Promethium
Element 61 · Lanthanide
Promethium is one of only two elements lighter than bismuth with no stable isotopes at all, so it barely exists in nature and is produced artificially in nuclear reactors instead. Like other lanthanides it favors the +3 state, but its radioactivity dominates every practical decision about it.
Its emitted beta particles once made watch dials and instrument markers glow without any external power, and the same steady energy powers beta-voltaic ‘nuclear batteries’ for remote devices. Because it is intensely radioactive, promethium is handled only in specialized facilities — it is never a consumer material.
Key facts
| Atomic number | 61 |
|---|---|
| Atomic weight | [145] |
| Category | Lanthanide |
| Group / Period | — / 6 |
| Block | f |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f5 |
| Common oxidation states | +3 |
| State at STP | Solid |
| Electronegativity | 1.13 |
| Density | 7.26 g/cm³ |
| Melting point | 1042 °C |
| Boiling point | 3000 °C |
Occurrence
Only trace amounts of promethium form in uranium ores as a fission product. Usable quantities are made by irradiating neodymium or uranium targets in nuclear reactors.
Applications
Luminous paint for instruments (historic)
Beta-voltaic nuclear batteries
Thickness gauges for industry
Research radiation sources
Common compounds
- promethium(III) oxide
- promethium(III) chloride
Promethium is one of only two elements lighter than bismuth with no stable isotopes — the other is technetium. It is named after Prometheus, who stole fire from the gods.
Data source: IUPAC standard atomic weights 2021