}

== Synthesis ==

=== Formation ===
<div style="float: left; margin: 0 1em 0 0; font-size: 90%;">
{| class="wikitable"
|+ Possible reactions after bombarding bismuth-209 with alpha particles
!rowspan=2| Reaction{{efn|A nuclide is commonly denoted by a symbol of the chemical element this nuclide belongs to, <!--if someone can come up with a better wording, please do, or, if you think it's okay, just remove this message--> preceded by a non-spaced superscript mass number and a subscript atomic number of the nuclide located directly under the mass number. (Neutrons may be considered as nuclei with the atomic mass of 1 and the atomic charge of 0, with the symbol being n.) With the atomic number omitted, it is also sometimes used as a designation of an isotope of an element in isotope-related chemistry.}}
!colspan=2| Energy of alpha particle
|-
! Threshold energy
! Maximum production
|-
| style="text-align:center;"| {{chem|209|83|Bi}} + {{chem|4|2|He}} → {{chem|211|85|At}} + 2 {{chem|1|0}}n
| style="text-align:center;"| 20.7&nbsp;MeV<ref name="Hermanne"/>
| style="text-align:center;"| 30<ref name="Gyehong"/>–31&nbsp;MeV<ref name="Hermanne"/>
|-
| style="text-align:center;"| {{chem|209|83|Bi}} + {{chem|4|2|He}} → {{chem|210|85|At}} + 3 {{chem|1|0}}n
| style="text-align:center;"| 28.4<ref name="zalutsky"/>–28.6&nbsp;MeV<ref name="Hermanne"/><ref name="Maiti"/>
| style="text-align:center;"| 37&nbsp;MeV<ref name="Hermanne">{{cite journal | last1=Hermanne | first1=A. | last2=Tárkányi | first2=F. | last3=Takács | first3=S. | last4=Szücs | first4=Z. | last5=Shubin | first5=Yu.N. | last6=Dityuk | first6=A.I. | title=Experimental study of the cross-sections of α-particle induced reactions on 209Bi | journal=Applied Radiation and Isotopes | publisher=Elsevier BV | volume=63 | issue=1 | year=2005 | issn=0969-8043 | doi=10.1016/j.apradiso.2005.01.015 | pages=1–9 | pmid=15866442 }}</ref>
|-
| style="text-align:center;"| {{chem|209|83|Bi}} + {{chem|4|2|He}} → {{chem|209|85|At}} + 4 {{chem|1|0}}n
| style="text-align:center;"| 35.9&nbsp;MeV<ref name="Maiti">{{cite journal | last1=Maiti | first1=Moumita | last2=Lahiri | first2=Susanta | last3=Kumar | first3=Deepak | last4=Choudhury | first4=Dibyasree | title=Separation of no-carrier-added astatine radionuclides from α-particle irradiated lead bismuth eutectic target: A classical method | journal=Applied Radiation and Isotopes | publisher=Elsevier BV | volume=127 | year=2017 | issn=0969-8043 | doi=10.1016/j.apradiso.2017.06.020 | pages=227–230| pmid=28649020 | bibcode=2017AppRI.127..227M }}</ref>
| style="text-align:center;"|
|}
</div>

[[File:Astatine-211 in bismuth target.jpg|thumb|left|The bismuth target after irradiation contains minuscule quantities of astatine-211.<ref>{{cite journal | last1=Naka | first1=Sadahiro | last2=Ooe | first2=Kazuhiro | last3=Shirakami | first3=Yoshifumi | last4=Kurimoto | first4=Kenta | last5=Sakai | first5=Toshihiro | last6=Takahashi | first6=Kazuhiro | last7=Toyoshima | first7=Atsushi | last8=Wang | first8=Yang | last9=Haba | first9=Hiromitsu | last10=Kato | first10=Hiroki | last11=Tomiyama | first11=Noriyuki | last12=Watabe | first12=Tadashi | title=Production of [211At]NaAt solution under GMP compliance for investigator-initiated clinical trial | journal=EJNMMI Radiopharmacy and Chemistry | volume=9 | issue=1 | date=2024-04-15 | article-number=29 | issn=2365-421X | pmid=38619655 | pmc=11018728 | doi=10.1186/s41181-024-00257-z | doi-access=free}}</ref>]]

