Astatine is one of the most mysterious elements of the periodic table. To begin with, it is the rarest naturally occurring element on Earth. It only exists as a minor spur on an obscure pathway of uranium and thorium decay. A pure sample of elemental astatine has not yet been isolated by scientists. The rarity and radioactive nature of element 85 grant to its mystery as it cannot be observed or weighed in a conventional way. The earlier studies of astatine were entirely theoretical and were based on a general notion that one can learn a lot about someone by meeting their family; the same is true for this element. The color of astatine is suggested to be black based on the increasingly dark shade from fluorine to iodine. Besides being a member of the halogen family, it also lies on a diagonal line containing metalloids like boron and silicon, and therefore, it can not be said with absolute certainty that astatine is either a non-metal or metalloid, perhaps it is a semiconductor.
Discovery and Naming
The periodic table published by Russian chemist Dmitri Mendeleev in 1869, suggested the first prediction of Astatine. Mendeleev used the prefixes “eka-, dvi- or dwi-, and tri-,” taken from the Sanskrit names of digits 1, 2, and 3, depending upon whether the predicted element was one, two, or three places down from the known elements of the same group in his table. After establishing the physical basis of the classification of chemical elements, Neil Bohr suggested the name “eka-iodine” for element no. 85. The search for Astatine goes back to the year 1922, and it includes a fascinating history in regards to its discovery, confirmation, and naming. The first claim about the discovery of Astatine was made around the year 1931 when Fred Allison, along with his associates, attempted to discover eka-iodine at the Alabama Polytechnic Institute. He suggested the name Alabamine to honor the state where the work was done. However, his claims were disproved later as his methods could not replicate the results, and the equipment was found to be faulty. Several unsuccessful claims were reported during the 1920s and 1930s by scientists from several countries including the UK, Germany, British India, Denmark, France, and Switzerland. By the end of the 1930s, Horia Hulubei and Yvette Cauchois claimed to have discovered X-ray wavelengths for three spectral lines of eka-iodine in the emission spectra of radon. Nevertheless, their claims were rejected by the Austrian chemist Fredrich Paneth due to the lack of substantial standards to enable identification. Another unsuccessful assertion to isolate astatine came about in 1940 by a Swiss chemist, Walter Minder. He announced the discovery of element 85 as the beta decay product of radium A (polonium-218) and chose the name “helvetium” as in Helvetia, the Latin for Switzerland. However, his claims also met the same fate once he was unable to reproduce his results.
Emilio Segrè, one of the discoverers of the main-group element astatine
Later in 1940, at the University of California, Berkeley, three scientists named Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè took a different approach rather than looking for eka-iodine in nature. They bombarded bismuth-209 with alpha particles in a cyclotron (particle accelerator) and saw several forms of radiation including the emission of α, γ, and X-rays, and also low energy electrons, all having a half-life of about 7.5 hours. Moreover, the confirmation of astatine took three more years after the announcement of its discovery. In early 1947, Nature (a British Weekly Scientific Journal) published a letter signed by Dale R. Corson, Kenneth Ross MacKenzie, and Emilio Segrè in which they suggested the name “astatine,” derived from a Greek word “astatos” for unstable for the element 85.
Isotopes
Currently, there are 39 known radioactive isotopes of astatine ranging from mass number 191 to 229 with the maximum half-life reaching 8.1 hours for _{ }^{210}{At}. However, 37 more isotopes are theoretically possible, which are yet to be confirmed, with none of them being stable. Like other heavy radionuclides, astatine also follows the same alpha decay energy trends. However, _{ }^{211}{At} makes an exception by having significantly high energy due to the presence of 126 neutrons that corresponds to a nucleus with magic no. of neutrons. Of all the possible isotopes, only five have half-lives greater than one hour. These are _{ }^{207}{At} (1.80 hr), _{ }^{208}{At} (1.63 hr), _{ }^{209}{At} (5.41 hr), _{ }^{210}{At} (8.1 hr), and _{ }^{211}{At} (7.21 hr). The shortest-lived isotope found to date is _{ }^{209}{At} and has a half-life of 125 nanoseconds. It undergoes α decay to the extremely long-lived _{ }^{209}{Bi} with the half-life of {1.9 × 10}^{19} years. Moreover, astatine has 24 nuclear isomers, a metastable state of an atomic nucleus in which one or more nucleons (protons or neutrons) occupy higher energy levels than the ground state energy of the same nucleus out of which the most stable isomer is _{ }^{202m1}{At} with a half-life of about 3 minutes.
