Francium

Francium is the heaviest alkali metal in group 1, below lithium, sodium, potassium, rubidium and caesium. Like them it has a single outer electron, 7s1, which it gives up easily to form the Fr+ ion. Simple trends would make it the most reactive metal of all, but relativistic effects pull its 7s electron slightly closer to the heavy nucleus. As a result, francium's ionisation energy (4.07 eV) is a little higher than caesium's (3.89 eV), so caesium is probably the more reactive of the two.

No one has ever seen a piece of francium. Its longest-lived isotope, francium-223, has a half-life of only 22 minutes, and any visible amount would vaporise itself with the heat of its own decay. Experiments instead trap a few hundred thousand atoms at a time with lasers and magnetic fields. Tracer studies show francium behaves like caesium in solution, forming soluble salts and coprecipitating with caesium perchlorate. Physicists use trapped francium atoms to test subtle predictions about the weak nuclear force.

Discovery

In 1939 Marguerite Perey, a former assistant of Marie Curie at the Radium Institute in Paris, noticed unexpected beta radiation while purifying actinium-227. She showed that about 1% of actinium atoms decay by emitting an alpha particle, forming element 87. She named it francium after France. Perey later became the first woman elected to the French Academy of Sciences, as a corresponding member in 1962.