Perey experimentally established that anomalous radiation in her purified actinium samples was caused by a new element, atomic number 87, later named francium.
On 7 January 1939, working as a laboratory assistant to Marie Curie's successor André Debierne at the Radium Institute in Paris, Marguerite Perey achieved a landmark result in nuclear chemistry. While purifying samples of actinium-227, she had noticed since late 1938 an anomalous form of radioactive decay that could not be explained by known decay chains. Rather than dismissing this as experimental error, Perey meticulously repeated her purifications and radiochemical separations, ultimately proving that the unexplained radiation came from a previously undiscovered element with atomic number 87.
This was an extraordinary achievement: element 87 had been theorized and searched for by chemists worldwide for decades, filling a persistent gap in the periodic table beneath cesium. Perey demonstrated that actinium underwent a rare alpha-decay branch (occurring only about 1% of the time) that produced this new element, which she initially called actinium-K.
Her work was rigorous and painstaking, conducted with the primitive radiochemical techniques of the era and considerable personal radiation exposure—exposure that would later contribute to her death from cancer. The discovery was historically significant as the last element discovered by extraction from a natural source rather than through synthetic nuclear reactions, marking the end of an era in element discovery. She later formally named the element francium, in honor of her home country, and it was accepted by the International Union of Pure and Applied Chemistry in 1949.