Francium is an element produced exclusively by the r-过程 (rapid neutron capture) during extreme astrophysical events such as supernovae or neutron star mergers. However, because all its isotopes are radioactive with very short half-lives (the most stable, \(^{223}\mathrm{Fr}\), has a half-life of only 22.00 minutes), 没有原始钫元素 remains in the universe since the formation of the solar system. All francium produced at that time decayed billions of years ago. The francium present on Earth today (in infinitesimal quantities) is constantly recreated in two ways:
\(^{223}\mathrm{Fr}\) (historically called AcK, for "actinium K") is the natural francium isotope with the longest half-life. It decays with a half-life of 22.00 minutes mainly by beta-minus decay (99.994%) to radium-223, and very weakly (0.006%) by alpha emission to astatine-219. Its presence is linked to secular equilibrium with its parent, actinium-227 (half-life 21.772 years). It is estimated that at any given time, less than one 克 of francium-223 is present in the entire Earth's crust, dispersed in uranium ores.
Despite (or because of) its extreme instability, francium is a fascinating object of study for physicists. As the 最后的碱, it has a single valence electron in an s orbital (7s¹), which makes it a "simple" atom from a quantum mechanical point of view, but with very pronounced relativistic effects due to the strong nuclear charge. Precise measurement of its atomic properties (energy levels, moments, hyperfine structure) allows testing with great precision the predictions of 量子电动力学（QED） in intense electromagnetic fields. These tests contribute to the search for new physics beyond the Standard Model.
The existence of element 87, an alkali heavier than cesium, was predicted by 德米特里·门捷列夫 who named it "类-铯". Its search was arduous and marked by several false discoveries in the early 20th century (such as "virginium" or "moldavium"), because its expected chemical properties (extreme reactivity, great instability) made it elusive.
The discovery goes to French physicist and chemist 玛格丽特·佩里 (1909-1975), then assistant to 玛丽·居里 at the Radium Institute (Paris). In 1939, while purifying a sample of actinium-227, she noticed an abnormal radioactive activity (beta emission) that could not be attributed to any known isotope. After months of meticulous chemical analyses, she demonstrated that this activity was due to a new element, produced by alpha decay of actinium-227 (1.38% branch):
\(^{227}\mathrm{Ac} \xrightarrow[\alpha]{} ^{223}\mathrm{Fr}\)
She confirmed that it was indeed the missing last alkali, and gave it the name 钫 in honor of her country, France, thus following the tradition of the Curies (polonium) and Debierne (actinium). Her thesis, defended in 1946, consolidated this discovery. Marguerite Perey was the first woman elected to the Academy of Sciences (1962), but not to the French Academy.
对钫的研究受限于自然界中极其微量的存在。20世纪70至80年代，粒子加速器的发展使得通过诸如\(^{197}\mathrm{Au} + ^{18}\mathrm{O} \rightarrow \,^{210}\mathrm{Fr} + 5n\)等反应，能够生产出数量更多（尽管在宏观尺度上仍属微量）的重同位素。这为更深入的物理研究铺平了道路。
Today, francium is produced exclusively artificially in a few specialized laboratories around the world (Stony Brook in the United States, TRIUMF in Canada, RIKEN in Japan, etc.). The most common method uses an accelerated 氧-18束流 at about 100 MeV to bombard a 金-197 target. The fusion-evaporation reaction produces heavy francium isotopes (such as \(^{210}\mathrm{Fr}\) to \(^{213}\mathrm{Fr}\)), which are then extracted, separated and trapped in experimental devices as individual atoms or small clouds.
The quantities produced are so small that they are measured in 每秒原子数 (typically \(10^4\) to \(10^6\) atoms/s), and never in grams. It is therefore impossible to have a visible or manipulable sample of metallic francium.
Francium (symbol Fr, atomic number 87) is an element of group 1, that of 碱金属. It is the heaviest and most radioactive member of this family, which includes lithium, sodium, potassium, rubidium, cesium and the very recent nihonium (probably not alkali). Its atom has 87 protons and, depending on the isotope, 123 to 150 neutrons. The natural isotope \(^{223}\mathrm{Fr}\) has 136 neutrons. Its electron configuration is [Rn] 7s¹, with a single valence electron in the 7s shell.
