Francium

Francium sits at the bottom of Group 1 — the alkali metals — and carries the distinction of being the most unstable element that forms naturally on Earth. Every atom present right now was born from the radioactive decay of actinium or uranium somewhere in the crust, and within a median time of 22 minutes it will transform into radium or astatine. The total amount on Earth at any moment is estimated at a few hundred grams at most, scattered in unimaginably dilute concentrations through uranium-bearing ore. Because so little exists and it vanishes so quickly, no one has ever seen francium with the naked eye. Scientists study it using extraordinarily sensitive laser and atomic-trap techniques that can manipulate individual atoms, probing the fundamental physics of the heaviest alkali metal and testing theories about how electrons behave near a nucleus containing 87 protons.

Francium has no practical commercial or industrial applications. Its extreme rarity and the absence of any long-lived isotope make it impossible to accumulate in useful quantities. Research use is confined entirely to atomic physics and nuclear structure studies. At institutions such as TRIUMF in Canada and SUNY Stony Brook in the United States, researchers use laser cooling and magneto-optical traps to capture clouds of a few thousand francium atoms at a time, cooled to within millionths of a degree of absolute zero. These trapped atoms are used to measure atomic parity non-conservation — subtle asymmetries in how atoms interact with light that test the Standard Model of particle physics. Because francium's large, loosely bound outer electron is especially sensitive to such effects, it offers a sensitive window into electroweak interactions that cannot be probed as cleanly in lighter elements. Future experiments may use francium to search for a permanent electric dipole moment of the electron.

Francium was discovered in 1939 by the French physicist Marguerite Perey at the Curie Institute in Paris. Perey was analyzing purified actinium-227 preparations and noticed an unexpected daughter activity that did not match any known element. She methodically ruled out contamination and misidentification and concluded she had found element 87, the long-sought eka-caesium below caesium in the periodic table. Perey named the element francium in honor of France, making her one of the few people ever to name an element after their own country. It was the last naturally occurring element to be discovered. Earlier claims by several researchers in the 1910s and 1930s — with proposed names such as virginium and moldavium — could not be substantiated. Perey's careful radiochemical work earned her a nomination for the Nobel Prize, and in 1962 she became the first woman elected to the French Academy of Sciences.

Francium occurs naturally as a decay product in the uranium-235 radioactive decay series, produced when actinium-227 undergoes alpha decay with a branching ratio of about 1.4 percent. It is also generated in much smaller amounts in the thorium-232 chain. The element exists only in vanishingly small concentrations — the entire inventory in Earth's crust at any moment is estimated at less than 30 grams, and some estimates put it lower still, perhaps only a few hundred grams distributed globally. Because Fr-223, its most abundant naturally occurring isotope, has a half-life of only 22.3 minutes, no geological accumulation is possible. Traces appear wherever uranium ore is found, including in minerals such as pitchblende and carnotite. For experimental work, francium is produced artificially by bombarding gold targets with oxygen-18 ions in particle accelerators, yielding short bursts of atoms that are immediately captured by laser traps.

Because no measurable quantity of francium has ever been collected as a bulk substance, its chemistry rests almost entirely on theoretical predictions and indirect experimental evidence gathered from single-atom studies. As the heaviest alkali metal, francium is expected to behave like an extreme version of caesium: highly electropositive, readily losing its single outer electron to form Fr+ ions, and vigorously reactive with water and oxygen. Calculations predict that relativistic effects — which become important for the innermost electrons of very heavy atoms — make francium's ionization energy slightly higher than simple extrapolation from lighter alkali metals would suggest. Francium hydroxide (FrOH) and francium chloride (FrCl) are expected to exist and to be highly soluble in water, following alkali metal trends. In the single-atom trapping experiments conducted so far, francium has been handled only as free atoms in vacuum, never in bulk chemical form.

• Marguerite Perey discovered francium while working as a laboratory assistant at the Curie Institute — a role she had taken as a teenager without a formal university degree, later earning her doctorate from research done at the bench.
• The largest number of francium atoms ever assembled in one place at one time is estimated at around 300,000 — still far too few to see or weigh, but enough to study spectroscopically.
• Francium's melting point has never been measured directly; it is estimated to be around 27 degrees Celsius, meaning it would be a liquid at room temperature if enough of it could ever be accumulated.
• Because francium is so similar to caesium in chemistry but far more radioactive, early attempts to hunt for it involved treating uranium ore residues with caesium-specific reagents and looking for an unusual radioactive fraction.
• Francium-223 decays in two ways: about 99 percent of the time by beta emission to radium-223, and about 1 percent of the time by alpha emission to astatine-219 — itself a radioactive element with a half-life of less than a minute.