Francium was the last naturally occurring element to be discovered, in 1939. Marguerite Perey identified it at the Curie Institute in Paris while studying actinium-227, a radioactive isotope.
Perey initially called the element eka-caesium because its position below caesium had been predicted from the periodic table. She found that actinium-227 produced a previously unknown alpha-emitting daughter, which was later identified as francium-223. The element was named for France, and IUPAC officially adopted the name in 1949.
Francium is exceptionally rare and short-lived. Its most stable isotope, francium-223, has a half-life of about 22 minutes, so it forms only in tiny amounts as part of radioactive decay chains, especially the actinium series. No weighable sample of the element exists in nature at one time.
A common mix-up is astatine, which was identified earlier through artificial nuclear experiments but was later found in trace quantities in nature. Francium remains the last element first discovered in nature, rather than by synthesis.