In 1935, Perey read a   paper by American scientists claiming to have discovered a type of radiation called beta particles being emitted by actinium and was skeptical because the reported energy of the beta particles didnt seem to match actinium. She decided to investigate for herself, theorizing that actinium was decaying into another element (a daughter atom) and that the observed beta particles were actually coming from that daughter atom. She confirmed this by isolating extremely pure actinium and studying its radiation very quickly; she detected a small amount of alpha radiation, a type of radiation that involves the loss of protons and therefore changes an atoms identity. Loss of an alpha particle (consisting of 2 protons and 2 neutrons) would turn actinium (element 89, with 89 protons) into the theorized but never-before-seen element 87.
Perey announced the discovery of the never-before-seen element 87 as a note  in the Comptes  Rendus presented  at the Académie des Sciences by Jean Baptiste Perrin on 9 January 1939 with the title "On an element 87, derived from actinium." Pereys discovery was announced by Perrin, not Perey herself, because she was only a laboratory assistant with no university degree. Perey named the element francium, after her home country, and it joined the other alkali metals in Group 1 of the periodic table of elements. Francium is the second rarest element (after astatine) - only about 550g exists in the entire Earths crust - and it was the last element to be discovered in nature. (Five elements that were discovered synthetically were later found to exist in nature: technetium, promethium, astatine, neptunium, and plutonium.)