Thulium
Element 69 · Lanthanide
Thulium is the scarcest of the naturally occurring lanthanides, a soft silvery metal that tarnishes slowly and is always +3 in compounds. Its rarity keeps it expensive and its uses specialized.
Irradiated thulium-170 emits X-rays, making it useful in portable X-ray sources that need no electricity — handy for field inspections. Thulium also dopes high-efficiency lamps and lasers, and its compounds show promise in specialized optical devices.
Key facts
| Atomic number | 69 |
|---|---|
| Atomic weight | 168.93 |
| Category | Lanthanide |
| Group / Period | — / 6 |
| Block | f |
| Electron configuration | 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13 |
| Common oxidation states | +3 |
| State at STP | Solid |
| Electronegativity | 1.25 |
| Density | 9.32 g/cm³ |
| Melting point | 1545 °C |
| Boiling point | 1950 °C |
Occurrence
Thulium occurs in monazite, xenotime, and euxenite in very small amounts, recovered as a byproduct of mining the more abundant rare earths.
Applications
Portable X-ray sources (Tm-170)
High-efficiency discharge lamps
Solid-state lasers
Research phosphors
Common compounds
- thulium oxide
- thulium chloride
Thulium is named after Thule, the ancient name for the far north — fitting for the scarcest of the stable rare earth metals.
Data source: IUPAC standard atomic weights 2021