Astatine holds a unique record: it is the 最稀有的天然元素 on Earth. Estimates suggest that the total amount of astatine present in the Earth's crust at any given time is less than 30克. This extreme rarity is due to the fact that all its isotopes are radioactive with very short half-lives. The longest-lived, astatine-210, has a half-life of only 8.1 hours. Thus, any primordial astatine present during the formation of the Earth has long since decayed. The astatine present today is constantly 重新创建 as an intermediate decay product in the natural uranium and thorium decay chains, but it disappears almost immediately.
砹的几种同位素出现在铀和钍的衰变链中，通过其母体元素钋或铋的β衰变形成。例如：
这些同位素以极微量产生，且寿命极短，无法累积。
Apart from radioactive decay, astatine can be produced naturally, in even smaller quantities, by 宇宙散裂: when high-energy cosmic rays strike heavier nuclei such as lead or bismuth in the Earth's crust, they can fragment them and produce exotic nuclei, including astatine.
砹位于元素周期表中核稳定性极低的区域。对其同位素（尤其是具有"幻数"中子的同位素，如含126个中子的\(^{211}\mathrm{At}\)）的研究，有助于核物理学家完善核结构模型，并理解重核稳定性的极限。
The existence of astatine was predicted by 德米特里·门捷列夫 as early as the creation of his periodic table in 1869. He gave it the provisional name of "eka-碘", predicting that it would be a halogen heavier than iodine, with similar chemical properties but a higher atomic mass and probably metallic characteristics. The search for this missing element mobilized several chemists for decades, without success, due to its extreme instability.
Astatine was finally artificially produced in 1940 by a team of researchers at the University of California, Berkeley: 戴尔·R·科森、肯尼斯·罗斯·麦肯齐和埃米利奥·塞格雷. They bombarded a bismuth-209 target with alpha particles accelerated in Berkeley's 60-inch cyclotron. The nuclear reaction produced astatine-211:
\(^{209}\mathrm{Bi} + \alpha \, (^{4}\mathrm{He}) \rightarrow \,^{211}\mathrm{At} + 2n\)
They identified it by its characteristic radioactivity and initially named it "alabamine" (symbol Ab), but this name was not retained.
After World War II, in 1943, 贝尔塔·卡利克与特劳德·贝尔纳特 succeeded in identifying traces of astatine (the isotopes \(^{218}\mathrm{At}\) and \(^{219}\mathrm{At}\)) in natural decay products of uranium and thorium, thus confirming that it does exist in nature, albeit in infinitesimal quantities. The final name, 砹 (derived from the Greek astatos, αστατος, meaning "unstable"), was proposed by the discoverers and adopted, emphasizing its most striking property.
如今，砹完全通过人工方式生产，主要借助粒子加速器（回旋加速器）。最常见的生产方法包括：
Global production is extremely low, on the order of 每年几微克到几毫克, mainly in specialized research laboratories (United States, Russia, Europe, Japan). Its cost is astronomical (millions of dollars per gram, if one could even accumulate a gram), and there is no "market" in the conventional sense.
Astatine (symbol At, atomic number 85) is an element of group 17, the 卤素. It is the heaviest and most radioactive member of this family, which includes fluorine, chlorine, bromine, iodine, and tennessine. Its atom has 85 protons and, depending on the isotope, 116 to 140 neutrons. The most used isotope, \(^{211}\mathrm{At}\), has 126 neutrons. Its electronic configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵, with seven valence electrons (6s² 6p⁵).
Due to its instability and the tiny amounts ever produced (it is estimated that less than a millionth of a gram of elemental astatine has ever been synthesized in total), most of its physical properties have 从未被直接测量过 on a macroscopic sample. They are deduced from theoretical calculations, extrapolations from halogen group trends, and studies on infinitesimal quantities.
Estimated melting point: ~575 K (~302 °C).
Estimated boiling point: ~610 K (~337 °C).
Chemically, astatine is expected to behave like a 卤素, but with marked differences due to its weight and relativistic effects. It is expected to be the 最不活泼的卤素 and to exhibit metallic character (tendency to form cations, At⁺). Its possible oxidation states range from -1 to +7, with -1, +1, +3, +5, and +7 being plausible.
Atomic number: 85.
Group: 17 (Halogens).
Electronic configuration: [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵.
Physical state (20°C): Solid (predicted).
Radioactivity: All isotopes are radioactive.