Occurrence
Astatine holds the title of the rarest naturally occurring element with the total amount in earth’s crust estimated to be less than one gram at any given time. It occurs only as a decay product of various heavier elements. A sample of the pure element has never been assembled because any macroscopic specimen, if put together, would be immediately vaporized by the heat of its own radioactivity. Four isotopes of astatine were subsequently found to be naturally occurring, although much less than one gram is present at any given point in time in the Earth’s crust. However, it can be produced in the lab by bombarding bismuth isotopes with alpha-particles. The resulting astatine is short-lived, with a half-life of just over 7 hours and hence, it is necessary to prevent it from being evaporated by cooling the bismuth target during irradiation. The quantity of thus produced astatine is limited to a few micrograms.
_{83}^{209}{Bi} + _{2}^{4}{He} → _{85}^{211}{At} +2 _{0}^{1}{n}
Properties of Astatine
Astatine is a very rare and radioactive element, which makes it quite difficult to work with. If one were to assemble a visible piece of astatine, it would immediately be vaporized due to heat generated by its own radioactive decay. Although the irradiation of astatine can be prevented to a certain degree by cooling, this method is not enough to produce a sample big enough for a complete understanding of its properties. It is also interesting that astatine may only be found in uranium-235 ores, nuclear facilities, and/or research laboratories. Otherwise, the element is undetectable.
Physical Properties
Astatine is an element in the periodic table with atomic no. 85 and atomic mass 210 grams per mol. Some of the physical properties of astatine are based on the appearances of the other halogen group elements. Hypothetical solid astatine may likely be very dark in colour, perhaps even black. Although astatine has not been yet produced in enough quantities to study its physical nature, at standard temperature and pressure conditions, it is predicted to be solid.
Based on the trends followed by the halogen elements, the melting point and the boiling point of astatine is estimated to be around 302 and 337 °C, respectively. Likewise, the structure of astatine is also uncertain. As an analogue of iodine, it may have an orthorhombic crystalline structure. Whether astatine is diatomic or not is debatable among the scientific community around the world.
Takahashi, a research scientist from Okayama University, reported interactions of astatine with benzene, toluene, and monochlorobenzene in his work in 1986. In this study, the chemical bond between {At}_{2} is quoted to cleave in a two-step mechanism that worked for benzene and toluene but not for monochlorobenzene. Condensed astatine is calculated to behave like a metal at 1 atm of pressure and might be a superconductor but would be monoatomic.
Chemical Properties
The electronic configuration of astatine is [Xe] {4f}^{14}{5d}^{10}{6s}^{2}{6p}^{5}. Extreme radioactivity and insufficient concentrations of astatine on earth cloud the degree of certainty up to which its chemical properties can be explored. Among the first 101 elements in the periodic table, only francium is less stable than astatine. Many of its apparent chemical properties have been observed using tracer studies on extremely dilute astatine solutions. Out of all the natural halogens, astatine is the least chemically reactive. Like other halogen elements, astatine also has several oxidation states, such as -1, 1, 3, 5, and 7, with -1 and 1 being the most common. Astatine can co-precipitate with metal sulphides in a solution of hydrochloric acid, and therefore, shows some metallic characteristics by forming a monoatomic cationic aqueous solution. Astatine has an electronegativity of 2.2 and first ionization energy of 899 kJ/mol. 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.
