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 promethium 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.
Size, to scale
Radius 183 pm — that is 0.183 nm, so about 2732 million of them side by side would span a millimetre.
Thermal range
Promethium is liquid over a 1958 K window, from 1315 K to 3273 K.
The story
The only lanthanide with no stable isotope — it decayed away and had to be made in a reactor.
When Dmitri Mendeleev organized the periodic table, he predicted the existence of an element between neodymium and samarium, calling it "eka-neodymium." For nearly 70 years, this remained chemistry's most elusive missing piece.
Multiple scientists claimed discovery of element 61:
All claims were later proven incorrect due to inadequate separation techniques and contamination with neighboring elements.
Charles Coryell, Jacob Marinsky, and Lawrence Glendenin at Oak Ridge National Laboratory finally isolated element 61 from uranium fission products during the Manhattan Project. Using advanced ion-exchange chromatography, they definitively identified and separated the radioactive element.
The discoverers initially proposed "atomium" after the atomic age, but the name was rejected. After years of debate, they chose "promethium" after Prometheus, the Greek Titan who stole fire from the gods - symbolizing humanity's mastery over nuclear fire.
The discovery was confirmed through:
Applications
Promethium, the only radioactive rare earth element with no stable isotopes, serves unique roles in specialized nuclear technologies. Promethium-147 generates beta radiation with a relatively safe energy level (225 keV), making it ideal for nuclear batteries that power deep-space missions where solar panels are ineffective.
NASA and other space agencies utilize promethium radioisotope thermoelectric generators (RTGs) for spacecraft venturing beyond Jupiter, where solar energy becomes insufficient. These compact power sources provide decades of reliable electricity for instruments on missions to Saturn, Uranus, and interstellar space.
In nuclear medicine, promethium-147 serves as a beta-emitting tracer for studying metabolic processes and organ function. Its relatively short half-life (2.6 years) allows for detailed imaging while minimizing long-term radiation exposure to patients.
High-end scientific instruments employ promethium as a thickness gauge for measuring extremely thin materials in semiconductor manufacturing. The consistent beta emission provides precise measurements critical for quality control in microelectronics production.
Emergency exit signs and aircraft instrument panels utilize promethium's phosphorescent properties, where beta radiation excites phosphor coatings to produce self-powered illumination lasting years without external power sources.
⚠️ Special Handling Required: All promethium applications require strict radiation safety protocols and specialized containment systems due to its radioactive nature.Promethium-147 powers betavoltaic cells used in pacemakers, remote weather stations, and scientific equipment in extreme environments. These batteries provide steady power for 10-20 years without maintenance, making them invaluable for inaccessible installations.
Military and aviation industries use promethium-based radioluminescent paint for instrument dials, compass markings, and emergency lighting systems. The self-luminous property eliminates dependence on external light sources in critical applications.
Scientific research facilities employ promethium sources for beta radiation calibration of detection equipment, ensuring accurate measurements in nuclear physics experiments and environmental monitoring systems.
Manufacturing industries utilize promethium-147 in thickness monitoring systems for paper, plastic, and metal sheet production, providing real-time quality assurance without physical contact with materials.
Where it comes from
0 mg/kg of Earth's crust · more abundant than 0% of elements
Promethium holds the distinction of being the only lanthanide with no stable isotopes. Unlike other rare earth elements found in mineral deposits, promethium exists naturally only in trace amounts as a fission product in uranium ores, with concentrations so minimal they're virtually undetectable.
Virtually all promethium used today originates from nuclear reactor operations. The most useful isotope, Promethium-147, is produced by neutron bombardment of neodymium-146 in specialized research reactors, yielding approximately 1-2 grams per year globally.
In the cosmos, promethium forms during stellar nuclear processes but decays too rapidly to accumulate in detectable quantities. Astronomers have identified promethium spectral lines in certain variable stars, providing insights into stellar interior conditions and nucleosynthesis mechanisms.
Natural uranium-235 fission occasionally produces promethium isotopes, but these decay within years to decades. Even in the most uranium-rich deposits, promethium concentrations remain below 10⁻¹⁸ grams per kilogram of ore.
Research facilities produce promethium through:
Handling
Promethium is radioactive. It has no stable isotope — every nucleus decays. Handling requires appropriate shielding and licensing.
Primary Risk: Promethium-147 emits beta radiation (electrons) with maximum energy of 225 keV. While external exposure is relatively low-risk due to beta particles' limited penetration, internal contamination poses serious health threats.
Work must be conducted in radiological fume hoods with negative pressure. Use remote handling tools when possible. Never eat, drink, or apply cosmetics in promethium work areas. Maintain strict inventory control of all sources.
Work surfaces must be covered with absorbent plastic-backed paper. Monitor hands, clothing, and work areas with radiation detection equipment before leaving the laboratory. Decontaminate surfaces with appropriate cleaning solutions.
Contamination incidents: Immediately notify radiation safety officer. Remove contaminated clothing and shower thoroughly. Seek medical evaluation for potential internal exposure. Spill cleanup requires specialized procedures and equipment.
Store in shielded containers within locked, posted radiological areas. Maintain temperature logs and visual inspection schedules. Sources must be inventoried monthly and leak-tested annually.
🚨 Emergency: Contact institutional radiation safety officer immediately for any promethium-related incidents or suspected contamination.Quick answers
Promethium (symbol Pm) is element 61 on the periodic table, a lanthanide in period 6. The only lanthanide with no stable isotope — it decayed away and had to be made in a reactor. At room temperature it is a solid, and it is radioactive.
[Xe] 4f⁵ 6s², giving 6 occupied shells holding 2, 8, 18, 23, 8, 2 electrons respectively.Promethium melts at 1315 K (1041.9 °C) and boils at 3273 K (2999.9 °C).
Promethium has no stable isotope, so it has no standard atomic weight. The figure quoted, 145, is the mass number of its longest-lived known isotope.
Promethium has a density of 7.26 g/cm³. Water is 1.0 g/cm³, so a block of promethium is about 7.3× heavier.
Promethium has a Pauling electronegativity of 1.13. 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.
Promethium was discovered in 1945 by Marinsky, Glendenin & Coryell. It is named after prometheus, who stole fire from the gods.
Promethium does not occur naturally on Earth in any meaningful quantity — it is made in a reactor or an accelerator.
Yes. Promethium has no stable isotope — every one of its nuclei decays. It does not occur in usable quantities in nature and must be synthesised.