Key medical isotope: \(^{211}\mathrm{At}\) (half-life 7.2 h, α emitter).
| 同位素 / 符号 | 质子（Z） | 中子（N） | 原子质量（u） | 生产/发生 | 半衰期 / 衰变模式 | 备注/应用 |
|---|---|---|---|---|---|---|
| 砹-210 — \(^{210}\mathrm{At}\) | 85 | 125 | 209.987148 u | 合成/天然痕量 | 8.1小时（α，99.8%；CE，0.2%） | 半衰期最长的同位素（但α纯度低于\(^{211}\mathrm{At}\)）。 |
| 砹-211 — \(^{211}\mathrm{At}\) | 85 | 126 | 210.987496 u | 合成（α在Bi-209上） | 7.214小时（α，100%） | 最重要的同位素. Pure high-energy alpha emitter (5.87 MeV). Ideal half-life for medicine (therapy). Target of choice for research. |
| 砹-217 — \(^{217}\mathrm{At}\) | 85 | 132 | 216.992420 u | 产生于 \(^{225}\mathrm{Ac}\) 的衰变链中 | 32.3 毫秒（α，99.99%） | 锕-225的衰变产物，用于靶向α疗法（TAT）。其衰变链可产生三个α粒子。 |
| 砹-218 — \(^{218}\mathrm{At}\) | 85 | 133 | 217.995350 u | 自然痕迹（铀-238衰变链） | 1.5秒（α，99.9%；β⁻，0.1%） | 非常短暂的自然同位素。 |
| 砹-219 — \(^{219}\mathrm{At}\) | 85 | 134 | 218.996590 u | 自然衰变链（铀-235链） | 56秒（α，97%；β⁻，3%） | 天然同位素，在天然砹中半衰期最长。 |
注意：:
Electron shells: 电子如何围绕原子核组织.
砹的85个电子分布在六个电子壳层中。其电子构型为[Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁵，第六壳层（s² p⁵）有七个价电子，这是卤素元素的特征构型。该构型也可表示为：K(2) L(8) M(18) N(32) O(18) P(7)，或完整形式：1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁵。
K壳层（n=1）: 2 electrons (1s²).
L壳层 (n=2): 8 electrons (2s² 2p⁶).
M层（n=3）: 18 electrons (3s² 3p⁶ 3d¹⁰).
N壳层（n=4）: 32 electrons (4s² 4p⁶ 4d¹⁰ 4f¹⁴).
O壳层 (n=5): 18 electrons (5s² 5p⁶ 5d¹⁰).
P壳层 (n=6): 7 electrons (6s² 6p⁵).
Astatine has 7 价电子 (6s² 6p⁵). Like other halogens, it tends to gain an electron to achieve the stable configuration of the noble gas (radon), thus forming the 砹离子（At⁻）. This is the -1 oxidation state, which should be the most stable. However, due to its large size and relativistic effects, astatine shows a strong tendency to exist in 正氧化态, unlike lighter halogens. The +1 (At⁺), +3 (AtO⁻ or At³⁺), +5 (AtO₃⁻), and +7 (AtO₄⁻) states are possible and have been observed in trace compounds. This chemistry is more similar to that of iodine than to chlorine or bromine.
The chemistry of astatine has never been studied with visible samples. It is explored using 放射化学示踪 techniques: the behavior of a few atoms or molecules labeled by the radioactivity of astatine in ultra-dilute solutions is tracked. This allows the determination of properties such as partition coefficients, redox potentials, or the stability of different complexes.
研究证实，砹的行为与卤素相似。
但它也显示出差异：
This is the most promising and practically the only application. Astatine-211 is a 纯α发射体 ideal for targeted internal radiotherapy:
To guide astatine-211 to the tumor, it must be covalently and stably attached to a vector molecule that specifically recognizes cancer cells. These 生物共轭物 are major chemical challenges because the At-C (carbon-astatine) bond is relatively weak and sensitive to deastatination (loss of the astatine atom). The vectors studied include:
\(^{211}\mathrm{At}\)的临床前和临床试验（I/II期）主要关注：
初步结果令人鼓舞，显示出抗肿瘤疗效且毒性有限。
Astatine-211 shares with polonium-210 an 摄入情况下的极端放射性毒性, due to its high-energy alpha emission. Its danger is even greater in some respects:
Handling of astatine-211 is done exclusively in 高安全性实验室（P3级别） equipped with sealed glove boxes under controlled atmosphere (nitrogen or argon). Protection against alpha emissions is simple (gloves, box), but prevention of incorporation (inhalation of vapors, ingestion, skin contact) is paramount. All operations are designed to work with activities on the order of gigabecquerels (GBq) in minuscule volumes.
There is no specific antidote. Prevention is the only effective strategy. In case of suspected contamination, emergency measures (decontamination, monitoring of excreta) and administration of 稳定碘 (to saturate the thyroid and limit astatine uptake) could be considered, although their efficacy is unproven.
与钋及其他第一类放射性物质一样，砹-211受国际原子能机构（IAEA）最严格的核安全与安保法规约束。其运输受到高度管制（遵循放射性物质ADR/RID法规）。全球仅有少数实验室获准生产与操作该物质。
The main obstacle to the development of astatine-211 therapies is its 限量生产. It requires a medium-energy cyclotron (~30 MeV) with a dedicated beamline for bismuth bombardment. The chemical separation of astatine from molten bismuth (distillation method) or in solution is complex and must be rapid. Developing more efficient production methods and logistics to deliver the isotope to hospitals within hours of production is an active area of research.
The future of astatine is almost entirely linked to 治疗诊断核医学:
砹，这个元素周期表中的“幽灵”，因此可能从实验室里的新奇事物转变为拯救生命的精准治疗工具，体现了放射性元素的悖论：将破坏力转化为治愈的力量。
原子的各种形态：从古代直觉到量子力学 
