Francium (Fr), a highly radioactive alkali metal with the atomic number 87, is the heaviest element in the alkali metal group, which includes lithium, sodium, and potassium. Possessing a single electron in its outermost shell, it exhibits extreme chemical reactivity. Francium is also one of the rarest elements in the Earth’s crust, making it the last naturally occurring element to be discovered. Its scarcity meant successful isolation remained a challenge for decades, even though its properties were well-established by chemical theory.
The Predicted Element: Eka-Cesium
The search for element 87 began long before its discovery, rooted in the predictive power of the periodic table developed by Dmitri Mendeleev in 1869. Mendeleev noticed a gap directly beneath Cesium in Group 1 and used the Sanskrit-derived prefix eka (meaning “one”) to provisionally name the missing element eka-cesium. He accurately predicted that this element would share the chemical characteristics of the alkali metal family, but be much heavier and more reactive.
The certainty of the element’s existence spurred numerous researchers to attempt its isolation, leading to several false claims over the following decades. In 1925, Soviet chemist Dmitry Dobroserdov incorrectly claimed its discovery and proposed the name russium. Later, American physicist Fred Allison announced he had found the element in 1930 using a magneto-optical machine, suggesting the name virginium. Another claim came in 1936 from Romanian physicist Horia Hulubei and his French colleague Yvette Cauchois, who proposed moldavium. These claims were ultimately discredited because the element’s extreme instability made it impossible to isolate using the methods of the time.
Marguerite Perey and the Successful Isolation
The true discovery of element 87 was made in 1939 by French chemist Marguerite Perey, a former student and assistant of Marie Curie, working at the Curie Institute in Paris. Perey was studying the decay chain of Actinium-227, which was known to decay almost entirely by emitting a beta particle to form Thorium-227. She became skeptical when a review of the decay process showed an unexpected type of radiation that did not match the known beta decay energy.
Perey hypothesized that a tiny fraction of the Actinium-227 atoms must decay through a different pathway, specifically by emitting an alpha particle (a helium nucleus). The loss of this particle transforms Actinium (element 89) into a new element with 87 protons, the long-sought eka-cesium. Her radiochemical work involved rapidly purifying Actinium-227 samples to separate the decay products before they decayed further. She successfully isolated the new product, which exhibited the chemical properties of an alkali metal, confirming it was element 87. Perey chose the name francium (Fr) in honor of her home country, France, a name officially adopted by the international scientific community in 1949.
Francium’s Extreme Instability
The reason Francium eluded discovery for so long is its profound nuclear instability, which severely limits its presence in nature. The most stable isotope, Francium-223, has a half-life of only 22 minutes, meaning half of any given sample decays into Radium-223 in less than half an hour.
This rapid decay makes it impossible for Francium to accumulate in macroscopic quantities, as it is constantly forming and disappearing in the natural decay chain of Uranium-235. Scientists estimate that only about 15 to 30 grams of Francium exist in the entire Earth’s crust at any single time, making it the second rarest naturally occurring element after Astatine. Due to its intense radioactivity, Francium has no commercial or practical applications outside of specialized physics research. Experiments must be conducted quickly, often using artificially produced atoms, to study its atomic structure and chemical behavior before it decays.