Francium

Francium, atomic number 87 and symbol Fr, is an extremely radioactive alkali metal with the electron configuration [Rn] 7s¹. Its most stable naturally occurring isotope, francium-223—originally called actinium K—has a half-life of just 22 minutes, making it one of the most unstable elements found in nature. Only astatine is rarer among naturally occurring elements, and francium is the second-most electropositive element after caesium. All francium isotopes quickly decay into astatine, radium, or radon.

Because of its intense radioactivity and short half-life, bulk francium has never been observed. Any visible quantity would instantly vaporize from the heat of its own decay. Based on the properties of other alkali metals, it is presumed to be a highly reactive metal, but obtaining a sample large enough to see is considered nearly impossible. Outside the lab, francium exists only in trace amounts within uranium ores, where francium-223 continually forms and decays as part of the uranium-235 decay chain. At any given moment, the entire Earth’s crust contains about one ounce (28 grams) of francium. Aside from francium-223 and francium-221, all other isotopes are synthetic; the largest laboratory sample ever produced contained a cluster of over 300,000 atoms.

Francium’s chemical behavior closely resembles that of caesium. It has the highest equivalent weight of any element and, if it could be liquefied, would have a surface tension of about 0.05092 N/m at its melting point. Its melting point is uncertain, with estimates ranging from 8 °C to 27 °C; a calculation based on binary ionic crystals gives 24.861 °C, while Mendeleev’s method suggests 20 °C. Boiling point estimates vary between 598 °C and 677 °C, with a common figure of 620 °C. Density predictions also differ, from 2.48 g/cm³ to 3.57 g/cm³, and the bulk modulus is estimated at 2.1–2.6 GPa. Pauling’s electronegativity for francium was set at 0.7, matching caesium, though caesium’s value has since been refined to 0.79. Francium actually has a slightly higher ionization energy than caesium (392.811 kJ/mol vs. 375.7041 kJ/mol), a relativistic effect that suggests caesium is the less electronegative of the two. Francium is also expected to have a higher electron affinity than caesium, and the Fr⁻ ion should be more polarizable than Cs⁻.

Francium’s instability means its salts are only poorly known. It coprecipitates with many caesium salts—such as caesium perchlorate, iodate, picrate, tartrate (also rubidium tartrate), chloroplatinate, and silicotungstate—which can be used to isolate it. Francium perchlorate, for example, forms when francium chloride reacts with sodium perchlorate and then coprecipitates with caesium perchlorate. This method, adapted from the radiocaesium coprecipitation technique of Lawrence E. Glendenin and C. M. Nelson, is not reliable for separating thallium, which also coprecipitates. Francium perchlorate’s entropy is predicted to be 42.7 e.u. (178.7 J mol⁻¹ K⁻¹). Francium halides are all water-soluble and expected to be white solids, produced by reacting francium with the corresponding halogen. Francium chloride has been studied as a way to separate francium from other elements due to its high vapor pressure, though francium fluoride would have an even higher vapor pressure. Other compounds, such as francium nitrate, sulfate, and hydroxide, are also known in trace amounts.

Francium was discovered by Marguerite Perey in France on January 7, 1939, and named after her country. Before its discovery, it was called eka-caesium because of its predicted position below caesium on the periodic table. It was the last element first discovered in nature rather than synthesized.