• Element 85 Astatine At, 1940
• Physicist Dale R. Corson, 1914
• Physicist Kenneth Ross MacKenzie, 1912
• Physicist Emilio Gino Segrè, 1905

Astatine is a radioactive chemical element with the symbol At and atomic number 85. It occurs on the Earth only as the result of decay of heavier elements, and decays away rapidly, so much less is known about this element than its upper neighbors in the periodic table. Earlier studies have shown this element follows periodic trends, being the heaviest known halogen, with melting and boiling points being higher than those of lighter halogens.

Until recently most of the chemical characteristics of astatine were inferred from comparison with other elements; however, important studies have already been done. The main difference between astatine and iodine is that the HAt molecule is chemically a hydride rather than a halide; however, in a fashion similar to the lighter halogens, it is known to form ionic astatides with metals. Bonds to nonmetals result in positive oxidation states, with +1 best portrayed by monohalides and their derivatives, while the higher are characterized by bond to oxygen and carbon. Attempts to synthesize astatine fluoride have been met with failure. The second longest living astatine - 211 is the only one to find a commercial use, being useful as an alpha emitter in medicine; however, only extremely small quantities are used, and in larger ones it is very hazardous, as it is intensely radioactive.

Astatine was first produced by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in the University of California, Berkeley in 1940. Three years later, it was found in nature; however, with an estimated amount of less than 28 grams (1 oz) at given time, astatine is the least abundant element in Earth's crust among non - transuranium elements. Among astatine isotopes, six (with mass numbers 214 to 219) are present in nature as the result of decay of heavier elements; however, the most stable astatine - 210 and the industrially used astatine - 211 are not.

Astatine is a highly radioactive element: its isotopes have their half-lives under half a day, decaying into those of bismuth, polonium, radon, or other astatine isotopes. Among the first 103 elements, only francium and nobelium, the latter of which does not occur in nature, are less stable than astatine. Its extremely short half-life allows only production in microscopic quantities, which limits research into astatine.

Astatine is often cited as either a nonmetal or a metalloid. Its most common properties are normal for a heavier (more metallic than iodine) halogen: Like other halogens, it is composed of diatomic At₂ molecules on standard conditions. Its melting and boiling point suit the trend in halogen series, increasing with the increase in atomic number, measured to be 302 °C (576 °F) and 337 °C (639 °F), respectively. Like iodine, it is also very volatile: at the room temperature, half of astatine vaporizes in an hour if put on clean glass. Most chemical properties, such as formation of the astatide anion, are in line with other halogen as well. However, it has a few metallic properties: it is allocated on a cathode, forms a cation in strong acidic solutions, and so on.

The element is often cited to have the electronegativity of 2.2 (Pauling scale), as this is stated in Pauling's work, lower than that of iodine (2.5 in the original work and 2.66 now) and same as hydrogen; however, experiments have shown that the actual astatine electronegativity is slightly below that of hydrogen. It sublimes more readily than iodine, with lower vapor pressure; it also dissolves in water.

There are 32 known isotopes of astatine, with atomic masses of 191 and 193 – 223. No stable or at least long lived astatine isotope is known, and no such isotope is expected to exist.

Alpha decay energy follows the same trend as other heavy elements. Lighter astatine isotopes have quite high energies of alpha decay, which get lower as the nuclei get heavier. However, astatine - 211, the nucleus with 126 neutrons (126 is a magic number and corresponds to a filled neutron shell) has a significantly higher energy than the previous one. Despite having a similar half-life time with the previous isotope (8.1 hours for ²¹⁰At and 7.2 hours for ²¹¹At), the alpha decay probability is much higher for the latter: 41.81% against only 0.18%. The two following isotopes release even more energy, with astatine - 213 releasing the highest amount of energy of all astatine isotopes. For this reason, it is the shortest lived astatine isotope. Even though heavier astatine isotopes release less energy, no long lived astatine isotope exists; this happens due to increasing role of beta decay. This decay mode is especially important for astatine: as early as 1950, it has been postulated that the element has no beta - stable isotopes (i.e., those that do not beta decay at all). The mode has already been found for all astatine isotopes, except for ²¹³At, ²¹⁴At, ²¹⁵At, ^216mAt, and ²¹⁷At.