Astatine was first produced by bombarding bismuth-209 with energetic alpha particles, and this is still the major route used to create the relatively long-lived isotopes astatine-209 through astatine-211. Astatine is only produced in minuscule quantities, with modern techniques allowing production runs of up to 6.6&nbsp;[[gigabecquerel]]s<ref name="zalutsky"/> (about 86&nbsp;[[nanogram]]s or 2.47{{e|14}} atoms). Synthesis of greater quantities of astatine using this method is constrained by the limited availability of suitable cyclotrons and the prospect of melting the target.<ref name="zalutsky"/><ref name="Larsen" />{{efn|See however Nagatsu et al.<ref>{{cite journal |first1=K.|last1=Nagatsu|first2=K. H.|last2=Minegishi|first3=M. |last3=Fukada |first4=H. |last4=Suzuki |first5=S.|last5=Hasegawa |first6=M. |last6=Zhang|year=2014 |title=Production of <sup>211</sup>At by a vertical beam irradiation method|journal=Applied Radiation and Isotopes |volume=94 |pages=363–371 |doi=10.1016/j.apradiso.2014.09.012|pmid=25439168|bibcode=2014AppRI..94..363N }}</ref> who encapsulate the bismuth target in a thin aluminium foil and place it in a niobium holder capable of holding molten bismuth.}} Solvent [[radiolysis]] due to the cumulative effect of astatine decay<ref>{{cite book | title = Therapeutic Nuclear Medicine| year = 2014 | pages = 95–104 (99) | publisher = Springer | isbn = 978-3-540-36718-5 | last1 = Barbet | first1 = J. | last2 = Bourgeois | first2 = M.|first3= J.|last3= Chatal|editor1 = R. P.|editor2 = Baum|chapter= Cyclotron-Based Radiopharmaceuticals for Nuclear Medicine Therapy}}</ref> is a related problem. With cryogenic technology, [[microgram]] quantities of astatine might be able to be generated via proton irradiation of [[thorium]] or [[uranium]] to yield radon-211, in turn decaying to astatine-211. Contamination with astatine-210 is expected to be a drawback of this method.<ref name="Wilbur">{{cite journal | last = Wilbur| first = D. S. | date = 2001| title = Overcoming the Obstacles to Clinical Evaluation of <sup>211</sup>At-Labeled Radiopharmaceuticals | journal = The Journal of Nuclear Medicine | volume = 42 | issue = 10 | pages = 1516–1518 | url=http://jnm.snmjournals.org/content/42/10/1516|pmid=11585866}}</ref>

The most important isotope is astatine-211, the only one in commercial use. To produce the bismuth target, the metal is [[sputtering|sputtered]] onto a gold, copper, or aluminium surface at 50 to 100 milligrams per square centimeter. [[Bismuth oxide]] can be used instead; this is forcibly fused with a copper plate.{{sfn|Lavrukhina|Pozdnyakov|1970|p=233}} The target is kept under a [[Nitrogen gas#Reactions|chemically neutral nitrogen]] atmosphere,<ref name="BiN2">{{cite book|title=Inorganic Chemistry for Undergraduates|first=R. |last=Gopalan |year=2009|page=547|publisher=Universities Press | isbn = 978-81-7371-660-7 | url = https://books.google.com/books?id=Fs4zQ-hNTz8C&pg=PA492}}</ref> and is cooled with water to prevent premature astatine vaporization.{{sfn|Lavrukhina|Pozdnyakov|1970|p=233}} In a particle accelerator, such as a cyclotron,<ref>{{cite book|title=Targeted Radionuclide Tumor Therapy: Biological Aspects|first1=T.|last1=Stigbrand |first2=J. |last2=Carlsson |first3=G. P.|last3=Adams|year=2008|page=150|publisher=Springer | isbn = 978-1-4020-8695-3 | url = https://books.google.com/books?id=-mT0Lthq_54C}}</ref> alpha particles are collided with the bismuth. Even though only one bismuth isotope is used (bismuth-209), the reaction may occur in three possible ways, producing astatine-209, astatine-210, or astatine-211. Although higher energies can produce more astatine-211, it will produce unwanted astatine-210 that decays to toxic polonium-210 as well. Instead, the maximum energy of the particle accelerator is set to be below or slightly above the threshold of astatine-210 production, in order to maximize the production of astatine-211 while keeping the amount of astatine-210 at an acceptable level.<ref name="zalutsky"/><ref name="Gyehong">{{cite journal |first1=G.|last1=Gyehong |first2=K.|last2=Chun|first3=S. H. |last3=Park |first4=B.|last4=Kim|year=2014 |title=Production of α-particle emitting <sup>211</sup>At using 45&nbsp;MeV α-beam| journal=Physics in Medicine and Biology |volume=59 |issue=11|pages=2849–2860| doi=10.1088/0031-9155/59/11/2849|pmid=24819557 |bibcode = 2014PMB....59.2849K |s2cid=21973246 }}</ref>

