Francium

Francium forms naturally only as a fleeting byproduct of uranium decay. Its most durable isotope has a half-life of only twenty-two minutes before disintegrating into other elements. Since it disappears almost as soon as it is born, on the entire planet Earth there are only about twenty to thirty grams at any time, distributed in microscopic amounts throughout the crust.

In nuclear physics facilities such as Stony Brook or TRIUMF, an accelerated beam of oxygen-18 ions collides against a hot molten gold-197 target. The generated francium atoms evaporate as ions and are magnetically guided toward a magneto-optical trap. There, six laser beams tuned with millimeter precision slow the thermal agitation of the atoms to almost absolute zero, immobilizing an invisible cloud of three hundred thousand atoms for quantum experiments.

Following the traditional periodic pattern of alkali metals, francium should be more electropositive and reactive than cesium. However, in such a heavy atom the 7s electrons travel at speeds close to the speed of light, increasing their relativistic mass and contracting their orbit near the nucleus. This contraction increases the energy required to remove the valence electron, causing francium to be theoretically somewhat less reactive and more electronegative than cesium.

Even if an entire milligram of francium-223 could be synthesized in a high-flux nuclear reactor, the intense radioactivity of its alpha and beta decays would release a devastating thermal energy density. Instantaneous radiogenic heat would vaporize and sublimate the metallic sample in milliseconds, destroying the container by thermal shock and vaporizing the metal before the human eye or any photographic camera could contemplate a visible drop of solid francium.

In 1939, Marguerite Perey worked in the Parisian laboratory founded by Marie Curie purifying mineral samples of actinium-227. When measuring the radioactivity of a newly purified sample free of thorium and radium, she detected an unexpected beta radiation that increased anomalously with an exact half-life of twenty-two minutes. After ruling out any known impurity, she demonstrated that actinium undergoes a minor alpha decay that originates the elusive element 87, naming it francium.

The laws of electromagnetism do not distinguish between a physical phenomenon and its mirror image reflected in a mirror. However, the weak nuclear force does violate this parity symmetry. Since francium has a heavy nucleus with eighty-seven protons and a simple atomic structure with a single outer electron, the orbital mixing effects induced by the weak force are amplified eighteenfold compared to cesium, allowing measurement of fundamental quantum constants of the universe.

Being the heaviest element of the alkali metal family, it would react with extreme violence, decomposing water instantly to release hydrogen gas and heat. However, gathering a visible piece of francium is physically impossible: its own radioactivity is so brutal that it would generate enough heat to evaporate the metal in a fraction of a second before being able to bring it near water.