Atomic number 61 · Lanthanide
Promethium Pm
Promethium (Pm), element 61, is the only lanthanide with no stable isotopes. Because virtually none of it survives in nature, its place in the periodic table stayed empty for decades until it turned up among the fission products of nuclear reactors. That is also why its atomic mass is written in brackets as [145].
Key properties
| Atomic number | 61 |
|---|---|
| Atomic weight | [145] (No stable isotope; mass number of a representative isotope in brackets) |
| Category | Lanthanide |
| Group | Lanthanides/actinides (no group number) |
| Period | 6 |
| Block | f |
| State (25 °C, 1 atm) | Solid |
| Melting point | 1315 K (1041.8 °C) |
| Boiling point | 3273 K (2999.8 °C) |
| Density | 7.26 g/cm³ |
| Electronegativity (Pauling) | — |
| Atomic radius (van der Waals) | 236 pm |
| First ionization energy | 5.55 eV |
| Electron affinity | — |
| Oxidation states | +3 |
| Discovered | 1945 |
Electron configuration
- Condensed
- [Xe]6s2 4f5
- Full
- 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 4f5 5s2 5p6 6s2
- Electrons per shell
- 2 · 8 · 18 · 23 · 8 · 2
Orbital diagram
Position in the table
Discovery
Once Henry Moseley tied X-ray spectra to atomic number in 1913, chemists could see a clear gap between neodymium (60) and samarium (62). In the 1920s, groups in the United States and Italy announced "illinium" and "florentium", but neither claim could be reproduced; the element simply was not present in their rare-earth samples in any detectable amount.
The real discovery came at Clinton Laboratories in Oak Ridge, Tennessee. In 1945 Jacob Marinsky, Lawrence Glendenin and Charles Coryell separated uranium fission products by ion-exchange chromatography and identified isotopes of element 61. Wartime secrecy delayed the announcement until 1947.
Origin of the name
The name honours Prometheus, the Titan of Greek myth who stole fire from the gods and gave it to humanity. It is usually credited to Grace Mary Coryell, the wife of one of the discoverers, and was meant to evoke both the power and the danger of nuclear energy, the very process that produced the element.
Main uses
Applications are small and specialised:
- Nuclear batteries: the gentle beta radiation of promethium-147 has been converted to electricity in betavoltaic cells, which once powered some cardiac pacemakers and space or military devices.
- Luminous paint: it replaced radium in some watch dials and instrument markings, where its beta particles made a phosphor glow.
- Thickness gauges: industry measures paper, plastic film and foil by how much beta radiation passes through.
Most of these uses have since been replaced by other isotopes or non-radioactive technology.
Isotopes
Promethium has no stable isotopes. The longest-lived is promethium-145, with a half-life of about 17.7 years. The workhorse isotope is promethium-147, with a half-life of about 2.6 years, which is recovered from spent reactor fuel or made by irradiating neodymium with neutrons. Promethium and technetium (43) are the two classic examples of unstable elements sitting between stable neighbours, a quirk explained by nuclear shell structure.
In everyday life
You are very unlikely to meet promethium. In the Earth's crust it forms only in trace amounts through spontaneous fission of uranium, so the total present at any moment is tiny. Old luminous instruments or gauges containing promethium should not be opened or thrown away with household waste; they belong in a proper radioactive-waste stream.
Good to know
- Promethium is the only lanthanide without a single stable isotope.
- The discovery was made in 1945 but kept secret until 1947 because of wartime security.
- Absorption lines attributed to promethium have been reported in the spectra of a few unusual stars, puzzling astronomers because it decays so quickly.
Same category (Lanthanide)
Data sources · Properties: PubChem Periodic Table (US NIH/NLM public data) · Atomic weights: CIAAW 2024 abridged standard atomic weights · Names: Wikidata (CC0); Korean names follow the Korean Chemical Society. Retrieved 2026-09-23.