The most stable of them is astatine - 210, which has a half-life of 8.1 hours. This isotope's primary decay mode is beta decay to a relatively long lived (compared to astatine isotopes) alpha emitter, polonium - 210. In total, only five isotopes have half-lives exceeding one hour, namely those with mass numbers between 207 and 211. The least stable ground state isotope is astatine - 213, with a half-life of 125 ns. It alpha decays to the extremely long lived (in practice, stable) bismuth - 209.

Astatine has 23 nuclear isomers (nuclei with of one or more nucleons – protons or neutrons – excited). A nuclear isomer may also be called a "meta state," which means 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 it. There may be more than one isomer for one isotope. The most stable of them is astatine - 202m1, which has a half-life of about 3 minutes; this is longer than those of all ground states, except for isotopes with mass numbers 203 – 211 and 220. The least stable one is astatine - 214m1; its half-life of 265 ns is shorter than those of all ground states, except for astatine - 213.

In 1869, 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 there could be the fifth halogen. Before 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, which led to erroneous discoveries.

The first claimed discovery of eka - iodine was made by Fred Allison and associates at the Alabama Polytechnic Institute (now Auburn University) in 1931; the discoverers named element 85 "alabamine" and assigned it the symbol Ab, which were used for a few years. In 1934, however, H.G. MacPherson of University of California, Berkeley, disproved the effectiveness of Allison's device and the validity of this discovery. This erroneous discovery was followed by another claim in 1937, by the chemist Rajendralal De. Working in Dhaka, British India (now 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. The properties he reported for dakin do not correspond to those of astatine, and its identity is not known.

In 1940, the Swiss chemist Walter Minder announced the discovery of element 85 as the product of the beta decay of Radium A (polonium - 218), choosing the name "helvetium" (from Helvetia, "Switzerland"). However, Berta Karlik and Traude Bernert were unsuccessful in reproducing his experiments, attributing the results to contamination of his radon stream (radon - 222 is the parent isotope of 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 A (polonium - 216) beta decay. They named this substance "anglo - helvetium," but Karlik and Bernert were again unable to reproduce these results.

In 1940, Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè finally isolated the element at the University of California, Berkeley. Instead of searching for the element in nature, the scientists actually created it by bombarding bismuth with alpha particles. The name "astatine" comes from the Greek word αστατος astatos, meaning "unstable," due to the created isotope's propensity for radioactive decay (later, all isotopes of the element were shown to be unstable), and the ending "-ine," found in the names of the four previously discovered halogens. Three years later, astatine was found as a product of natural decay chains by Karlik and Bernert. Since then, astatine has been found in three out of the four natural decay chains.

Astatine is the rarest naturally occurring element among those that do not belong to the transuranium elements series, with the total amount in Earth's crust estimated to be less than 28 grams (1 oz) at given time. Astatine present at the formation of the Earth has long since decayed; natural astatine at present has formed through the decay of heavier elements. Previously thought to be the rarest element occurring on the Earth, astatine has lost this status to berkelium, atoms of which can be produced by neutron capture reactions and beta decay in very highly concentrated uranium - bearing deposits.

Six astatine isotopes occur naturally: these are astatine - 214 to - 219. Because of their short half-lives, they are found only in trace amounts. Although astatine is found on the Earth, there is no data of astatine occurrence in stars.

Four out of these isotopes (²¹⁵At, ²¹⁷At, ²¹⁸At, and ²¹⁹At) are found there due to their production in major natural decay chains. The father isotope of astatine - 219, francium - 223, alpha decays with a probability of only 0.006%, making this astatine isotope extremely rare even compared to other astatine isotopes, although its half-life is the longest of the natural astatine isotopes at 56 seconds. This rare isotope decays to polonium - 215, which itself beta decays to astatine - 215 with an even smaller probability of 0.00023%; for this reason, the Americas to a depth of sixteen kilometers (10 mi) contains only a trillion astatine - 215 atoms at given time. Astatine - 218 is found in nature as result of polonium - 218 beta decay; like francium - 223 and polonium - 215, decay to an astatine isotope is not the primary decay mode. Therefore, most Earth's astatine is astatine - 217, whose father (francium - 221) decays exclusively to this nuclide; its fathers, grandfathers, so on, decay to exclusively to one exact nuclide, to make only one possible way for the starting nuclide in the neptunium series, neptunium - 237, to decay to astatine - 217.

