Fr · Element 87 · Alkali metal
The second rarest natural element, and one of the most electropositive — but no one has ever seen a visible sample.
Structure
Not a diagram of dots on rings — a Monte-Carlo sample of the actual probability density |ψ|² for each occupied subshell. Drag to rotate. Blue and violet mark opposite signs of the wavefunction, which is what makes bonding possible.
Measured values
Every bar shows where francium sits among all 118 elements for that property.
Sources: IUPAC 2021 standard atomic weights · CRC Handbook of Chemistry and Physics · NIST. Values marked ~ are predicted rather than measured.
Same element, different neutrons
All francium atoms have the same number of protons, 87, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, Fr-223, has 136 neutrons and a half-life of 22.00 min.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Fr-212 | 125 | — | 20.0 min | ε/β⁺ (57%), α (43%) |
| Fr-221 | 134 | — | 4.9 min | α |
| Fr-222 | 135 | — | 14.2 min | β⁻ |
| Fr-223 longest-lived | 136 | — | 22.00 min | β⁻ |
| Fr-225 | 138 | — | 3.95 min | β⁻ |
Here the classroom shortcut, rounded atomic mass (223) minus atomic number, gives the same answer: 136.
Decay modes
Data: IAEA Nuclear Data Section, LiveChart of Nuclides, ground-state evaluations, retrieved October 10, 2026. Abundance is the share of atoms in the natural element; a dash means the isotope has no standard natural abundance.
Size, to scale
Radius 260 pm — that is 0.26 nm, so about 1.92 million of them side by side would span a millimetre.
Thermal range
Francium is liquid over a 650 K window, from 300 K to 950 K.
The story
The second rarest natural element, and one of the most electropositive — but no one has ever seen a visible sample.
Francium was discovered in 1939 by French chemist Marguerite Perey at the Curie Institute in Paris, making her the last person to discover a naturally occurring element. Working as Marie Curies laboratory assistant, Perey was studying the radioactive decay of actinium-227 when she noticed that some alpha particles had energies that didnt match known decay products. Her meticulous analysis revealed the existence of element 87, which she initially called "actinium-K."
Pereys discovery required great patience and precision. She had to distinguish francium from other radioactive products by carefully measuring decay energies, half-lives, and chemical behavior. Using only basic radiometric equipment available in 1939, she proved that element 87 behaved like an alkali metal and had the predicted properties for the missing element below cesium in the periodic table.
Perey initially proposed the name "catium" for element 87, but later chose "francium" to honor her homeland. Her discovery was confirmed by other researchers and officially recognized by the International Union of Pure and Applied Chemistry. Perey became the first woman elected to the French Academy of Sciences in 1962, largely based on her discovery of francium and subsequent research in nuclear chemistry.
Pereys discovery came at a crucial time in nuclear physics, just as scientists were beginning to understand radioactive decay chains and nuclear structure. Her work contributed to the theoretical framework that would soon lead to nuclear weapons and nuclear power. The discovery of francium helped complete the natural periodic table and validated Mendeleevs predictions about undiscovered elements.
Applications
Francium serves as a demanding test case for atomic physics theories and quantum mechanical calculations. As the second most electropositive element, after caesium, francium atoms provide unique insights into electron behavior in harsh conditions. Researchers use laser cooling and magnetic trapping techniques to study individual francium atoms, testing fundamental physics principles and searching for physics beyond the Standard Model. These experiments require only a few thousand atoms and help validate theoretical models of atomic structure.
Francium is particularly valuable for studying parity violation in atomic systems - a fundamental asymmetry in nature where physical laws change under mirror reflection. Fr-210 and Fr-212 isotopes are used in precision measurements that test the Standard Model of particle physics and search for new physics phenomena. These experiments help scientists understand why the universe contains more matter than antimatter.
Researchers use francium atoms to test whether fundamental constants of nature (like the fine structure constant) change over time or vary with location. These measurements require very precise spectroscopy of francium atomic transitions and help address some of the deepest questions in physics about the stability and universality of natural laws.
While actual francium samples are impossibly rare, computer simulations and theoretical models of francium help train atomic physicists and test computational methods. Understanding franciums predicted properties helps researchers prepare for studies of superheavy elements and validates theoretical approaches used throughout the periodic table.
Francium has no practical applications outside of highly specialized atomic physics research. Its rarity (less than 30 grams exist on Earth at any time) and short half-life (longest-lived isotope Fr-223 lasts only 22 minutes) make it impossible to accumulate in useful quantities. All research uses involve individual atoms or collections of up to a few hundred thousand atoms held in laser traps in ultra-high vacuum systems.
Scientists use sophisticated laser systems to trap and study francium atoms one at a time. These experiments measure atomic properties like hyperfine structure, isotope shifts, and electric dipole transitions with high precision. The data helps refine atomic theory and provides benchmarks for testing quantum mechanical calculations in heavy atoms.