=== Separation methods ===
Since astatine is the main product of the synthesis, after its formation it must only be separated from the target and any significant contaminants. Several methods are available, "but they generally follow one of two approaches—dry distillation or [wet] acid treatment of the target followed by solvent extraction." The methods summarized below are modern adaptations of older procedures, as reviewed by Kugler and Keller.{{sfn|Kugler|Keller|1985|pp=95–106, 133–139}}{{efn|See also Lavrukhina and Pozdnyakov.{{sfn|Lavrukhina|Pozdnyakov|1970|pp=243–253}}}} Pre-1985 techniques more often addressed the elimination of co-produced toxic polonium; this requirement is now mitigated by capping the energy of the cyclotron irradiation beam.<ref name="zalutsky"/>

==== Dry ====
The astatine-containing cyclotron target is heated to a temperature of around 650&nbsp;°C. The astatine [[Volatilization|volatilizes]] and is condensed in (typically) a [[cold trap]]. Higher temperatures of up to around 850&nbsp;°C may increase the yield, at the risk of bismuth contamination from concurrent volatilization. Redistilling the condensate may be required to minimize the presence of bismuth{{sfn|Kugler|Keller|1985|p=97}} (as bismuth can interfere with astatine [[radioactive tracer|labeling reactions]]). The astatine is recovered from the trap using one or more low concentration solvents such as [[sodium hydroxide]], [[methanol]] or [[chloroform]]. Astatine yields of up to around 80% may be achieved. Dry separation is the method most commonly used to produce a chemically useful form of astatine.<ref name="Larsen">{{cite journal|last1=Larsen|first1=R. H.|last2=Wieland|first2=B. W.|last3=Zalutsky|first3=M. R. J.|year=1996|title=Evaluation of an Internal Cyclotron Target for the Production of <sup>211</sup>At via the <sup>209</sup>Bi (α,2n)<sup>211</sup>At reaction|journal=Applied Radiation and Isotopes|volume=47|issue=2|pages=135–143|doi=10.1016/0969-8043(95)00285-5|pmid=8852627|bibcode=1996AppRI..47..135L }}</ref><ref>{{cite journal|last1=Lindegren|first1=S.|last2=Bäck|first2=T.|last3=Jensen|first3=H. J.|year=2001|title=Dry-distillation of Astatine-211 from Irradiated Bismuth Targets: A Time-saving Procedure with High Recovery Yields|journal=Applied Radiation and Isotopes|volume=55|issue=2|pages=157–160|doi=10.1016/S0969-8043(01)00044-6|pmid=11393754|bibcode=2001AppRI..55..157L }}</ref>

==== Wet ====
The irradiated bismuth (or sometimes [[bismuth trioxide]]) target is first dissolved in, for example, concentrated nitric or perchloric acid. Following this first step, the acid can be distilled away to leave behind a white residue that contains both bismuth and the desired astatine product. This residue is then dissolved in a concentrated acid, such as hydrochloric acid. Astatine is extracted from this acid using an organic solvent such as [[dibutyl ether]], [[diisopropyl ether]] (DIPE), or [[thiosemicarbazide]]. Using liquid-liquid extraction, the astatine product can be repeatedly washed with an acid, such as HCl, and extracted into the organic solvent layer. A separation yield of 93% using nitric acid has been reported, falling to 72% by the time purification procedures were completed (distillation of nitric acid, purging residual [[nitrogen oxide]]s, and redissolving [[bismuth nitrate]] to enable [[liquid–liquid extraction]]).<ref>{{cite journal|last1=Yordanov|first1=A. T.|last2=Pozzi|first2=O.|last3=Carlin|first3=S.|last4=Akabani|first4=G. J.|last5=Wieland|first5=B.|last6=Zalutsky|first6=M. R.|year=2005|title=Wet Harvesting of No-carrier-added <sup>211</sup>At from an Irradiated <sup>209</sup>Bi Target for Radiopharmaceutical Applications|url=https://link.springer.com/article/10.1007/s10967-004-0481-z|journal=Journal of Radioanalytical and Nuclear Chemistry|volume=262|issue=3|pages=593–599|doi=10.1007/s10967-005-0481-7|bibcode=2005JRNC..262..593Y |s2cid=93179195|url-access=subscription}}</ref><ref name="Balkin-2013">{{Cite journal|last1=Balkin|first1=Ethan|last2=Hamlin|first2=Donald|last3=Gagnon|first3=Katherine|last4=Chyan|first4=Ming-Kuan|last5=Pal|first5=Sujit|last6=Watanabe|first6=Shigeki|last7=Wilbur|first7=D.|date=2013-09-18|title=Evaluation of a Wet Chemistry Method for Isolation of Cyclotron Produced [211At]Astatine|journal=Applied Sciences|language=en|volume=3|issue=3|pages=636–655|doi=10.3390/app3030636|issn=2076-3417|citeseerx=10.1.1.383.1903|doi-access=free}}</ref> Wet methods involve "multiple radioactivity handling steps" and have not been considered well suited for isolating larger quantities of astatine. However, wet extraction methods are being examined for use in production of larger quantities of astatine-211, as it is thought that wet extraction methods can provide more consistency.<ref name="Balkin-2013" /> They can enable the production of astatine in a specific [[oxidation state]] and may have greater applicability in experimental [[radiochemistry]].<ref name="zalutsky"/>

