astatine

astatine 化学特性,用途語,生産方法

• 解説

  At．原子番号85の元素．電子配置[Xe]4f 145d106s26p5の周期表17族人工放射性ハロゲン元素．ギリシア語の“不安定”を意味するαστατοσから名づけられた．1940年，カリフォルニア大学でD.R. Corson，K.R. MacKenzie，E. SegréによってBiをα粒子で衝撃して得られた．現在，知られている同位体核種は，194～223の範囲に核異性体を含めて41種．質量数210の核種がもっとも長寿命で半減期8.1 h．211Atは7.2 h，209Atは5.4 h．209Atと210Atは主として β＋ 崩壊，211Atは β＋ 崩壊とα崩壊がほぼ同じ割合で起こる．天然にも，ごく微量の218Atがウラン系列，215At，219Atがアクチニウム系列，216Atがトリウム系列の分岐中間崩壊生成核種として存在する．Biのα粒子衝撃後，ターゲットから昇華法で比較的長寿命の209,210,211Atが得られるが，化学実験用のトレーサーとしてしか用途はない．融点302 ℃，沸点337 ℃ のハロゲン．一般的性質はヨウ素に似るが，ヨウ素より金属性である．ヨウ素と同様，人体に入ると甲状腺に集中する．ハロゲン間化合物AtCl，AtBr，AtIが合成された．酸化数－1，1，3，5，7．水に微溶，ベンゼン，四塩化炭素，二硫化炭素に可溶．[CAS 7440-68-8]
  森北出版「化学辞典（第2版）

• 物理的性質

  Astatine is located just below iodine, which suggests that it should have some of the samechemical properties as iodine, even though it also acts more like a metal or semimetal thandoes iodine. It is a fairly heavy element with an odd atomic number, which assisted chemistsin learning more about this extremely rare element. The 41 isotopes are man-made in atomicreactors, and most exist for fractions of a second. The element’s melting point is about 302°C,its boiling point is approximately 337°C, and its density is about 7g/cm³.

• 同位体

  All 41 isotopes of astatine are radioactive, with half-lives ranging from 125nanoseconds to 8.1 hours. The isotope As-210, the most stable isotope with an 8.1-hour half-life, is used to determine the atomic weight of astatine. As-210 decays byalpha decay into bismuth-206 or by electron capture into polonium-210.

• 名前の由来

  From the Greek word astatos, which means “unstable.” All of its isotopes are unstable.

• 天然物の起源

  Chemists of the early twentieth century tried to find the existence of element 85, whichwas given the name “eka-iodine” by Mendeleev in order to fill the space for the missing elementin the periodic table. Astatine is the rarest of all elements on Earth and is found in onlytrace amounts. Less than one ounce of natural astatine exists on the Earth at any one time.There would be no astatine on Earth if it were not for the small amounts that are replenishedby the radioactive decay process of uranium ore. Astatine produced by this uranium radioactivedecay process soon decays, so there is no long-term build up of astatine on Earth. Theisotopes of astatine have very short half-lives, and less than a gram has ever been produced forlaboratory study.

• 特性

  Astatine is the heaviest and densest of the elements in group 17 (VIIA). It is difficult todetermine the chemical and physical properties and characteristics of astatine because it ispresent in such small quantities that exist for extremely short periods of time. Many of itscharacteristics are inferred through experiments rather than by direct observations.

• 来歴

  Synthesized in 1940 by D. R. Corson, K. R. MacKenzie, and E. Segre at the University of California by bombarding bismuth with alpha particles. The longest-lived isotope, ²¹⁰At, has a half-life of only 8.1 hours. Thirty-six other isotopes and isomers are now known. Minute quantities of ²¹⁵At, ²¹⁸At, and ²¹⁹At exist in equilibrium in nature with naturally occurring uranium and thorium isotopes, and traces of ²¹⁷At are in equilibrium with ²³³U and ²³⁹Np resulting from interaction of thorium and uranium with naturally produced neutrons. The total amount of astatine present in the Earth’s crust, however, is probably less than 1 oz. Astatine can be produced by bombarding bismuth with energetic alpha particles to obtain the relatively long-lived 209–211At, which can be distilled from the target by heating it in air. Only about 0.05 μg of astatine has been prepared to date. The “time of flight” mass spectrometer has been used to confirm that this highly radioactive halogen behaves chemically very much like other halogens, particularly iodine. The interhalogen compounds AtI, AtBr, and AtCl are known to form, but it is not yet known if astatine forms diatomic astatine molecules. HAt and CH3At (methyl astatide) have been detected. Astatine is said to be more metallic that iodine, and, like iodine, it probably accumulates in the thyroid gland.

• 使用

  In water solution astatine resembles iodine in some of its chemical and physical properties.Both are powerful oxidizing agents.
  It has limited use in medicine as a radioactive source. It concentrates in the thyroid glandjust like iodine, which makes it a useful radioisotope tracer.
  Because of its isotopes’ short half-lives and its scarcity, astatine has few practical uses outsideof the laboratory for research, where less than a gram has ever been produced.

• 定義

  Nonmetallic element of atomic number 85. Group VIIA of periodic table, aw 211. Heaviest member of the halogen family, has 20 isotopes, all radioactive; derived by α-bombardment of bismuth. The two most stable isotopes have halflives of approximately 8 hours. Astatine occurs in nature to the extent of approximately one ounce in the entire earth’s crust. Like iodine, it concentrates in the thyroid gland. Its use in medicine is still experimental.

• 危険性

  The major hazard is from the radiation of astatine’s isotopes. However, given that theseisotopes have very short half-lives, they do not pose a great long-term danger. Even so, astatineis considered a dangerous element that is a radioactive poison and carcinogen. It has beendemonstrated that astatine causes cancer in laboratory animals.

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