Due to the impossibility of obtaining a macroscopic quantity, most of the physical properties of francium have 从未被直接测量过. They are deduced by extrapolation of alkali group trends, theoretical calculations and spectroscopic studies on individual atoms.
Estimated melting point: ~300 K (~27 °C).
Estimated boiling point: ~950 K (~677 °C).
These values are very uncertain.
Atomic number: 87.
Group: 1 (Alkali metals).
Electron configuration: [Rn] 7s¹.
Oxidation state: +1 (exclusive).
Most stable isotope: \(^{223}\mathrm{Fr}\) (T½ = 22.00 min).
Appearance (predicted): Silvery metal, extremely reactive.
| 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 |
|---|---|---|---|---|---|---|
| 钫-212 — \(^{212}\mathrm{Fr}\) | 87 | 125 | 212.012秒 | 合成 | 20.0分钟（β⁻，99.45%；α，0.55%） | 中等寿命的合成同位素。 |
| 钫-221 — \(^{221}\mathrm{Fr}\) | 87 | 134 | 221.014秒 | 自然痕量（Np-237链） | 4.9分钟（α，99.65%；β⁻，0.35%） | 存在于含有镎-237的矿石中，含量极微。 |
| 钫-222 — \(^{222}\mathrm{Fr}\) | 87 | 135 | 222.017秒 | 合成 | 14.2分钟（β⁻） | 合成同位素。 |
| 钫-223 — \(^{223}\mathrm{Fr}\) | 87 | 136 | 223.019736 u | 天然（铀-235链）与合成 | 22.00分钟（β⁻，99.994%；α，0.006%） | 最稳定的天然同位素. Discovered by Marguerite Perey. Longest half-life. Used for certain studies. |
注意：:
Electron shells: 电子如何围绕原子核组织排列.
钫有87个电子，分布在七个电子壳层中。其电子排布 [Rn] 7s¹ 极为简单：由氡（一种稀有气体）的排布加上7s壳层中的一个额外电子组成。也可写作：K(2) L(8) M(18) N(32) O(18) P(8) Q(1)，或完整形式：1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 4f¹⁴ 5s² 5p⁶ 5d¹⁰ 6s² 6p⁶ 7s¹。
K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层（n=4）: 32 electrons (4s² 4p⁶ 4d¹⁰ 4f¹⁴).
O壳层（n=5）: 18 electrons (5s² 5p⁶ 5d¹⁰).
P壳层 (n=6): 8 electrons (6s² 6p⁶).
Q壳层 (n=7): 1 electron (7s¹).
Francium has a single 价电子 (7s¹). This electron is very far from the nucleus and is weakly bound due to the significant shielding created by the 86 electrons of the inner shells (noble gas configuration). Consequently:
这些特性使钫成为极端碱金属的典型代表：电正性最强、反应性最高，且其化学性质主要由Fr⁺离子主导。
Like other alkalis, francium would exist chemically only in the +1 oxidation state. The Fr⁺ ion would be the largest alkali cation, with an estimated ionic radius of 180 pm. Its chemistry in aqueous solution would be simple and similar to that of cesium (Cs⁺), but with some differences:
如果能分离出金属态的钫，它将具有爆炸性反应活性。
实际上，这些反应永远无法在可见的样本上观察到。
The chemistry of francium is studied by 放射化学示踪 and 冷阱中冷原子的光谱学 techniques. The behavior of a few atoms is monitored (by their radioactivity) in ion exchange columns or during coprecipitations. These studies confirmed that its behavior is very close to that of cesium, with perhaps a slight difference in partition coefficients due to its larger size.
Francium has 严格来说，没有实际应用 outside of fundamental research, due to its extreme rarity and instability. Its "applications" are therefore limited to the field of pure science:
与任何β/α辐射体一样，进入体内的钫会具有毒性。然而，这种风险纯属理论上的：
操作在受控核实验室中进行，配备离子束屏蔽装置以及管理活化靶材的流程。分离化学操作在手套箱或封闭容器内完成。
钫将永远是一种实验室元素，一种处于稳定边缘的科学奇观。它的价值在于它教会我们关于基本物理定律的知识。当前和未来的研究旨在：
原子的各种形态：从古代直觉到量子力学 
