Promethium
Promethium, originally prometheum, is a chemical element with the symbol Pm and atomic number 61. All of its isotopes are radioactive; it is one of only two such elements that are followed in the periodic table by elements with stable forms, a distinction shared with technetium. Chemically, promethium is a lanthanide, which forms salts when combined with other elements.
Identity
- Atomic mass
- 145 u
- Category
- Lanthanides
- Group
- 3
- Period
- 6
- Block
- f
- Oxidation States
- +3
- Phase (25 °C)
- Solid
- Appearance
- metallic
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f5
- Short configuration
- [Xe] 4f5 6s2
- Electrons per shell
- 2, 8, 18, 23, 8, 2
- Electronegativity (Pauling)
- 1.13
- Electron affinity
- 12.4 kJ/mol
- 1st ionization energy
- 540 kJ/mol
- Ionization energies
- 540 · 1050 · 2150 · 3970 kJ/mol
Physical properties
- Melting point
- 1041.8 °C (1315 K)
- Boiling point
- 2999.8 °C (3273 K)
- Density
- 7.26 g/cm³
- Molar heat capacity
- —
Chemical behavior of promethium
Promethium has five outer electrons ([Xe] 4f⁵ 6s²) and almost always acts as +3, with very low electronegativity (1.13) and bonds of markedly ionic character. It is the only lanthanide with no stable isotopes: all its chemistry must be studied in radioactive samples that decay. Like the other lanthanides it is highly electropositive: it oxidizes in air, reacts with water and dissolves readily in dilute acids.
Reactivity
4 Pm + 3 O₂ → 2 Pm₂O₃
In air it oxidizes rapidly to pinkish-violet promethium(III) oxide. The heat from its own decay accelerates the surface oxidation.
2 Pm + 6 H₂O → 2 Pm(OH)₃ + 3 H₂
It reacts with water releasing hydrogen and forming the hydroxide. The reaction must be carried out in shielded cells because of the metal's beta radiation.
2 Pm + 6 HCl → 2 PmCl₃ + 3 H₂
It dissolves in dilute acids giving pinkish promethium(III) salts. The chloride is the starting point for separating Pm-147 from spent nuclear fuel.
Pm₂O₃ + 6 HF → 2 PmF₃ + 3 H₂O
With hydrofluoric acid it gives the insoluble fluoride, useful to precipitate and purify promethium against other fission products.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Promethium oxide | Pm₂O₃ | Beta source for batteries and phosphors |
| Promethium chloride | PmCl₃ | Separation from nuclear fuel |
| Promethium fluoride | PmF₃ | Precipitation and purification |
| Promethium oxalate | Pm₂(C₂O₄)₃ | Intermediate in radiochemical purification |
| Promethium boride | PmB₆ | Studies of radioactive materials |
Electron shell distribution
View in the atom builderIsotopes
No isotope of promethium is stable. The longest-lived is promethium-145 (¹⁴⁵Pm), with 84 neutrons and a half-life of about 17.7 years. The most used is promethium-147 (¹⁴⁷Pm), a pure beta emitter with a half-life of 2.62 years that decays to samarium-147 with almost no gamma. Promethium-151 (¹⁵¹Pm, 28 hours) is used as a tracer in research.
Occurrence and production
It barely exists in nature: only fleeting traces appear from the spontaneous fission of uranium-238 or the decay of europium-151, on the order of picograms per ton. All useful promethium is produced artificially: by fission of uranium-235 in reactors or by neutron capture on neodymium-146 to neodymium-147, which decays to Pm-147. Oak Ridge and Dimitrovgrad are the historical producers.
History and origin of the name
Square 61 was the last rare earth to be filled. Moseley predicted it by X-rays and dozens of false discoveries looked for it (florenium, illinium). In 1945 Marinsky, Glendenin and Coryell isolated it by ion exchange among the fission products of the Clinton Pile and announced it in 1947. They named it promethium, after Prometheus, who stole fire from the gods: a tragic and luminous name for an element that glows by radioactivity.
Applications and relevance of promethium
Uses of the element
As a pure beta source, Pm-147 powers miniature nuclear batteries that convert its decay into electricity for years without recharging, for remote sensors, space equipment and experimental pacemakers. Its decay heat has also been studied as an auxiliary heat source.
Uses of its compounds
Mixed with zinc sulfide phosphors it produces luminous paints and signs that glow without electricity, an alternative to radium in watch dials and panels. As a calibrated beta source it measures the thickness of paper, plastic and metal sheets on industrial lines without contact.
Health, safety and environment
Biological role
It has no biological function and is doubly toxic: lanthanide chemistry and beta radiotoxicity. Pm-147 fixes in bone and liver if it is taken up, so its handling requires dosimetric control and full containment.
Safety
Its low-energy beta radiation does not penetrate intact skin, but ingested or inhaled it irradiates internal tissues. It is handled only in licensed facilities, encapsulated in steel, with acrylic shielding, gloves and monitors. It does not exist in current consumer products.
Environment
There is no significant natural background nor routine spills: all inventory is artificial and accounted for. Pm-147 waste decays to harmless levels in a few decades, but until then it is managed as intermediate-level radioactive waste with full traceability.
Frequently asked questions about Promethium
Why is promethium radioactive?
Promethium is the only lanthanide that has no stable isotopes: all of its atoms decay over time and emit radiation. For that reason it does not occur naturally in appreciable quantities, and the promethium that is usable is produced mainly in nuclear reactors. Its best-known isotope, promethium-147, emits low-energy beta particles, a radiation that can be harnessed in tightly controlled technical applications.
Is promethium dangerous to people?
Yes, promethium is dangerous if it is not handled properly, because its radioactivity can damage tissues and increase health risks upon significant exposure. However, it is not an element present in everyday objects or in the home environment. Its use is restricted to licensed facilities, where it is encapsulated, shielded, subject to dose controls and strict storage and disposal protocols.
What is a promethium nuclear battery?
A promethium nuclear battery is a device that converts the energy of the radioactive decay of promethium-147 into electricity. It does not work by a chain reaction, but by harnessing beta emissions or the heat generated to power very low-power circuits. These sources can last for years without recharging, so they have been studied for remote sensors, space equipment and experimental medical applications.
Why is promethium so difficult to study?
Its difficulty comes from its radioactivity and the absence of stable isotopes. Any sample decays continuously, so it must be produced, handled and stored under controlled conditions. In addition, because it is scarce and requires nuclear facilities to obtain, it is not available as a common laboratory metal. This limits its chemical study and explains why it is one of the lanthanides least known to the general public.
What is promethium-147 used for?
Promethium-147 has been used in miniature nuclear batteries, luminous sources and industrial thickness gauges. In the batteries, the energy released by its decay is converted into electricity for devices that must run for years without recharging. In instrumentation, its emissions allow the thickness of sheets and materials to be measured. Its current uses are highly specialized because of its radioactivity, its scarcity and the strict regulation that surrounds radioactive materials.
Is promethium found in nature?
Promethium barely occurs naturally. Although traces can form during the decay of other radioactive elements, its half-life is relatively short and it disappears quickly. For this reason the promethium used in research or technology is obtained artificially, above all in nuclear reactors. This combination of instability, natural scarcity and artificial production distinguishes it from most of the other lanthanides.
Why was promethium used in luminous paints?
Promethium-147 emits beta radiation that can excite a phosphorescent material and make it emit visible light. That principle was used in luminous indicators, panels and clocks, as a safer alternative to the radium used in the past. The beta particles of promethium are less penetrating, which reduced the risks when the material was properly encapsulated. Over time, many of these applications were replaced by non-radioactive technologies.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.