Francium (Atomic Number 87), the Last Discovered Natural Element

In 1935 the three natural radioactive series (238U, 235U, and 232Th) were coherent and complete, but the elements with atomic numbers 85 and 87 were still missing in the periodic table. It was anticipated that an isotope of element 87 could be formed by α-decay of actinium. In 1939 a young technician, Marguerite Perey (1909–1975), the 30th anniversary of whose death we celebrate this year, was recruited at the Institut du Radium in Paris as the personal laboratory assistant of Marie Curie (1867–1934). For many years Perey worked on actinium and in 1939 discovered the first isotope of element 87, Dmitrii Ivanovich Mendeleev’s eka-cesium, the heaviest alkali metal, for which she proposed the name “francium.” The events leading up to Perey’s discovery, her work, and its consequences are recounted.