Astatine

Astatine 化学特性,用途語,生産方法

• 解説

  アスタチン，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．

• 化学的特性

  radioactive crystal(s) halogen with 20 isotopes; heaviest of the halogens; 209At, t1/2=5.5 h; 210At, t1/2=8.3 h; more metallic than iodine; preparation: from Bi by α-particle bombardment; possible medical uses, concentrates in thyroid gland [HAW93] [MER06]

• 物理的性質

  Physical properties of this element have not been well investigated due to short half-lives of isotopes. The element is volatile; may be distilled in vacuum at room temperature in a glass apparatus; and condensed in a dry ice trap. It is soluble in chloroform, ether, hexane and many other organic solvents. Solubility in water should be of low order.

• 天然物の起源

  Astatine is one of the rarest elements in nature. Extremely small amounts of short-lived isotopes At-215, At-217, At-218 and At-219 are naturally found occurring in equilibrium with uranium, neptunium and thorium isotopes. The element was named by Corson, MacKenzie and Segre who produced the first of its isotope At-211 in 1940 by bombarding bismuth with alpha particles. Since then many isotopes in the mass range 200 to 219 have been synthesized. All isotopes, however, are unstable, their half-lives ranging between a few microseconds to less than ten hours. The most stable ones are At-210, At-211 and At-209. No use of this element is known so far.

• 危険性

  Exposure to radiation may cause cancer. Studies on experimental animals show it induces tumors.