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Astatine, 85At
Astatine
Pronunciation/ˈæstətiːn, -tɪn/ ⓘ ​(AS-tə-teen, -⁠tin)
Appearanceunknown, probably metallic
Mass number[210]
Astatine in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
 I 
↑
At
↓
Ts
polonium ← astatine → radon
Atomic number (Z)85
Groupgroup 17 (halogens)
Periodperiod 6
Block  p-block
Electron configuration[Xe] 4f14 5d10 6s2 6p5
Electrons per shell2, 8, 18, 32, 18, 7
Physical properties
Phase at STPsolid (predicted)
Density (near r.t.)8.91–8.95 g/cm3 (estimated)[1]
Molar volume23.6 cm3/mol (estimated)[1]
Atomic properties
Oxidation statescommon: −1, +1
+3,[2] +5,[2] +7[2]
Ionization energies
  • 1st: 899.003 kJ/mol[3]
Other properties
Natural occurrencefrom decay
Crystal structure ​face-centered cubic (fcc)
Face-centered cubic crystal structure for astatine

(predicted)[4]
CAS Number7440-68-8
History
NamingAncient Greek ástatos (ἄστατος) 'unstable'
DiscoveryDale R. Corson, Kenneth Ross MacKenzie, Emilio Segrè (1940)
Isotopes of astatine
Main isotopes[5] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
207At synth 1.81 h β+90% 207Po
α10% 203Bi
208At synth 1.63 h β+99.5% 208Po
α0.55% 204Bi
209At synth 5.41 h β+96.1% 209Po
α3.9% 205Bi
210At synth 8.1 h β+99.8% 210Po
α0.175% 206Bi
211At synth 7.214 h ε58.2% 211Po
α41.8% 207Bi
 Category: Astatine
| references

Astatine is a chemical element; it has symbol At and atomic number 85. It is the rarest naturally occurring element in the Earth's crust, occurring only as the decay product of various heavier elements. All of astatine's isotopes are short-lived; the most stable is astatine-210, with a half-life of 8.1 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 halogens. 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 anionic 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 ástatos (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 particles.

Characteristics

[edit]

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 (205–211At) are synthetic and do not occur in nature.[6]

The bulk properties of astatine are not known with any certainty.[7] Research is limited by its short half-life, which prevents the creation of weighable quantities.[8] A visible piece of astatine would immediately vaporize itself because of the heat generated by its intense radioactivity.[9] It remains to be seen if, with sufficient cooling, a macroscopic quantity of astatine could be deposited as a thin film.[4] Astatine is usually classified as either a nonmetal or a metalloid;[10][11] metal formation has also been predicted.[4][12]

Physical

[edit]

Most of the physical properties of astatine have been estimated (by interpolation or extrapolation), using theoretically or empirically derived methods.[13] For example, halogens get darker with increasing atomic weight – 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).[14][15][16]

Astatine sublimes less readily than iodine, having a lower vapor pressure.[8] 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.[a] The absorption spectrum of astatine in the middle ultraviolet region has lines at 224.401 and 216.225 nm, suggestive of 6p to 7s transitions.[18][19]

The structure of solid astatine is unknown.[20] As an analog of iodine it may have an orthorhombic crystalline structure composed of diatomic astatine molecules, and be a semiconductor (with a band gap of 0.7 eV).[21][22] Alternatively, if condensed astatine forms a metallic phase, as has been predicted, it may have a monatomic face-centered cubic structure; in this structure, it may well be a superconductor, like the similar high-pressure phase of iodine.[4] Metallic astatine is expected to have a density of 8.91–8.95 g/cm3.[1]

Evidence for (or against) the existence of diatomic astatine (At2) is sparse and inconclusive.[23][24][25][26][27] Some sources state that it does not exist, or at least has never been observed,[28][29] while other sources assert or imply its existence.[30][31][32] Despite this controversy, many properties of diatomic astatine have been predicted;[33] for example, its bond length would be 300±10 pm, dissociation energy <50 kJ/mol,[34] and heat of vaporization (∆Hvap) 54.39 kJ/mol.[35] Many values have been predicted for the melting and boiling points of astatine, but only for At2.[36]

Chemical

[edit]

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".[21] Many of its apparent chemical properties have been observed using tracer studies on extremely dilute astatine solutions,[32][37] typically less than 10−10 mol·L−1.[38] Some properties, such as anion formation, align with other halogens.[8] Astatine has some metallic characteristics as well, such as plating onto a cathode,[b] and coprecipitating with metal sulfides in hydrochloric acid.[40] It forms complexes with EDTA, a metal chelating agent,[41] 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.[42] It has been suggested that astatine can form a stable monatomic cation in aqueous solution.[40][43]

Astatine has an electronegativity of 2.2 on the revised Pauling scale – 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.[44][45][46][47] That would be consistent with the electronegativity of astatine on the Allred–Rochow scale (1.9) being less than that of hydrogen (2.2).[48][c] 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−1, is 21% less than that of iodine.[50] 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 interactions.[38] The first ionization energy of astatine is about 899 kJ mol−1, which continues the trend of decreasing first ionization energies down the halogen group (fluorine, 1681; chlorine, 1251; bromine, 1140; iodine, 1008).[3]

Compounds

[edit]
Main article: Astatine compounds

Less reactive than iodine, astatine is the least reactive of the halogens;[51] the chemical properties of tennessine, the next-heavier group 17 element, have not yet been investigated, however.[52] Astatine 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) to work.[53][54][d] Like iodine, astatine has been shown to adopt odd-numbered oxidation states ranging from −1 to +7.[57]

