Protactinium is not synthesized in significant quantities in stars. Like all heavy actinides, its formation is linked to extreme astrophysical processes such as the r-过程 (rapid neutron capture) during neutron star mergers or supernova explosions. In the solar system, its existence is fleeting and dependent on the decay chains of longer-lived parent elements. The isotope \(\,^{231}\mathrm{Pa}\) (half-life of 32,760 years) is a key link in the decay chain of 铀-235. Its presence in trace amounts in uranium minerals and marine sediments serves as a powerful 地质年代学工具. The \(\,^{231}\mathrm{Pa}\)/\(\,^{235}\mathrm{U}\) ratio is used to date geological processes on timescales of 10,000 to 300,000 years, complementing the thorium-230/uranium-238 pair.
The history of protactinium is marked by its fleeting nature. In 1913, physicists 卡西米尔·法扬斯（Kasimir Fajans，1887-1975） and 奥斯瓦尔德·赫尔穆特·格林（Oswald Helmuth Göhring，1889-1915） discovered a new short-lived element in the decay chain of uranium-238. They named it "brevium" (from the Latin 简短, short) in reference to its short half-life (1.17 minutes for the isotope 234mPa). However, the true element 91, with a longer-lived isotope, was isolated later. In 1917-1918, two groups of scientists discovered it independently: 莉泽·迈特纳（1878-1968） and 奥托·哈恩（1879-1968） in Germany, and 弗雷德里克·索迪（1877-1956） and 约翰·克兰斯顿（1891-1972） in the United Kingdom. They identified it in the uranium-235 chain and named it 镤 (from the Greek 原语, first, and 锕), as it decays into actinium-227. It was not until 1934 that 阿里斯蒂德·冯·格罗斯（1905-1985） isolated 2 mg of pure protactinium oxide (Pa2O5) from 5.6 tons of pitchblende, a feat of radiochemistry.
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Protactinium is one of the last natural elements to be discovered: for decades, it was the 最稀有且最昂贵的天然元素 in the world. Before the 1960s, global stocks did not exceed a few hundred grams, obtained from the reprocessing of tons of uranium residues. Its price was astronomical. It was only with the advent of large-scale nuclear industry and the processing of massive amounts of spent fuel that kilograms of protactinium could be isolated.
Protactinium (symbol Pa, atomic number 91) is an actinide, located between thorium and uranium. It is a dense, malleable metal, silvery-gray in color, which slowly tarnishes in air by forming a protective oxide. It has a complex crystalline structure (body-centered tetragonal at room temperature). Its chemistry is particularly rich and complex for an early actinide, mainly exhibiting the +5 oxidation state (Pa5+), but also the +4 (Pa4+) stably, and sometimes +3 in certain compounds. This duality makes it unique among its immediate neighbors. All its isotopes are radioactive.
Density: 15.37 g/cm³.
Melting point: ≈ 1841 K (1568 °C).
Boiling point: ≈ 4300 K (≈ 4027 °C, estimate).
| 同位素 / 符号 | 质子（Z） | 中子（N） | 原子质量（u） | 天然丰度 | 半衰期/稳定性 | 主要衰变模式/备注 |
|---|---|---|---|---|---|---|
| 镤-231 — \(\,^{231}\mathrm{Pa}\,\) | 91 | 140 | 231.035884 u | 痕量（铀-235中） | 32,760年 | α (100%). Most stable natural isotope. Crucial link in the 235U chain. Geological dating tool (231Pa/235U ratio). |
| 镤-234m — \(\,^{234m}\mathrm{Pa}\) | 91 | 143 | 234.043308 u | 痕量（在铀-238中） | 1.17分钟 | β– (99.84%) and IT (0.16%). Metastable isomer. Daughter of uranium-238 via thorium-234. First discovered ("brevium"). |
| 镤-233 — \(\,^{233}\mathrm{Pa}\) | 91 | 142 | 233.040247 u | 非天然的（合成的） | 26.967天 | β– (100%). 钍循环中的关键同位素. Produced by neutron capture on 232Th. Decays into fissile uranium-233. |
| 镤-230 — \(\,^{230}\mathrm{Pa}\) | 91 | 139 | 230.034541 u | 非天然的（合成的） | 17.4天 | β– and ε. Produced in accelerators. Studies of fundamental chemistry and nuclear properties. |
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Electron shells: 电子如何在原子核周围组织.
Protactinium has 91 electrons. Its ground state electronic configuration is [Rn] 5f2 6d1 7s2. It is the first actinide where the 5f orbitals begin to be unambiguously populated in the ground state, marking a transition in the series. This configuration gives it a 独特的双化学体系: it stably exhibits the +5 and +4 oxidation states, and to a lesser extent +3. In aqueous solution, Pa(V) is the most stable state, generally existing as the oxycation PaO2+. The Pa(IV) ion is stable in non-oxidizing environments. This duality makes its solution chemistry complex and highly dependent on redox potential and pH.
镤的化学性质以其强烈的水解倾向和形成多核或胶体络合物的特性为主导，尤其是在五价镤状态下。这使得其溶液中的行为难以预测且实验操作困难。镤能与氟化物、草酸盐和碳酸盐等阴离子形成稳定的络合物。从其他锕系元素（特别是钍、铀和镎）中化学分离镤是放射化学领域的一大挑战，通常利用四价与五价状态行为的细微差异，或采用甲基异丁基酮（MIBK）等特定溶剂。
In the solid state, protactinium mainly forms compounds in the +5 and +4 oxidation states. The white oxide Pa2O5 is the most stable. Mixed oxides (PaO2) and various halides (PaF5, PaCl4, PaBr4, etc.) also exist. Protactinium pentachloride (PaCl5) is a yellow solid used as a starting point for the synthesis of other compounds. The complexity of its solid-state chemistry reflects the richness of its transitional electronic configuration.
Protactinium does not exist in exploitable deposits. It is always produced as a byproduct of 铀的提取与加工. It concentrates in the residues (tailings) of uranium ore processing plants. The most important source for obtaining weighable quantities (on the order of grams to kilograms) is the 乏核燃料的后处理, where it accumulates as a fission and activation product. Isolating protactinium from these complex matrices is a long and costly process, involving a succession of precipitation, solvent extraction, and ion chromatography steps. There is no commercial market for protactinium; its production is solely driven by scientific research or specific technological development needs. If commercialized, its cost would be extremely high.
Protactinium is a 高放射性且有毒 element. The isotope 231Pa, the most relevant in the long term, is a pure alpha emitter. As with other alpha emitters, the main danger is 内部合并 (inhalation, ingestion). Once in the body, it preferentially accumulates in the bones (chemical behavior similar to that of actinium and thorium), where its alpha decay irradiates bone marrow cells in a highly localized and damaging manner, significantly increasing the risk of cancer. Its handling, even in trace amounts, absolutely requires 受控气氛设施 (glove boxes or shielded cells) to prevent any contamination of the operator or the environment. Storage is done in chemically stable form (usually oxide or insoluble salt) in sealed and shielded containers.
原子的各种形态：从古代直觉到量子力学 