== Uses and precautions ==
<div style="float: right; margin: 2px; font-size: 90%;">
{| class="wikitable"
|+ Several <sup>211</sup>At-containing molecules and their experimental uses{{sfn|Vértes|Nagy|Klencsár|2003|p=337}}
! Agent
! Applications
|-
| [<sup>211</sup>At]astatine-tellurium colloids
| Compartmental tumors<!-- Please do not add fact tags to the table; all (all) info may be seen in the source in the title of the table -->
|-
| 6-[<sup>211</sup>At]astato-2-methyl-1,4-naphtaquinol diphosphate
| [[Adenocarcinoma]]s
|-
| <sup>211</sup>At-labeled [[methylene blue]]
| [[Melanoma]]s
|-
| Meta-[<sup>211</sup>At]astatobenzyl guanidine
| Neuroendocrine tumors
|-
| 5-[<sup>211</sup>At]astato-2'-deoxyuridine
| Various
|-
| <sup>211</sup>At-labeled biotin conjugates
| Various pretargeting
|-
| <sup>211</sup>At-labeled [[octreotide]]
| [[Somatostatin receptor]]
|-
| <sup>211</sup>At-labeled monoclonal antibodies and fragments
| Various
|-
| <sup>211</sup>At-labeled [[bisphosphonates]]
| [[Bone metastases]]
|}
</div>
Newly formed astatine-211 is the subject of ongoing research in [[nuclear medicine]].{{sfn|Vértes|Nagy|Klencsár|2003|p=337}} It must be used quickly as it decays with a half-life of 7.2&nbsp;hours; this is long enough to permit [[radioactive tracer|multistep labeling strategies]]. Astatine-211 has potential for [[targeted alpha-particle therapy]], since it decays either via emission of an alpha particle (to bismuth-207),<ref>{{cite journal|last1=Zalutsky|first1=Michael|last2=Vaidyanathan|first2=Ganesan|title=Astatine-211-Labeled Radiotherapeutics An Emerging Approach to Targeted Alpha-Particle Radiotherapy|journal=Current Pharmaceutical Design|date=1 September 2000|volume=6|issue=14|pages=1433–1455|doi=10.2174/1381612003399275|pmid=10903402}}</ref> or via electron capture (to an extremely short-lived nuclide, polonium-211, which undergoes further alpha decay), very quickly reaching its stable granddaughter lead-207. Polonium X-rays emitted as a result of the electron capture branch, in the range of 77–92&nbsp;[[keV]], enable the tracking of astatine in animals and patients.{{sfn|Vértes|Nagy|Klencsár|2003|p=337}} Although astatine-210 has a slightly longer half-life, it is wholly unsuitable because it usually undergoes beta plus decay to the extremely toxic polonium-210.<ref>{{cite journal |last=Wilbur |first=D. Scott |date=20 February 2013 |title=Enigmatic astatine |journal=Nature Chemistry |volume=5 |issue=3 |page=246 |doi=10.1038/nchem.1580 |pmid=23422568 |doi-access=free |bibcode=2013NatCh...5..246W }}</ref>