Only a few compounds with metals have been reported, in the form of astatides of sodium,[9] palladium, silver, thallium, and lead.[58] 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.[59]

Hydrogen astatide space-filling model

The formation of an astatine compound with hydrogen – usually referred to as hydrogen astatide – was noted by the pioneers of astatine chemistry.[60] 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 At0 or At+ 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.[8][61]

Astatine is known to bind to boron,[62] carbon, and nitrogen.[63] Various boron cage compounds have been prepared with At–B bonds, these being more stable than At–C bonds.[64] Astatine can replace a hydrogen atom in benzene to form astatobenzene C6H5At; this may be oxidized to C6H5AtCl2 by chlorine. By treating this compound with an alkaline solution of hypochlorite, C6H5AtO2 can be produced.[65] The dipyridine-astatine(I) cation, [At(C5H5N)2]+, forms ionic compounds with perchlorate[63] (a non-coordinating anion[66]) and with nitrate, [At(C5H5N)2]NO3.[63] This cation exists as a coordination complex in which two dative covalent bonds separately link the astatine(I) centre with each of the pyridine rings via their nitrogen atoms.[63]

With oxygen, there is evidence of the species AtO− and AtO+ 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.[8][67] The species previously thought to be AtO−2 has since been determined to be AtO(OH)−2, a hydrolysis product of AtO+ (another such hydrolysis product being AtOOH).[68] The well characterized AtO−3 anion can be obtained by, for example, the oxidation of astatine with potassium hypochlorite in a solution of potassium hydroxide.[65][69] Preparation of lanthanum triastatate La(AtO3)3, following the oxidation of astatine by a hot Na2S2O8 solution, has been reported.[70] Further oxidation of AtO−3, such as by xenon difluoride (in a hot alkaline solution) or periodate (in a neutral or alkaline solution), yields the perastatate ion AtO−4; this is only stable in neutral or alkaline solutions.[71] 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.[65][72]

Astatine may form bonds to the other chalcogens; these include S7At+ and At(CSN)−2 with sulfur, a coordination selenourea compound with selenium, and an astatine–tellurium colloid with tellurium.[73]

Structure of astatine iodide, one of the astatine interhalogens 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 compounds with formulas AtI, AtBr, and AtCl.[55] The first two compounds may also be produced in water – 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 AtBr−2 and AtI−2 ions,[55] or in a chloride solution, they may produce species like AtCl−2 or AtBrCl− via equilibrium reactions with the chlorides.[56] 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, AtOCl−2 or AtCl−2 may be produced.[55] The polyhalides PdAtI2, CsAtI2, TlAtI2,[74][75][76] and PbAtI[77] are known or presumed to have been precipitated. In a plasma ion source mass spectrometer, the ions [AtI]+, [AtBr]+, and [AtCl]+ 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.[55] No astatine fluorides 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.[e] 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.[55][71]

History

[edit]
Periodic table by Mendeleev (1871), with astatine missing below chlorine, bromine and iodine ("J")
Dmitri Mendeleev's table of 1871, with an empty space at the eka-iodine position

In 1869, when Dmitri Mendeleev published his 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 eka 'one') to imply it was one space under iodine (in the same manner as eka-silicon, eka-boron, and others).[81] Scientists tried to find it in nature; given its extreme rarity, these attempts resulted in several false discoveries.[82]

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.[83][84][85] In 1934, H. G. MacPherson of University of California, Berkeley disproved Allison's method and the validity of his discovery.[86] 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 F (polonium-210) in the radium series.[87] The properties he reported for dakin do not correspond to those of astatine,[87] and astatine's radioactivity would have prevented him from handling it in the quantities he claimed.[88] Moreover, astatine is not found in the thorium series, and the true identity of dakin is not known.[87]

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.[88] He had also been involved in an earlier false claim as to the discovery of element 87 (francium) and this is thought to have caused other researchers to downplay his work.[89]

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 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).[90][f] 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 A (polonium-216) beta decay. They named this substance "anglo-helvetium",[91] but Karlik and Bernert were again unable to reproduce these results.[53]

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 particles in a cyclotron (particle accelerator) to produce, after emission of two neutrons, astatine-211.[92] 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.[93] In 1943, astatine was found as a product of two naturally occurring decay chains by Berta Karlik and Traude Bernert, first in the so-called uranium series, and then in the actinium series.[94][95] (Since then, astatine was also found in a third decay chain, the neptunium series.[96]) 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 published the discoverers' suggestions; a letter from Corson, MacKenzie, and Segrè suggested the name "astatine"[93] coming from the Ancient Greek ástatos (ἄστατος) meaning '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.[97]

Corson and his colleagues classified astatine as a metal on the basis of its analytical chemistry.[98] Subsequent investigators reported iodine-like,[99][100] cationic,[101][102] or amphoteric behavior.[103][53] In a 2003 retrospective, Corson wrote that "some of the properties [of astatine] are similar to iodine ... it also exhibits metallic properties, more like its metallic neighbors Po and Bi."[97]

Isotopes

[edit]
Main article: Isotopes of astatine
Alpha decay characteristics for sample astatine isotopes[g]
Mass
number 		Half-life[6] Probability
of alpha
decay[6] 		Alpha-
decay
half-life
207 1.80 h 8.6% 20.9 h
208 1.63 h 0.55% 12.3 d
209 5.41 h 4.1% 5.5 d
210 8.1 h 0.175% 193 d
211 7.21 h 41.8% 17.2 h
212 0.31 s ≈100% 0.31 s
213 125 ns 100% 125 ns
214 558 ns 100% 558 ns
219 56 s 97% 58 s
220 3.71 min 8% 46.4 min
221 2.3 min experimentally
alpha-stable 		∞