The other remaining isotopes (²¹⁴At and ²¹⁶At, as well as ²¹⁵At) are found as the result of triple alpha decay of the naturally present protactinium isotopes: ²²⁶Pa, ²²⁷Pa, and ²²⁸Pa. These, however, are extremely rare, so they are often even not cited as natural astatine isotopes.

Astatine is the least reactive of the halogens, being less reactive than iodine; however, multiple compounds of astatine have been synthesized in microscopic amounts and studied as intensively as possible before their inevitable radioactive disintegration. The reactions are normally tested with dilute solutions of astatine mixed with larger amounts of iodine. The iodine acts as a carrier, ensuring that there is sufficient material for laboratory techniques such as filtration and precipitation to work.

The most common compound of the element is hydrogen astatide. The hydrogen astatide molecule has been calculated to have a dipole moment of 0.06 debyes, with hydrogen carrying the partial negative charge. Because astatine has a lower electronegativity when compared to hydrogen (unlike the other halogens), the molecule should more properly be called astatine monohydride; this reversal of polarity partially explains its lower stability compared to the other hydrogen halides. As it is easily oxidized, it is precipitated in aqueous nitric acid / silver (I) solution, forming silver (I) astatide, AgAt.

Astatine is known to react with its lighter homologues iodine, bromine, and chlorine in the vapor state; this reaction produces diatomic interhalogen compounds, with formulas AtI, AtBr, and AtCl. 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; in a chloride solution, they may turn to species like AtCl−
2 or AtBrCl⁻ via equilibrium. No report of gas phase AtCl preparation has been shown, but oxidation of the element with dichromate (in nitric acid solution) showed that adding chloride turned the astatine into a molecule, either AtCl or AtOCl; similarly, AtOCl−
2 or AtCl−
2 may be produced. In a plasma ion source mass spectrometer, similar ions [AtI]⁺, [AtBr]⁺, and [AtCl]⁺ have been formed by introducing vapors of the lighter halogens to the helium filled camera where astatine is situated, supporting the existence of stable neutral molecules in the plasma ion state. No astatine fluoride has been discovered yet, and although its synthesis is thought to be possible, it may require a liquid halogen fluoride solvent; this has already been used for characterization of radon fluorides.

The lower oxidation states are the starting point for astatine – oxygen bonds: treating them with an oxygen containing oxidizer leads to the formation of astatate ions, AtO−
3. Further oxidation, e.g., by hypochlorite or electrochemical oxidation, was originally thought to form unstable astatine (VII), either as perastatic acid H₅AtO₆ (analogous to periodic acid) or perastatate AtO−
4; however, this has never been confirmed. The intermediate states coprecipitate with several silver (I), thallium (I) or caesium oxysalts to form cationic astatine. Astatine may also replace a hydrogen atom in benzene to form C₆H₅At, which may be oxidized to C₆H₅AtCl₂ by chlorine; by treating this compound in an alkaline solution of hypochlorite, C₆H₅AtO₂ may be produced. Astatine may form bonds to the other chalcogens, such as S₇At⁺ and At(CSN)−
2 with sulfur, a coordination selenocarbamide compound with selenium, and astatine – tellurium colloid with tellurium. Additionally, astatine is known to bind to nitrogen, lead, and boron.

Astatine was first produced by bombarding bismuth with energetic alpha particles; this is still the major route used to create the relatively long lived isotopes astatine - 209 through - 211. Astatine is only produced in microscopic quantities, with modern techniques allowing production runs of 2 terabecquerel (about 25 μg).