The principal medicinal difference between astatine-211 and [[iodine-131]] (a radioactive iodine isotope also used in medicine) is that iodine-131 emits high-energy beta particles, and astatine does not. Beta particles have much greater penetrating power through tissues than do the much heavier alpha particles. An average alpha particle released by astatine-211 can travel up to 70&nbsp;μm through surrounding tissues; an average-energy beta particle emitted by iodine-131 can travel nearly 30 times as far, to about 2&nbsp;mm.{{sfn|Lavrukhina|Pozdnyakov|1970|p=233}} The short half-life and limited penetrating power of alpha radiation through tissues offers advantages in situations where the "tumor burden is low and/or malignant cell populations are located in close proximity to essential normal tissues."<ref name="zalutsky"/> Significant morbidity in cell culture models of human cancers has been achieved with from one to ten astatine-211 atoms bound per cell.{{sfn|Vértes|Nagy|Klencsár|2003|p=338}}

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Several obstacles have been encountered in the development of astatine-based [[radiopharmaceuticals]] for [[cancer]] treatment. [[World War II]] delayed research for close to a decade. Results of early experiments indicated that a cancer-selective carrier would need to be developed and it was not until the 1970s that [[monoclonal antibodies]] became available for this purpose. Unlike iodine, astatine shows a tendency to [[dehalogenation|dehalogenate]] from molecular carriers such as these, particularly at [[Orbital hybridisation#sp3|sp<sup>3</sup> carbon sites]]{{efn|In other words, where carbon's one s [[atomic orbital]] and three p orbitals [[Orbital hybridisation|hybridize]] to give four new orbitals shaped as intermediates between the original s and p orbitals.}} (less so from [[Orbital hybridisation#sp2|sp<sup>2</sup> sites]]). Given the toxicity of astatine accumulated and retained in the body, this emphasized the need to ensure it remained attached to its host molecule. While astatine carriers that are slowly metabolized can be assessed for their efficacy, more rapidly metabolized carriers remain a significant obstacle to the evaluation of astatine in nuclear medicine. Mitigating the effects of astatine-induced radiolysis of labeling chemistry and carrier molecules is another area requiring further development. A practical application for astatine as a cancer treatment would potentially be suitable for a "staggering" number of patients; production of astatine in the quantities that would be required remains an issue.<ref name="Wilbur" /><ref>{{cite journal |first1=G. |last1=Vaidyanathan |first2=M. R. |last2=Zalutsky |year=2008 |title=Astatine Radiopharmaceuticals: Prospects and Problems |journal=Current Radiopharmaceuticals |volume=1 |issue=3 |pages=177–196 | doi=10.2174/1874471010801030177 | pmc=2818997 |pmid=20150978}}</ref>{{efn|"Unfortunately, the conundrum confronting the ... field is that commercial supply of <sup>211</sup>At awaits the demonstration of clinical efficacy; however, the demonstration of clinical efficacy requires a reliable supply of <sup>211</sup>At."<ref name="zalutsky">{{cite journal|first1=M. R.|last1=Zalutsky|first2=M.|last2=Pruszynski|year=2011|title=Astatine-211: Production and Availability|pmc=3503149|journal=Current Radiopharmaceuticals|volume=4|issue=3|pages=177–185|doi=10.2174/1874471011104030177|pmid=22201707}}</ref>}}

Animal studies show that astatine, similarly to iodine—although to a lesser extent, perhaps because of its slightly more metallic nature<ref name="Stwertka">Stwertka, Albert. ''A Guide to the Elements'', Oxford University Press, 1996, p. 193. {{ISBN|0-19-508083-1}}</ref>—is preferentially (and dangerously) concentrated in the [[thyroid gland]]. Unlike iodine, astatine also shows a tendency to be taken up by the lungs and spleen, possibly because of in-body oxidation of At<sup>–</sup> to At<sup>+</sup>.<ref name="Guerard" /> If administered in the form of a radiocolloid it tends to concentrate in the [[liver]]. Experiments in rats and monkeys suggest that astatine-211 causes much greater damage to the thyroid gland than does iodine-131, with repetitive injection of the nuclide resulting in necrosis and cell [[dysplasia]] within the gland.{{sfn|Lavrukhina|Pozdnyakov|1970|pp=232–233}} Early research suggested that injection of astatine into female rodents caused morphological changes in breast tissue;<ref>{{cite book |last1= Odell | first1= T. T. Jr. |last2= Upton |first2= A. C. |year= 2013 |orig-date= Softcover reprint of the hardcover 1st edition 1961 |chapter= Late Effects of Internally Deposited Radioisotopes |editor1-last= Schwiegk | editor1-first= H.|editor2-last= Turba | editor2-first= F. | title= Radioactive Isotopes in Physiology Diagnostics and Therapy | trans-title= Radioaktive Isotope in Physiologie Diagnostik Und Therapie | publisher= Springer-Verlag | pages= 375–392 (385)| isbn= 978-3-642-49477-2}}</ref> this conclusion remained controversial for many years. General agreement was later reached that this was likely caused by the effect of breast tissue irradiation combined with hormonal changes due to irradiation of the ovaries.<ref name="Fisher">{{cite conference | first = D. | last = Fisher | year = 1995 | book-title = Human Radiation Studies: Remembering the Early Years | title = Oral History of Dr. Patricia Wallace Durbin, PhD| publisher = United States Department of Energy, Office of Human Radiation Experiments | url = https://ehss.energy.gov/ohre/roadmap/histories/0458/0458toc.html#0458_Dr_J | access-date = 25 March 2015}}</ref> Trace amounts of astatine can be handled safely in fume hoods if they are well-aerated; biological uptake of the element must be avoided.<ref name="Ullmann"/>