There are 41 known isotopes of astatine, with mass numbers of 188 and 190–229.[104][105] Theoretical modeling suggests that about 37 more isotopes could exist.[104] No stable or long-lived astatine isotope has been observed, nor is one expected to exist.[106]

Astatine's alpha decay energies follow the same trend as for other heavy elements.[106] Lighter astatine isotopes have quite 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 corresponding to a filled neutron shell. Despite having a similar half-life to the previous isotope (8.1 hours for astatine-210 and 7.2 hours for astatine-211), the alpha decay probability is much higher for the latter: 41.81% against only 0.18%.[6][h] 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.[106] Even though heavier astatine isotopes release less energy, no long-lived astatine isotope exists, because of the increasing role of beta decay (electron emission).[106] This decay mode is especially important for astatine; as early as 1950 it was postulated that all isotopes of the element undergo beta decay,[107] though nuclear mass measurements indicate that 215At is in fact beta-stable, as it has the lowest mass of all isobars with A = 215.[6] 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.[5]

The most stable isotope is astatine-210, which has a half-life of 8.1 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.[6]

Astatine has 24 known nuclear isomers, which are nuclei with one or more nucleons (protons or neutrons) in an excited state. A nuclear isomer may also be called a "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,[i] 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 nanoseconds is shorter than 125 nanoseconds for astatine-213, the shortest-lived ground state.[5]

Natural occurrence

[edit]
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.[j] The total amount of astatine in the Earth's crust (quoted mass 2.36 × 1025 grams)[108] is estimated by some to be less than one gram at any given time.[8] Other sources estimate the amount of ephemeral astatine, present on earth at any given moment, to be up to one ounce[109] (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)[110] are instead continuously produced as a result of the 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 × 10−10 grams).[111] 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 ores.[112] Astatine-218 was the first astatine isotope discovered in nature.[113] Astatine-219, with a half-life of 56 seconds, is the longest lived of the naturally occurring isotopes.[6]

Isotopes of astatine are sometimes not listed as naturally occurring because of misconceptions[103] that there are no such isotopes,[114] or discrepancies in the literature. Astatine-216 has been counted as a naturally occurring isotope but reports of its observation[115] (which were described as doubtful) have not been confirmed.[116]

Synthesis

[edit]

Formation

[edit]
Possible reactions after bombarding bismuth-209 with alpha particles
Reaction[k] Energy of alpha particle
Threshold energy Maximum production
209
83Bi + 4
2He → 211
85At + 2 1
0n 		20.7 MeV[117] 30[118]–31 MeV[117]
209
83Bi + 4
2He → 210
85At + 3 1
0n 		28.4[119]–28.6 MeV[117][120] 37 MeV[117]
209
83Bi + 4
2He → 209
85At + 4 1
0n 		35.9 MeV[120]
The bismuth target after irradiation contains minuscule quantities of astatine-211.[121]

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 gigabecquerels[119] (about 86 nanograms or 2.47×1014 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.[119][122][l] Solvent radiolysis due to the cumulative effect of astatine decay[124] 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.[125]

The most important isotope is astatine-211, the only one in commercial use. To produce the bismuth target, the metal is 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.[126] The target is kept under a chemically neutral nitrogen atmosphere,[127] and is cooled with water to prevent premature astatine vaporization.[126] In a particle accelerator, such as a cyclotron,[128] 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.[119][118]

Separation methods

[edit]

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.[129][m] 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.[119]

Dry

[edit]

The astatine-containing cyclotron target is heated to a temperature of around 650 °C. The astatine volatilizes and is condensed in (typically) a cold trap. Higher temperatures of up to around 850 °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[131] (as bismuth can interfere with astatine 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.[122][132]

Wet

[edit]

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 oxides, and redissolving bismuth nitrate to enable liquid–liquid extraction).[133][134] 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.[134] They can enable the production of astatine in a specific oxidation state and may have greater applicability in experimental radiochemistry.[119]

Uses and precautions

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Several 211At-containing molecules and their experimental uses[135]
Agent Applications
[211At]astatine-tellurium colloids Compartmental tumors
6-[211At]astato-2-methyl-1,4-naphtaquinol diphosphate Adenocarcinomas
211At-labeled methylene blue Melanomas
Meta-[211At]astatobenzyl guanidine Neuroendocrine tumors
5-[211At]astato-2'-deoxyuridine Various
211At-labeled biotin conjugates Various pretargeting
211At-labeled octreotide Somatostatin receptor
211At-labeled monoclonal antibodies and fragments Various
211At-labeled bisphosphonates Bone metastases

Newly formed astatine-211 is the subject of ongoing research in nuclear medicine.[135] It must be used quickly as it decays with a half-life of 7.2 hours; this is long enough to permit 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),[136] 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 keV, enable the tracking of astatine in animals and patients.[135] 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.[137]

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 μm through surrounding tissues; an average-energy beta particle emitted by iodine-131 can travel nearly 30 times as far, to about 2 mm.[126] 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."[119] Significant morbidity in cell culture models of human cancers has been achieved with from one to ten astatine-211 atoms bound per cell.[138]

Astatine ... [is] miserable to make and hell to work with.[139]

P Durbin, Human Radiation Studies: Remembering the Early Years, 1995

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 dehalogenate from molecular carriers such as these, particularly at sp3 carbon sites[n] (less so from sp2 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.[125][140][o]

