Pm · Element 61 · Lanthanide
The only lanthanide with no stable isotope — it decayed away and had to be made in a reactor.
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.
Same element, different neutrons
All promethium atoms have the same number of protons, 61, and differ only in their number of neutrons: those variants are its isotopes. None of them is stable. The longest-lived known isotope, Pm-145, has 84 neutrons and a half-life of 17.7 yrs.
| ISOTOPE | Neutrons | Natural Abundance | Half-life | Decay Mode |
|---|---|---|---|---|
| Pm-143 | 82 | — | 265 days | ε/β⁺ |
| Pm-144 | 83 | — | 363 days | ε/β⁺ |
| Pm-145 longest-lived | 84 | — | 17.7 yrs | ε |
| Pm-146 | 85 | — | 5.53 yrs | ε (65.7%), β⁻ (34.3%) |
| Pm-147 | 86 | — | 2.6234 yrs | β⁻ |
Here the classroom shortcut, rounded atomic mass (145) minus atomic number, gives the same answer: 84.
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 183 pm — that is 0.183 nm, so about 2.73 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.
In 1902 Bohuslav Brauner noticed that neodymium and samarium differed more than any other neighbouring rare earths and proposed an element between them. For over 40 years, this remained chemistry's most elusive missing piece.
Several scientists predicted or claimed element 61:
All discovery 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 "clintonium" after Clinton Laboratories; Grace Mary Coryell, Charles Coryell's wife, suggested "prometheum", later spelled 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 useful for small nuclear (betavoltaic) batteries with a useful life of about five years.
NASA's deep-space radioisotope thermoelectric generators (RTGs) run on plutonium-238; promethium has only been proposed as an auxiliary power source for probes. These compact power sources provide decades of reliable electricity for instruments on missions to Saturn, Uranus, and interstellar space.
Promethium-147 has no established role in nuclear medicine; as a nearly pure beta emitter it cannot be imaged.
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 harsh environments. These batteries have a useful lifetime of about five years, their output halving every 2.6 years, 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.
Spontaneous fission of natural uranium-238, and rare alpha decay of europium-151, occasionally produce promethium isotopes, but these decay within years to decades. Even in the most uranium-rich deposits, promethium makes up only about 4 parts per quintillion by mass (about 4 × 10⁻¹⁵ 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 and leak-tested at the intervals the license sets, typically at least every six months for sealed sources.
🚨 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 with the distribution 2, 8, 18, 23, 8, 2.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 no stable isotope. Its longest-lived known isotope, Pm-145, has 84 neutrons (mass number 145 minus atomic number 61) and a half-life of 17.7 yrs. The classroom shortcut, rounded atomic mass (145) minus atomic number, gives the same 84.
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. Origin of the name: 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.