Francium atoms can be slowed and trapped using laser cooling techniques, allowing detailed study of their behavior at temperatures near absolute zero. These experiments provide insights into quantum mechanics at the single-atom level and help develop techniques later applied to other alkali metals with practical applications in quantum computing and atomic clocks.
Francium's position as the heaviest alkali metal makes it valuable for testing and refining theoretical models used throughout chemistry and physics. Accurate predictions of francium behavior validate computational methods that are then applied to design new materials, predict chemical reactions, and understand the behavior of other heavy elements that are difficult or dangerous to study experimentally.
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Francium exists naturally in uranium and thorium ores as an extremely rare intermediate product in radioactive decay chains. At any given moment, less than 30 grams of francium exist in the entire Earths crust - making it the second rarest naturally occurring element, after astatine. The element occurs as Fr-223 in the actinium decay series (uranium-235 chain) and as Fr-221 in the neptunium decay series, but these isotopes decay rapidly with half-lives of 22 minutes and 4.9 minutes respectively.
Natural francium forms when actinium-227 undergoes alpha decay (a 1.38% branch), and francium-223 then beta-decays into radium-223. These nuclear reactions occur deep within uranium-bearing minerals, but the francium atoms created quickly decay into radium or astatine before they can accumulate. The continuous creation and destruction of francium atoms maintains a steady-state concentration that is vanishingly small.
Because francium forms through radioactive decay rather than geological processes, it has no specific geographic concentration. Trace amounts exist wherever uranium and thorium deposits occur, including the Athabasca Basin in Canada, Olympic Dam in Australia, and uranium deposits in Kazakhstan, Niger, and the American Southwest. However, the concentrations are so low that extraction is physically impossible.
Scientists estimate franciums abundance by calculating the balance between its formation rate from radioactive decay and its destruction rate through further decay. In uranium ore containing 1% uranium, francium concentrations reach only about 10^-20 to 10^-19 grams per gram of ore - roughly equivalent to finding a single franc coin in a pile of money worth the entire global economy.
Handling
Francium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
All francium isotopes are extremely radioactive, with the longest-lived (Fr-223) having a half-life of only 22 minutes. Francium-223 decays almost entirely by beta emission (99.994 percent), with a tiny alpha branch and accompanying gamma rays. If enough francium could ever be gathered, its chemical similarity to potassium means it would concentrate in vital organs, causing severe radiation poisoning.
Francium research requires the most sophisticated containment systems available, including ultra-high vacuum chambers, laser cooling apparatus, and remote manipulation equipment. All work must be conducted in specialized facilities designed for handling the most dangerous radioactive materials. Personnel must be trained in advanced radiation safety protocols and wear full protective equipment.
There are no safe procedures for handling macroscopic amounts of francium because such quantities cannot exist - they would immediately decay into other elements. Even the atomic-scale quantities used in research (up to a few hundred thousand atoms held in laser traps) require great caution. Because francium-223 has a half-life of 22 minutes and research samples hold only atoms, any released francium decays away within hours and cannot cause wide-area contamination.
While direct human exposure to francium is impossible due to its rarity, theoretical assessments suggest it would be one of the most toxic substances known. Its chemical similarity to potassium and cesium means it would disrupt cellular processes throughout the body, while its intense radioactivity would deliver a severe internal radiation dose if a weighable amount could ever be gathered, which its 22-minute half-life prevents.
Quick answers
Francium (symbol Fr) is element 87 on the periodic table, an alkali metal in period 7, group 1. The second rarest natural element, and one of the most electropositive — but no one has ever seen a visible sample. At room temperature it is a solid, and it is radioactive.
[Rn] 7s¹, giving 7 occupied shells with the distribution 2, 8, 18, 32, 18, 8, 1.Francium melts at 300 K (26.9 °C) and boils at 950 K (676.9 °C). It will melt in a warm room or in the palm of your hand.
Francium has no stable isotope, so it has no standard atomic weight. The figure quoted, 223, is the mass number of its longest-lived known isotope.
Francium has no stable isotope. Its longest-lived known isotope, Fr-223, has 136 neutrons (mass number 223 minus atomic number 87) and a half-life of 22.00 min. The classroom shortcut, rounded atomic mass (223) minus atomic number, gives the same 136.
Francium is predicted to have a density of about 1.87 g/cm³, though no sample large enough to weigh has ever been made. Water is 1.0 g/cm³ for comparison.
Francium has a Pauling electronegativity of 0.7. The scale runs from 0.70 (francium, the least greedy for electrons) to 3.98 (fluorine, the most). A value this low means it readily gives its outer electrons away, forming positive ions.
Francium was discovered in 1939 by Marguerite Perey. Origin of the name: France.
Francium does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Francium has no stable isotope — every one of its nuclei decays. Trace amounts occur naturally as decay products of heavier elements.