Animal studies show that astatine, similarly to iodine—although to a lesser extent, perhaps because of its slightly more metallic nature[109]—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– to At+.[42] 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.[141] Early research suggested that injection of astatine into female rodents caused morphological changes in breast tissue;[142] 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.[139] Trace amounts of astatine can be handled safely in fume hoods if they are well-aerated; biological uptake of the element must be avoided.[29]

See also

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Notes

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References

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Bibliography

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Astatine

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Astatine is a radioactive with the symbol At and 85, belonging to the group (group 17) of the periodic table and positioned in period 6. As the rarest naturally occurring element in the , with an estimated total amount of less than 1 gram in the , astatine occurs only in trace amounts as a of and . All 41 known isotopes of astatine are highly unstable and radioactive, lacking any stable nuclides; the longest-lived is astatine-210, with a of 8.1 hours. First synthesized in by Dale R. Corson, Kenneth R. Mackenzie, and at the , through the bombardment of with alpha particles, astatine derives its name from the Greek word astatos, meaning "unstable." Physically, astatine exists as a solid at , appearing as a black or metallic solid with a of approximately 7 g/cm³, a of 302 °C, and a of 337 °C. Chemically, it exhibits halogen-like behavior similar to iodine but displays more metallic characteristics, with an of [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, possible oxidation states of +7, +5, +3, +1, and -1, and an of 2.2 on the Pauling scale. It accumulates in the thyroid gland like other and can form compounds such as astatine iodide (AtI). Due to its extreme scarcity and intense radioactivity, astatine has no established commercial applications, though it is produced in cyclotrons for purposes. Notably, the radioisotope astatine-211, with a of 7.2 hours and alpha-particle emission, is under investigation for targeted alpha in , where it can be attached to antibodies to selectively destroy tumor cells with high precision. As of 2025, first-in-human clinical trials have shown preliminary efficacy in treating radioiodine-refractory differentiated .

Properties

Physical Properties

Astatine (At) is a chemical element with atomic number 85, occupying the position of the heaviest in group 17 and period 6 of the periodic table. Its is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, consistent with the filling of the 6p subshell by one electron short of a core. The of astatine on the Pauling scale is 2.2, lower than that of lighter like iodine (2.66), indicating reduced electron-attracting power. The covalent is estimated at 150 pm, while the is 202 pm; for ionic forms, the radius of At(VII) in octahedral coordination is 76 pm. Due to its extreme scarcity and radioactivity, direct measurements of bulk physical properties are limited, with most values derived from theoretical calculations, extrapolations from neighboring elements, or tracer-scale experiments. Astatine is predicted to exist as a solid at (20°C, 1 atm), likely appearing as a metallic gray or dark lustrous solid, exhibiting or even metallic character unlike the molecular solids of lighter . First-principles calculations indicate that condensed astatine adopts a monatomic face-centered cubic structure and displays metallic conductivity at ambient conditions, potentially with a Debye temperature of 195 K. The estimated density of solid astatine ranges from 6.35 to 7 g/cm³, reflecting uncertainties in structural models. The is approximately 302°C (575 K), and the around 337°C (610 K), suggesting a narrow range similar to other heavy but with potential deviations due to . These phase behaviors are extrapolated from limited spectroscopic data and computational simulations, as experimental confirmation is hindered by the short half-lives of astatine's isotopes, which cause rapid decay and sample vaporization during handling, restricting studies to quantities below 10⁻¹⁰ g.

Chemical Properties

Astatine, as the heaviest naturally occurring halogen in group 17 of the periodic table, follows the general trend of increasing metallic character down the group, exhibiting amphoteric behavior and post-transition metal-like properties that deviate from the non-metallic nature of lighter such as and . This shift is attributed to its larger atomic size and the influence of relativistic effects on its 6p electrons, which stabilize the 6s orbital and destabilize the 6p orbitals, leading to a more metallic electronic configuration. The element displays multiple oxidation states, including -1, +1, +5, and +7, with +1 being the most stable in due to the formation of like At⁺ and AtO⁺. In its -1 state, astatine forms the astatide ion (At⁻), analogous to other ions, but these astatides are less stable than iodides when reacting with metals to produce compounds such as astatides. Astatine's reactivity as an is weaker than that of iodine, reflecting its reduced tendency to gain electrons, as evidenced by its lower electron affinity of 2.41578(7) eV compared to iodine's 3.059 eV. Relativistic effects significantly alter astatine's chemical behavior by contracting the 6s electrons and expanding the 6p electrons, which diminishes its oxidizing power relative to iodine and contributes to its amphoteric nature, allowing both anionic and cationic species to form stably. In terms of electronegativity, astatine has a Pauling value of 2.2, lower than iodine's 2.66 but higher than polonium's 2.0, indicating a transitional position between and metallic character. Its first is approximately 9.5 eV, lower than iodine's 10.45 eV and higher than polonium's 8.41 eV, further underscoring this intermediate behavior. Astatine exhibits moderate solubility in water, where the At⁻ ion persists, but it tends to hydrolyze readily, particularly in higher oxidation states; for instance, AtO⁺ hydrolyzes to form species like AtO(OH) and AtO(OH)₂⁻ in basic conditions. It also shows solubility in organic solvents, consistent with halogen trends, though its compounds are generally less stable than those of iodine due to relativistic stabilization of lower oxidation states.