Compounds of Astatine
Although astatine is the least reactive member in the halogen family, it has the ability to form compounds with elements including metals, hydrogen, boron, and carbon. It can also react with oxygen and other chalcogens, and form compound with chlorine, bromine, and iodine. Though there are only a few compounds of astatine with metals such as sodium, magnesium, palladium, silver, thallium and lead. However, given the extraordinarily limited amount of available astatine, estimations, by extrapolation, of the characteristics of AgAt and NaAt have been made based on other metal halides. A variety of boron cage compounds have been made with At-B bonds stronger than At-C bonds. Scientists have also been able to prepare the carbon compounds of astatine such as tetraastatide ({C}{At}_{4}) and astatobenzene ({C}_{6}{H}_{5}{At}). Astatobenzene can also be oxidized to {C}_{6}{H}_{5}{At}{Cl}_{2} by exposure to {Cl}_{2}. This chlorinated compound can then be treated with a basic hypochlorite solution to give {C}_{6}{H}_{5}{At}{O}_{2}.
Reactions of astatine in the vapour state with chlorine, bromine, and iodine have produced the diatomic compounds AtCl, AtBr, and AtI. AtBr is also formed by interacting astatine with iodine monobromide and bromide solution, whereas AtI is prepared by reacting astatine with iodine/iodide solution in an aqueous media. To ensure a reaction, dilute solutions of astatine are mixed with larger amounts of iodine. Iodine, acting as a carrier, ensures that there is sufficient material for techniques such as filtration and precipitation to be properly conducted. Excess of iodide or bromide could lead to the formation of {At}{Br}_{2} – and/or {At}{I}_{2} – ions or if in a chloride solution, species such as AtBrCl- or {At}{Cl}_{2} – can come about via equilibrium reactions with the chlorides.
Uses of Astatine
Although scientists have not been able to synthesize astatine in notable amounts, several studies have shown that even the fairly small amounts could lead to its significant potential applications in various sectors. However, astatine is fairly expensive to be employed on a vast scale. Several factors, such as the cost of production, extreme rarity, high radioactivity, and the cost of handling and transportation, results in the per gram cost of astatine being around 100$. So far, the most common application of astatine is in the medical sector.
Lab-technician preparing equipment for endoradiotherapy
The most appealing approach for cancer treatment is targeting radiotherapy and endoradiotherapy due to their potential for delivering curative doses of radiation to the tumour while sparing normal tissues. An important consideration in targeted radiotherapy is the selection of a radionuclide that decays by the emission of radiation with a tissue range and cytotoxicity that is appropriate for a particular clinical application. Radionuclides that decay by the emission of α-particles such as the heavy halogen astatine-211 (_{}^{211}{At}) offer the exciting prospect of combining cell-specific molecular targets with radiation having a range in the tissue of only a few cell diameters. 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. Also, the short half-life and limited penetrating power of alpha radiation emitted from _{}^{211}{At} through tissues offer advantages over _{}^{131}{At}. A typical α-particle released by _{}^{211}{At} may travel only 70 mm through bodily tissues, whereas an average β-particle emitted by _{}^{131}{At} can travel more than 25 times farther inside the tissues. Given this short path length of α-particles in the human body and their high energy (6.0 to 7.5 MeV), such particles are very effective at killing cells bound by carrier-targeting agents in situations where the tumour burden is low and/or malignant cell populations are located close to essential normal tissues.
Radiotherapy
Astatine is far too rare to have many applications. Apart from medicine and research, there are currently no uses of astatine.
Health Aspects
Regardless of its extreme radioactivity, the health effects of astatine are not much of concern because it is present in very minute amounts in the atmosphere. However, many animal studies have shown that overexposure of astatine can lead to severe damage to the thyroid gland. Trace amounts of astatine can be handled safely in fume hoods if they are well-aerated; however, it is advised that the biological uptake of astatine must be avoided as it can cause morphological effects on various organs such as lungs, liver, and spleen.