The most important isotope is now astatine - 211, being the only one to find a commercial use. To produce the bismuth target, the metal is sputtered on a gold, copper, or aluminum surface, to form a 50 – 100 milligrams per centimeter squared bismuth layer (or, alternatively, bismuth oxide is pressed into a copper plate). The target is kept under a chemically neutral nitrogen atmosphere and is cooled with water to prevent the premature astatine vaporization. In a particle accelerator, such as a cyclotron, alpha particles are collided with bismuth. Even though bismuth is composed of only one isotope, bismuth - 209, the reaction may occur in three possible ways, producing astatine - 209, - 210, and - 211. In order to eliminate the undesired nuclides, the maximum energy of the reaction is set to 28 MeV.

Since the element is the main product of the synthesis, after its formation, it only must be separated from the target and the traces of other radioisotopes. The target (with astatine in it) is heated to 270 °C (520 °F) to vaporize away the volatile traces of various radioisotopes, after which the temperature is raised to 800 °C (1450 °F). 80% of astatine may be distilled, but bismuth begins to vaporize as well. Its vaporization does not occur on temperatures below 600 °C (1100 °F), but on these temperatures, astatine volatility from bismuth surface decreases significantly. The distillate is collected on a water cooled surface, which is put in a U-like quartz vessel. The vessel is heated to 130 °C (270 °F) to remove the further traces of impurities (namely polonium) and then to 500 °C (930 °F) to remove astatine, which is collected on a cold finger. The element is then washed off it with a weak nitric acid solution. Using this technique, up to 30% of astatine may result.

The newly formed astatine - 211 is important in nuclear medicine. Once produced, astatine should be used quickly, as astatine - 211 decays with a half-life of 7.2 hours; this, however, is long enough to permit multi - step labeling strategies. Astatine - 211 can be used for targeted alpha particle radiotherapy, since it decays either via alpha decay to bismuth - 207, or via electronic capture to an extremely short lived nuclide of polonium - 211, which itself alpha decays.

Similarly to iodine, astatine is collected by the thyroid gland, even though to a lesser extent; however, it concentrates in the liver if released to the body if the form of a radiocolloid. The principled behavior difference between astatine - 211 and iodine - 131 (a radioactive iodine isotope, also used in medicine) is that astatine does not destroy the neighboring parathyroid gland, as it does not emit beta particles: an average alpha particle released by astatine - 211 runs about 70 µm, while a beta particle emitted by iodine - 131 runs about 2 mm.

Because of its short half-life and small particle run, astatine is considered preferable to iodine - 131 in the diagnosis of diseases. However, it attacks the thyroid gland much more strongly, and the repetitive injection of the nuclide caused tissue destruction in the gland, followed by dysplasia in rats and monkeys; this should be true for all living organisms. When lethal quantities are added, morphological changes in other tissues are not found, with the possible exception of the breasts.

Dale R. Corson led the university through the final years of the Vietnam War and student activism, and through the economic recession of the 1970s. His role was to return the university to stability: to concentration on research, teaching, and scholarship.

Corson brought together the state and endowed components of Cornell, forming one university enjoying public and private support, as envisioned by White and Cornell and articulated by Jacob Gould Schurman. Significant support was provided for the research programs at Arecibo, the Wilson Synchrotron Laboratory, and the Nanofabrication Facility. He revitalized the Department of Geology, expanded the Division of Biological Sciences, and added new programs, such as Medieval studies. The I.M. Pei designed Herbert F. Johnson Museum of Art was completed. He encouraged such multidisciplinary programs as Science, Technology, and Society, the Materials Science Center, environmental programs, radio physics, and space research.

The status of women on campus was greatly improved during the Corson presidency. A Women's Studies Program was formally established in 1972. A Provost's Advisory Committee on the Status of Women was created and presented specific recommendations. The university's policy statement on equal opportunity was changed to include gender among the proscribed criteria with regard to admission to the university. New employment procedures were implemented, and increasing numbers of women were appointed to the faculty and to high administrative positions. Corson provided support for the Africana Studies and Research Center, which had developed from the black studies movement. He recommended the formation of an Affirmative Action Advisory Board to monitor the status of women and minorities and to propose more effective procedures.

During his presidency, university governance was overhauled including the establishment of a faculty - student - employee University Senate and the addition of Student and Employee representatives to the Board of Trustees. A new campus judicial system and campus code of conduct were established.