History

Discovery

The existence of element 85, positioned as the heaviest member of the group in the periodic table, was anticipated by in his 1869 formulation, where he left a gap below iodine and predicted properties akin to other such as volatility and reactivity with metals. Subsequent chemists, including those in the early , reinforced this prediction by expecting a rare, potentially radioactive analog to iodine in natural minerals. Prior to its synthesis, multiple unsuccessful searches for element 85 in natural sources occurred during . In 1931, American physicist Fred Allison and colleagues at Alabama Polytechnic Institute claimed its detection in minerals using a magneto-optic method, proposing the name "alabamine," but the technique was later proven unreliable and the claim invalidated. Similarly, in 1936, Romanian physicist Horia Hulubei and French physicist Yvette Cauchois reported X-ray spectral lines suggestive of element 85 in radioactive minerals like and , yet these observations could not be reproducibly confirmed. Astatine was first unequivocally produced in 1940 by physicists Dale R. Corson, Kenneth R. MacKenzie, and at the , Berkeley's Radiation Laboratory. They irradiated a target with 32 MeV alpha particles from a 60-inch , generating the astatine-211 through the reaction 209Bi+4He→211At+2n^{209}\text{Bi} + ^4\text{He} \rightarrow ^{211}\text{At} + 2n. The product exhibited alpha radioactivity with an emission energy of 5.87 MeV, and a of 7.3 ± 0.2 hours. Confirmation of its identity as element 85 involved meticulous chemical separations, including with and tests, which demonstrated halogen-like behavior and distinguished the activity from contaminants like and thallium-206. The new element was not carrier-free, but yields were sufficient to verify its through these procedures. This synthetic approach marked the definitive discovery, as natural traces proved too minute for prior isolation.

Naming and Early Research

The name astatine was proposed in 1947 by its discoverers, Dale R. Corson, Kenneth R. Mackenzie, and , deriving from the Greek word astatos, meaning "unstable," to reflect the element's highly radioactive nature. This nomenclature was chosen in keeping with the tradition of naming radioactive elements to emphasize their instability, and the chemical symbol At was suggested alongside the name in the same publication. The International Union of Pure and Applied Chemistry (IUPAC) has since formally recognized At as the standard symbol for the element. Following the initial synthesis in 1940, Segrè and collaborators conducted foundational studies in the 1940s to identify astatine's isotopes and measure their half-lives, confirming the production of astatine-211 through alpha-particle bombardment of bismuth. Their work established that At-211 has a half-life of approximately 7.2 hours, primarily decaying via alpha emission, which provided the first evidence of the element's nuclear properties and distinguished it from lighter halogens. In 1942–1943, Berta Karlik and Traude Bernert identified the natural occurrence of astatine isotopes, such as At-218 and At-215, as decay products in the uranium and actinium series. These experiments laid the groundwork for understanding astatine's position in the periodic table as the heaviest halogen. In the , researchers such as A. H. W. Aten Jr. advanced the chemical characterization of astatine through separation techniques like co-precipitation with silver halides and chromatographic methods, demonstrating its analogy to other in forming similar ionic compounds while exhibiting some metallic traits due to relativistic effects. These studies confirmed astatine's oxidation states, including +1, +3, +5, and +7, and highlighted its tendency to behave more like iodine or in aqueous solutions, though with reduced volatility. Handling astatine presented significant challenges in the mid-20th century owing to its extremely low production yields—typically on the order of or less—and intense , which necessitated remote manipulation and limited sample sizes for experimentation. The first production of larger quantities on the microgram scale was achieved in the 1960s using improved irradiations of targets, enabling more detailed chemical investigations despite ongoing hazards.

Occurrence and Production

Natural Occurrence

Astatine is the rarest naturally occurring element in the , with an estimated total abundance of approximately 25 grams at any given time. This trace presence results exclusively from its formation as short-lived radioactive intermediates in the decay chains of primordial heavy elements, particularly . In the decay series, astatine-218 arises from the of polonium-218, while astatine-219 forms in the series via the of francium-219. All isotopes of astatine are radioactive, with no stable forms, and their half-lives—ranging from milliseconds to a maximum of 8.1 hours for astatine-210—ensure rapid decay that prevents any meaningful accumulation. As a result, all primordial astatine formed during the Earth's early history has long decayed, and current natural astatine exists only in within and thorium-rich minerals. This inherent instability imparts high fugacity to the element, limiting its persistence in geological systems. Astatine has been detected in trace quantities in uranium ores through alpha spectrometry, which identifies the characteristic alpha emissions from its isotopes amid the signatures. Due to these short half-lives and localized production, astatine's distribution across the broader environment remains negligible, with no significant concentrations in the atmosphere, , or .

