|Pronunciation||/ - /, |
|Appearance||unknown, probably metallic|
|Astatine in the periodic table|
Ancient Greek ἄστατος (ástatos) 'unstable'
|Discovery||Dale R. Corson, Kenneth Ross MacKenzie, Emilio Segrè (1940)|
|Isotopes of astatine|
Astatine is a
The bulk properties of astatine are not known with certainty. Many of them have been estimated from its position on the
The first synthesis of astatine was in 1940 by
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 are in any case synthetic and do not occur in nature.
The bulk properties of astatine are not known with any certainty.
Most of the physical properties of astatine have been estimated (by interpolation or extrapolation), using theoretically or empirically derived methods. 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).
Astatine sublimes less readily than does iodine, having a lower
The structure of solid astatine is unknown.
Evidence for (or against) the existence of diatomic astatine (At2) is sparse and inconclusive.
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".
Astatine has an
Less reactive than iodine, astatine is the least reactive of the halogens; the chemical properties of tennessine, the next-heavier group 17 element, have not yet been investigated, however. 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.[d] Like iodine, astatine has been shown to adopt odd-numbered oxidation states ranging from −1 to +7.
Only a few compounds with metals have been reported, in the form of states of sodium, palladium, silver, thallium, and lead. 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.
The formation of an astatine compound with hydrogen – usually referred to as
Astatine is known to bind to
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
Astatine is known to react with its lighter homologs iodine,
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). Scientists tried to find it in nature; given its extreme rarity, these attempts resulted in several false discoveries.
The first claimed discovery of eka-iodine was made by
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. 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.
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).[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", but Karlik and Bernert were again unable to reproduce these results.
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. 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. 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. (Since then, astatine was also found in a third decay chain, the neptunium series.) 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" coming from the Ancient Greek αστατος (astatos) 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.
Corson and his colleagues classified astatine as a metal on the basis of its analytical chemistry. Subsequent investigators reported iodine-like, cationic, or amphoteric behavior. 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."
|Alpha decay characteristics for sample astatine isotopes[g]|
|207||−13.243 MeV||1.80 h||8.6%||20.9 h|
|208||−12.491 MeV||1.63 h||0.55%||12.3 d|
|209||−12.880 MeV||5.41 h||4.1%||5.5 d|
|210||−11.972 MeV||8.1 h||0.175%||193 d|
|211||−11.647 MeV||7.21 h||41.8%||17.2 h|
|212||−8.621 MeV||0.31 s||≈100%||0.31 s|
|213||−6.579 MeV||125 ns||100%||125 ns|
|214||−3.380 MeV||558 ns||100%||558 ns|
|219||10.397 MeV||56 s||97%||58 s|
|220||14.350 MeV||3.71 min||8%||46.4 min|
|221[h]||16.810 MeV||2.3 min||experimentally
There are 41 known isotopes of astatine, with mass numbers of 188 and 190–229. Theoretical modeling suggests that about 37 more isotopes could exist. No stable or long-lived astatine isotope has been observed, nor is one expected to exist.
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.
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,[j] 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-214m1; its half-life of 265 nanoseconds is shorter than those of all ground states except that of astatine-213.
Astatine is the rarest naturally occurring element.[k] The total amount of astatine in the Earth's crust (quoted mass 2.36 × 1025 grams) is estimated by some to be less than one gram at any given time. Other sources estimate the amount of ephemeral astatine, present on earth at any given moment, to be up to one ounce (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)
Isotopes of astatine are sometimes not listed as naturally occurring because of misconceptions that there are no such isotopes, or discrepancies in the literature. Astatine-216 has been counted as a naturally occurring isotope but reports of its observation (which were described as doubtful) have not been confirmed.
|Reaction[l]||Energy of alpha particle|
83Bi + 4
2He → 211
85At + 2 1
83Bi + 4
2He → 210
85At + 3 1
83Bi + 4
2He → 209
85At + 4 1
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
The most important isotope is astatine-211, the only one in commercial use. To produce the bismuth target, the metal is
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.[n] 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.
The astatine-containing cyclotron target is heated to a temperature of around 650 °C. The astatine
The irradiated bismuth (or sometimes
Uses and precautions
Several 211At-containing molecules and their experimental uses 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-labeledbisphosphonates Bone metastases
Newly formed astatine-211 is the subject of ongoing research in nuclear medicine. 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), 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. 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.
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. 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." Significant morbidity in cell culture models of human cancers has been achieved with from one to ten astatine-211 atoms bound per cell.
Astatine ... [is] miserable to make and hell to work with.
P Durbin, Human Radiation Studies: Remembering the Early Years, 1995
Several obstacles have been encountered in the development of astatine-based
Animal studies show that astatine, similarly to iodine—although to a lesser extent, perhaps because of its slightly more metallic nature
- 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 metals.
- It is also possible that this is sorption on a cathode.
- 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.
- Iodine can act as a carrier despite it reacting with astatine in water because these reactions require iodide (I−), not (only) I2.
- 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. Ten years later, the compound was predicted to be non-volatile, out of line with the lighter halogens but similar to radon fluoride; by this time, the latter had been shown to be ionic.
- In other words, some other substance was undergoing beta decay (to a different end element), not polonium-218.
- In the table, under the words "mass excess", the energy equivalents are given rather than the real mass excesses; "mass excess daughter" stands for the energy equivalent of the mass excess sum of the daughter of the isotope and the alpha particle; "alpha decay half-life" refers to the half-life if decay modes other than alpha are omitted.
- The value for mass excess of astatine-221 is calculated rather than measured.
- This means that, if decay modes other than alpha are omitted, then astatine-210 has an alpha decay half-life of 4,628.6 hours (128.9 days) and astatine-211 has one of only 17.2 hours (0.7 days). Therefore, astatine-211 is very much less stable toward alpha decay than astatine-210.
- "m1" means that this state of the isotope is the next possible one above – with an energy greater than – 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.
- Emsley 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.
- A nuclide is commonly denoted by a symbol of the chemical element this nuclide belongs to, 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.
- See however Nagatsu et al. who encapsulate the bismuth target in a thin aluminium foil and place it in a niobium holder capable of holding molten bismuth.
- See also Lavrukhina and Pozdnyakov.
- In other words, where carbon's one s atomic orbital and three p orbitals hybridize to give four new orbitals shaped as intermediates between the original s and p orbitals.
- "Unfortunately, the conundrum confronting the … field is that commercial supply of 211At awaits the demonstration of clinical efficacy; however, the demonstration of clinical efficacy requires a reliable supply of 211At."
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