Marguerite Perey discovered francium at the Curie Institute in 1939. The element was the previously unknown element 87, identified while Perey studied the radioactive decay of actinium.
Perey had worked as Marie Curie’s laboratory assistant at the Radium Institute in Paris since 1929. By carefully purifying actinium, she detected alpha radiation showing that actinium was transforming into a lighter, undiscovered element. Because alpha decay removes two protons, actinium’s atomic number 89 becomes 87.
Perey named the new element francium after France. It belongs to Group 1, the alkali metals, alongside lithium, sodium, and cesium. Francium is extraordinarily rare because all of its isotopes are radioactive and short-lived; it occurs naturally only in tiny traces within uranium and thorium decay chains.
A common mix-up is astatine, another extremely rare radioactive element. Astatine was discovered earlier, in 1940, by researchers who produced it artificially, whereas francium was the last naturally occurring element to be discovered.