Synthetic Production

Astatine isotopes, particularly the medically relevant ^{211}At, are primarily produced through artificial nuclear reactions in particle accelerators, as natural sources provide only trace amounts. The standard method involves the of a target with alpha particles in a , following the reaction 209Bi(α,2n)211At^{209}\mathrm{Bi}(\alpha, 2n)^{211}\mathrm{At}. This process was first used to synthesize astatine in by Corson, Mackenzie, and Segrè, who irradiated foil with 32 MeV alpha particles from the 60-inch at the , yielding minuscule quantities sufficient only for initial chemical identification. Over the decades, production techniques have evolved significantly with advancements in accelerator technology, transitioning from low-yield early experiments in the 1940s—where activities were on the order of microcuries—to modern high-current cyclotrons capable of generating curie-level outputs. Contemporary setups use thick bismuth targets, often in metallic form or electrodeposited on substrates, irradiated with alpha beams of 28–29.5 MeV energy to optimize the cross-section while minimizing unwanted ^{210}At production, which has a higher proton emission rate. Yield optimization depends on beam current, irradiation duration, and target cooling; for instance, a 4-hour irradiation at 55 μA and 28 MeV can produce up to 6.6 GBq of ^{211}At at the end of bombardment. Thick-target yields typically range from 16 to 41 mCi/μA·h under these conditions, enabling routine production for preclinical and clinical studies. Alternative synthetic routes exist but are less commonly employed due to lower efficiencies and higher impurity levels. These include proton or deuteron of targets via such as 209Bi(p,π−xn)210−xAt^{209}\mathrm{Bi}(p, \pi^- xn)^{210-x}\mathrm{At}, which generate various astatine isotopes as byproducts but require high-energy beams ( of MeV) and yield far less ^{211}At per incident particle. Similarly, proton of targets, such as 232Th(p,x)211At^{232}\mathrm{Th}(p, x)^{211}\mathrm{At}, produces astatine isotopes alongside other actinides, though this method is primarily explored in accelerator-driven facilities rather than dedicated production. Isotope-specific production, like ^{211}At from , remains favored for targeted alpha therapy applications due to its favorable and higher purity.

Isolation Methods

Isolation of astatine, particularly the radioisotope astatine-211 (At-211), from production targets such as irradiated involves separating it from co-produced contaminants like and while preserving its short of 7.2 hours. Techniques are broadly classified as dry or wet methods, each offering trade-offs in yield, time, and complexity. Dry methods primarily rely on thermal , where the irradiated target is heated to volatilize astatine while leaving behind due to its higher . The process typically occurs in a furnace at 600–750°C under a stream of , with astatine condensing in a cold trap as a dry residue. Electrodeposition from targets at 500–600°C has also been employed, depositing astatine onto electrodes for collection. These approaches complete in under 20 minutes, minimizing decay losses. Wet methods dissolve the target in concentrated , followed by reduction and extraction to isolate astatine in solution. Solvent extraction commonly uses from 8 M HCl medium after target dissolution and acid removal by at ~300°C, with back-extraction into 4 M NaOH. Ion-exchange , such as using tellurium-packed columns, adsorbs astatine from HCl solutions and elutes it with 1–2 M NaOH, achieving separation from and impurities. These processes take 90–120 minutes but allow for finer chemical control. Recent advancements from 2023–2025 focus on to enhance purity and for At-211. Automated systems using extraction dissolve targets in and process through multiple columns, yielding 95% recovery in under 20 minutes while reducing manual handling. In November 2025, researchers at reported a patent-pending automated resin-column trapping method for isolating At-211 from targets, enabling efficient loading and shipping to support national distribution for targeted alpha therapy. Thiol-based labeling post-isolation improves stability of At-211 conjugates, minimizing deastatination for downstream applications. Overall recovery rates for both methods range from 80–90%, though optimized automated wet processes can reach 93%. Challenges include removing residual (target material) and (decay product of At-211's daughter ), which can contaminate samples and reduce purity. Safety considerations emphasize the volatility of astatine , which can lead to airborne contamination; handling occurs in sealed gloveboxes or hot cells with directed gas flows to contain vapors. Strict protocols, including remote operation in automated setups, minimize to personnel.

Isotopes

Known Isotopes

Astatine has 41 known isotopes, spanning mass numbers 188 and 190 to 229, all of which are radioactive and highly unstable, with no stable isotopes observed. The isotopes exhibit a wide range of half-lives, from microseconds to several hours, and decay primarily via alpha emission, electron capture, or beta decay processes. The most stable isotope is ^{210}At, with a half-life of 8.1 hours, decaying predominantly by electron capture to ^{210}Po (99.825%) and to a lesser extent by alpha decay to ^{206}Bi (0.175%). Another key isotope, ^{211}At, has a half-life of 7.214 hours and decays by alpha emission to ^{207}Bi (41.80%) or electron capture to ^{211}Po (58.20%), making it notable for potential medical applications due to its alpha decay properties. The first astatine isotope discovered was ^{211}At in 1940, produced by alpha-particle bombardment of by Corson, Mackenzie, and Segrè. Subsequent isotopes were identified through various methods, including neutron capture on , alpha emission from heavier elements, and fission fragments from or , with discoveries extending into the 2020s via heavy-ion fusion-evaporation reactions at facilities like the National Superconducting Laboratory and the Accelerator Laboratory of the . For instance, the lightest known ^{188}At (half-life 190 μs, decaying by to ^{187}Po followed by ) was synthesized in 2025 using the reaction ^{107}Ag(^{84}Sr,3n)^{188}At, while lighter isotopes such as ^{191}At ( 2.1 ms, ) were synthesized in 2003 using multinucleon transfer reactions. Heavier isotopes like ^{217}At ( 32.3 ms, primarily to ^{213} with 99.99% branching ratio) appear briefly in natural decay chains, such as the actinium series derived from uranium-235. The following table summarizes selected astatine isotopes, focusing on those with relatively longer half-lives or significance in production and decay studies:
Mass NumberHalf-LifePrimary Decay Modes and Branching Ratios (%)
^{209}At5.41(10) hEC/β⁺ (95.9), α (4.1)
^{210}At8.1(4) hEC/β⁺ (99.825), α (0.175)
^{211}At7.214(7) hα (41.80), EC (58.20)
^{207}At1.80(10) hEC/β⁺ (91.4), α (8.6)
^{208}At1.63(10) hEC/β⁺ (99.45), α (0.55)
^{218}At1.5(1) sα (~100)
^{217}At32.3(3) msα (99.99), β⁻ (0.01)
^{216}At0.31(3) msα (~100)

