Thulium
lanthanideProperties
| Property | Value |
|---|---|
| Atomic Mass | 168.93 amu |
| Category | lanthanide |
| Period | 6 |
| Electron Configuration | [Xe] 4f13 6s2 |
| Electronegativity | 1.25 (Pauling) |
| Oxidation States | 2, 3 |
| Melting Point | 1818 K (1544.8 °C) |
| Boiling Point | 2223 K (1949.8 °C) |
| Density | 9.32 g/cm³ |
| Discovered By | Per Teodor Cleve (1879) |
About Thulium
Thulium is the scarcest of the natural lanthanides — Cleve isolated it in 1879 from erbia residues by counting fractional crystallizations until the spectral lines stopped shifting. It still costs more per kilogram than most rare earths, which is why it shows up in places where you genuinely need its specific photonics rather than as a bulk material. The headline application is Tm:YAG and Tm-doped silica fiber lasers running on the ³F₄ → ³H₆ transition near 1.94 µm — a wavelength water absorbs strongly enough that surgeons use it for prostate enucleation (ThuLEP), kidney-stone fragmentation, and soft-tissue cutting with very shallow thermal penetration. Tm-doped fiber amplifiers also extend optical-comm gain into the S-band beyond what erbium can cover. ¹⁷⁰Tm pellets, activated to about 1 Ci by a few weeks in a reactor, become palm-sized 84 keV γ sources that field crews use for industrial radiography on welds where lugging an iridium camera isn't practical.
Fun Fact
Thulium is named after Thule, the legendary land at the edge of the world in ancient mythology — a fitting name for an element that sat at the frontier of chemical discovery for decades.
Common Uses
- Tm:YAG and Tm:fiber surgical lasers at 1.94 µm for ThuLEP and lithotripsy
- Tm-doped silica fiber amplifiers extending dense WDM into the S-band
- ¹⁷⁰Tm portable γ-radiography sources for pipeline and weld inspection
- Tm:YLF blue-green upconversion lasers for biomedical imaging
- Anti-counterfeiting upconversion phosphors in passport security inks
- Co-dopant with Ho in 2 µm laser glasses for eye-safe LIDAR rangefinders