Dale R. Corson also served on NACA"s Special Committee on Space Technology also called the Stever Committee, named after its chairman. It was a special steering committee that was formed with the mandate to coordinate various branches of the Federal government, private companies as well as universities within the United States with NACA's objectives and also harness their expertise in order to develop a space program. Dr. Corson therefore played a pivotal role in the process of establishing the nascent United States space program.

He was the author of a very important textbook on electromagnetism with the following two editions:

• Corson, D.R. and Lorrain, P. Introduction to electromagnetic fields and waves, W. H. Freeman, 1962.
• Lorrain, P. and Corson, Dale R. Electromagnetic Fields and Waves, 2nd ed., W. H. Freeman, 1970.

The latter incorporated some of the ideas of relativistic electromagnetism.

As part of his Ph.D. work in UC Berkeley, Corson was the co-discoverer of the element Astatine. In 1987 he was awarded the Public Welfare Medal from the National Academy of Sciences.

Segrè was born into a Sephardic Jewish family in Tivoli, near Rome, and enrolled in the University of Rome La Sapienza as an engineering student. He switched to physics in 1927 and earned his doctorate in 1928, having studied under Enrico Fermi.

After a stint in the Italian Army from 1928 to 1929, he worked with Otto Stern in Hamburg and Pieter Zeeman in Amsterdam as a Rockefeller Foundation fellow in 1930. Segrè was appointed assistant professor of physics at the University of Rome in 1932 and served until 1936. From 1936 to 1938 he was Director of the Physics Laboratory at the University of Palermo. After a visit to Ernest O. Lawrence's Berkeley Radiation Laboratory, he was sent a molybdenum strip from the laboratory's cyclotron deflector in 1937 which was emitting anomalous forms of radioactivity. After careful chemical and theoretical analysis, Segrè was able to prove that some of the radiation was being produced by a previously unknown element, dubbed technetium, and was the first artificially synthesized chemical element which does not occur in nature.

He was a colleague and close friend of Ettore Majorana, who disappeared mysteriously in 1938.

While Segrè was on a summer visit to California in 1938, Benito Mussolini's fascist government passed anti - Semitic laws barring Jews from university positions. As a Jew, Segrè was now rendered an indefinite émigré. At the Berkeley Radiation Lab, Lawrence offered him a job as a Research Assistant — a relatively lowly position for someone who had discovered an element — for US$ 300 a month. However, in Segrè's recollection, when Lawrence learned that Segrè was legally trapped in California, he reduced his salary to US$ 116 a month which many, including Segrè, saw as exploiting the situation. Segrè also found work as a lecturer of the physics department at the University of California, Berkeley.

While at Berkeley, he helped discover the element astatine and the isotope plutonium - 239 (which was later used to make Fat Man, the atom bomb dropped on Nagasaki). He found in April 1944 that Thin Man, the proposed plutonium "gun - type" bomb, would not work (because of the presence of Pu-240 impurities), and priority was given to Fat Man, the plutonium "implosion" bomb.

From 1943 to 1946 he worked at the Los Alamos National Laboratory as a group leader for the Manhattan Project. In 1944, he became a naturalized citizen of the United States. He taught at Columbia University, University of Illinois and University of Rio de Janeiro. On his return to Berkeley in 1946, he became a professor of physics and of the history of science, serving until 1972.

Professors Emilio Segrè and Owen Chamberlain were co-heads of a research group at the Lawrence Radiation Laboratory. Their group proposed the experiment to discover the anti - proton and this was the chief reason that the Bevatron was built at LRL. The Bevatron was designed to reach proton energies of 6.2 m₀c²where m_o is the rest mass of the proton. With the new Bevatron, the Segrè / Chamberlain group produced the first anti - proton (as seen in bubble chamber pictures) and the two shared the 1959 Nobel Prize in Physics for their work.

In 1970, Segrè published a biography of Fermi (Enrico Fermi: Physicist, University of Chicago Press)

In 1974 he returned to the University of Rome as a professor of nuclear physics.

Segrè was also active as a photographer, and took many photos documenting events and people in the history of modern science. The American Institute of Physics named its photographic archive of physics history in his honor. Segrè died at the age of 84 of a heart attack.