Nuclear Properties

Astatine's isotopes primarily undergo alpha decay and electron capture as their dominant decay modes, reflecting the neutron-deficient nature of most known nuclides in this region of the nuclear chart. Alpha decay involves the emission of a helium-4 nucleus, leading to daughter products like polonium or bismuth isotopes, while electron capture results in the capture of an inner-shell electron by the nucleus, often producing characteristic X-rays. Beta decay, which includes both beta-minus and positron emission, is comparatively rare and typically observed only in a limited number of neutron-richer isotopes, such as those beyond mass 215, where it competes weakly with the other modes. Half-lives of astatine isotopes follow a pronounced trend, peaking around mass number 210 and diminishing rapidly with increasing or decreasing deviation from this value, due to heightened nuclear instability away from the line of beta stability. For instance, isotopes near A=210 exhibit half-lives on the order of hours, whereas those at the extremes span from microseconds to seconds. This pattern arises from the interplay of nuclear binding energies, where deviations amplify Coulomb repulsion among protons (Z=85) relative to the neutron-proton balance. Relativistic effects in the nuclear structure of such heavy elements subtly influence fission barriers and decay widths, though shell corrections dominate the overall stability profile. Specific half-life values for individual isotopes are detailed in the Known Isotopes section. The fissionability of astatine nuclei varies with isotopic mass, remaining low for lighter isotopes (A < 210) where the fission barrier is high due to pronounced shell stabilization, but increasing for heavier ones (A > 215) as the barrier lowers, enhancing competition with evaporation or in compound nucleus reactions. This is evident in spallation-fission studies of astatine compound nuclei formed by heavy-ion bombardments, where fission cross-sections become significant at higher excitation energies. Production cross-sections for key astatine isotopes, such as At-211, are well-characterized for reactions like 209Bi(α,2n), which yield optimal values of about 1 barn at incident alpha-particle energies of 28-30 MeV, enabling efficient synthesis in cyclotrons. Theoretical nuclear models, particularly those incorporating shell effects near Z=85, predict the absence of long-lived isotopes, as the proton subshell closures (e.g., at Z=82) do not extend stability to this atomic number, resulting in all observed half-lives being under 9 hours and reinforcing astatine's inherent radioactivity.

Compounds

Inorganic Compounds

Astatine forms a limited number of inorganic compounds, primarily studied through tracer-scale experiments due to its radioactivity and scarcity. The halides, including astatine monoiodide (AtI), monochloride (AtCl), and monobromide (AtBr), are interhalogen species synthesized by reacting elemental astatine with the corresponding halogen in the vapor phase or via aqueous reactions involving halide ions and oxidants such as chlorine or bromine. For instance, AtI is produced by equilibrating astatine with iodine/iodide solutions, where it exhibits moderate extractability into carbon tetrachloride (distribution constant KD≈5.5K_D \approx 5.5), while AtBr shows lower extractability (KD≈0.04K_D \approx 0.04) and AtCl forms extractable species like HAtCl4_4 in hydrochloric acid. These compounds are unstable in aqueous media, prone to oxidation by atmospheric oxygen or trace impurities, leading to decomposition into higher oxidation states such as At(III) or At(V), which limits their isolation to nano-scale quantities. Interhalogen compounds beyond the monohalides, such as astatine trifluoride (AtF) and trichloride (AtCl3_3), have been inferred from extraction behaviors in solutions. AtCl3_3 is suggested to form in concentrated HCl, where it is extractable into , indicating volatility similar to other interhalogens, though direct synthesis remains challenging due to astatine's tendency to disproportionate. AtF is postulated from reactions with fluorine carriers but lacks confirmed isolation. These species undergo in neutral or basic conditions, forming oxyanions like AtOCl2−_2^- or AtOCl2−^{2-}, and their volatility allows chromatographic separation, with AtCl and AtBr showing reactivity toward aromatic substrates in electrophilic substitutions. Other inorganic compounds include oxides such as astatine trioxide (At2_2O3_3) and possibly astatate (AtO3−_3^-), along with astatanous acid (HAtO). AtO3−_3^- is synthesized by oxidizing astatine with cerium(IV) or hot persulfate in alkaline media, coprecipitating with iodates like Pb(IO3_3)2_2, while At2_2O3_3 appears as an intermediate in milder oxidations. HAtO, representing the +1 oxidation state, is detected as a transient species in photochemical or mild oxidation reactions but hydrolyzes readily. Characterization of these is limited, relying on coprecipitation and electrophoretic mobility rather than bulk properties. Bonding in astatine inorganic compounds is predominantly covalent with partial ionic character, weaker than in iodine analogs due to astatine's increasing metallic tendencies and relativistic effects that reduce bond polarity; for example, the At-I bond in AtI is less polar than I-I. Spectroscopic data derive from At-211 tracer studies, employing alpha-particle and X-ray counting to track distribution coefficients and redox equilibria, with no resolved molecular spectra due to low concentrations (typically 10−10^{-10} to 10−14^{-14} M).