{{clear}}

== See also ==
{{portal|Chemistry}}
* [[Radiation protection]]

== Notes ==
{{notelist|30em}}

== References ==
{{reflist|30em}}

== Bibliography ==
* {{cite journal | title = Artificially Radioactive Element 85 | first1 = D. R. | last1 = Corson | author-link1=Dale R. Corson | first2 = K. R. | last2 =MacKenzie | author-link2=Kenneth Ross MacKenzie | first3 = E. | last3 = Segrè | author-link3=Emilio Segrè | journal = Physical Review | volume = 58 | pages = 672–678 | year = 1940 | doi = 10.1103/PhysRev.58.672 | issue = 8| bibcode = 1940PhRv...58..672C }} {{subscription required}}
* {{cite book | title = Chemistry of the Elements|edition=2nd| year = 2002| publisher = Butterworth-Heinemann | isbn = 978-0-7506-3365-9 | last1 = Greenwood | first1 = N. N. | last2 = Earnshaw | first2 = A.}}
* {{cite book | series = Gmelin Handbook of Inorganic and Organometallic Chemistry | title = 'At, Astatine', System No. 8a | edition=8th | year = 1985 | publisher = Springer-Verlag | isbn = 978-3-540-93516-2 | last1 = Kugler | first1 = H. K. | last2 = Keller | first2 = C. | volume = 8}}
* {{cite book |title=Analytical Chemistry of Technetium, Promethium, Astatine, and Francium |first1=Avgusta Konstantinovna |last1=Lavrukhina |first2=Aleksandr Aleksandrovich |last2=Pozdnyakov |year=1970 |publisher=Ann Arbor–Humphrey Science Publishers |others=Translated by R. Kondor |isbn=978-0-250-39923-9}}
* {{cite book |title=A Tale of Seven Elements|first1= Eric|last1= Scerri|year=2013|publisher=Oxford University Press, ISBN 9780195391312 }}
* {{cite book|title=Handbook of Nuclear Chemistry|volume=4|first1=A.|last1=Vértes |first2=S.|last2=Nagy |first3=Z.|last3=Klencsár|year=2003|publisher=Springer | isbn = 978-1-4020-1316-4 | url = https://books.google.com/books?id=0skQvMEa8EYC}}
* {{cite book | title = Inorganic Reactions and Methods, Volume 3, The Formation of Bonds to Halogens (Part 1)| first1 = J. J. | last1 = Zuckerman | first2 = A. P. | last2 = Hagen | year = 1989 | publisher = John Wiley & Sons | isbn = 978-0-471-18656-4}}
* {{cite book | title = Inorganic Reactions and Methods, Volume 4, The Formation of Bonds to Halogens (Part 2)| first1 = J. J. | last1 = Zuckerman | first2 = A. P. | last2 = Hagen | year = 1990 | publisher = John Wiley & Sons | isbn = 978-0-471-18657-1}}

== External links ==
{{Sister project links |wikt=astatine |commons=Astatine |n=no |q=no |s=no |b=no |v=Astatine atom}}
* [http://www.periodicvideos.com/videos/085.htm Astatine] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://quantumchymist.blogspot.com.au/2014/02/astatine-halogen-or-metal-part-1.html Astatine: Halogen or Metal?]

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[[Category:Chemical elements]]
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