Organic Derivatives

Organoastatine compounds, particularly aryl astatides such as phenylastatine (PhAt), are synthesized through the reaction of astatine with aromatic diazonium salts, enabling the formation of stable carbon-astatine (C-At) bonds suitable for radiolabeling applications. This method, pioneered in early studies, involves the where astatine preferentially reacts with diazonium salts that decompose via radical mechanisms, yielding aryl astatides with high efficiency under mild conditions. These compounds exhibit notable stability compared to aliphatic derivatives, retaining the astatine label for extended periods in biological systems due to the robustness of the aromatic C-At bond. For targeted applications, astatine is incorporated into bioconjugates such as antibodies or peptides using prosthetic groups like N-succinimidyl , which facilitate conjugation to residues on biomolecules. A common example is N-succinimidyl 3-(tri-n-methylstannyl)benzoate, which undergoes astatodestannylation to form the astatine-labeled , followed by coupling to proteins with radiochemical yields often exceeding 70%. This approach allows precise delivery of astatine-211 for therapeutic purposes while maintaining the integrity of the targeting vector. Recent advancements from 2023 to 2025 have focused on thiol-reactive agents, such as maleimide-based probes, to form more stable At-C bonds resistant to detachment . These agents enable conjugation to residues on biomolecules, improving retention rates and reducing off-target effects, with studies demonstrating enhanced stability in serum compared to traditional aryl methods. A pathway for organoastatine compounds in biological environments is oxidative deastatination, particularly in , where oxidants like peroxides cleave the C-At bond, leading to free astatide release. This process, akin to , underscores the need for bond-strengthening strategies in design. Purity of organoastatine derivatives is assessed using (HPLC) for separation and quantification, often achieving resolutions that confirm radiochemical purity above 99%. Autoradiography complements HPLC by visualizing radioactive spots in chromatographic analyses, ensuring detection of trace impurities without mass-based interference.

Applications and Safety

Medical and Research Applications

Astatine-211 (At-211) has emerged as a promising for (TAT) in , particularly for treating cancers such as , neuroendocrine tumors, and radioiodine-refractory differentiated . In TAT, At-211 delivers high-energy alpha particles to tumor cells, enabling precise destruction of malignant tissue while sparing surrounding healthy cells due to the particles' limited penetration depth. This approach leverages At-211's , which emits alpha particles with an average energy of 6.8 MeV, resulting in a short tissue range of 50–100 μm and high (LET) of approximately 100 keV/μm, facilitating dense ionization and irreparable DNA double-strand breaks in targeted cells. Labeling strategies for At-211 typically involve electrophilic destannylation of organotin precursors to attach the to biomolecules, ensuring stable conjugation for delivery. For tumor-specific targeting, At-211 is conjugated to monoclonal antibodies (mAbs) that recognize surface antigens or to small molecules such as prostate-specific membrane antigen (PSMA) inhibitors, allowing selective accumulation in tumors like those in or neuroendocrine cancers. These constructs have demonstrated potent antitumor effects in preclinical models, including tumor growth suppression in and xenografts. Recent advancements from 2020 to 2025 have focused on improving production, stability, and clinical translation of At-211-based therapies. In 2025, researchers at Texas A&M University developed an automated protocol for efficient production and purification of At-211 using cyclotron irradiation of bismuth targets, reducing processing time and enabling shipment of clinically relevant quantities (up to 3700 MBq) to support broader therapeutic access. Enhanced chelation strategies, including the use of macrocyclic ligands and neighboring-group stabilization, have improved in vivo stability of At-211 radiopharmaceuticals, minimizing deastatination and off-target effects. Phase I clinical trials have advanced, notably a first-in-human study of PSMA-targeted [211At]PSMA-5 for metastatic castration-resistant prostate cancer (as of December 2024), which demonstrated preliminary efficacy in tumor uptake via SPECT/CT imaging and was well tolerated in the initial patient, with no severe toxicities reported. Preclinical toxicity studies in mice showed no severe toxicities at doses up to 35 MBq/kg. Similarly, a Phase I trial for refractory thyroid cancer using [211At]NaAt (reported September 2025) showed tolerability and preliminary efficacy, with preclinical data indicating superior DNA damage induction compared to iodine-131. Beyond , At-211 serves as a tracer in research to study uptake mechanisms, revealing enhanced accumulation in differentiated cells when co-administered with iodide. In environmental , At-211 investigations explore its volatility and adsorption behaviors in aqueous and organic media, informing nuclear waste management and atmospheric transport models. A key limitation of At-211 applications is its 7.2-hour , necessitating on-site or regional production to ensure timely delivery for therapy.

Handling and Precautions

Astatine poses significant radioactivity risks due to its isotopes' emissions, primarily and x-rays from , with At-211 undergoing 41% and 59% , leading to internal hazards if or ingested as it concentrates in the thyroid gland. Elemental astatine and its compounds can volatilize, forming aerosols that increase the potential for exposure, while gamma emissions from daughter products like those in At-210 decay chains exacerbate external concerns. Chemically, astatine acts as a strong oxidant similar to other , potentially reacting aggressively with reducing agents or organic materials, though its primary hazard stems from rather than . Its volatility, particularly in the zero , allows it to adsorb onto surfaces or escape into the air, necessitating measures to prevent formation during manipulation. Handling astatine requires stringent precautions, including work in glove boxes or hot cells with high-efficiency ventilation systems to contain volatile species, alongside such as gloves and shielding to minimize exposure. For At-211 handlers, using alpha counters or scintillation detectors monitors levels, adhering to principles of time, , and shielding to reduce dose. Regulatory guidelines for radiohalogens like astatine follow international standards from the (IAEA), emphasizing safe transport, storage, and disposal of radioactive materials to prevent environmental release. Waste management involves segregating astatine-contaminated materials in designated shielded containers for decay or specialized disposal, ensuring compliance with protocols. Historical incidents involving astatine exposure are minimal, attributable to the element's production and use at scales in controlled nuclear laboratories, though general concerns over persist in such settings.

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