Thulium (Tm)
“The rarest stable lanthanide, named for a mythical land at the top of the world — and the isotope that takes X-rays where electricity cannot reach.”
How its electrons are arranged
Computed from the aufbau principle, with known real-world exceptions applied automatically.
Characteristics
- Electron configuration [Xe] 4f¹³ 6s² — thirteen of the 4f sub-shell’s fourteen slots are filled, and its working ion Tm³⁺ carries [Xe] 4f¹².
- The rarest stable lanthanide: thulium never occurs free in nature, and only a whisper of it hides in the common rare-earth ores.
- Metal soft enough to slice with a knife; it tarnishes slowly in air and dissolves in dilute acids to give pale green Tm³⁺ salts.
- Its laser lines near 2 micrometres — 2,013 nanometres from Tm:YAG among them — land where water absorbs light most fiercely, the physical secret of its surgical edge.
A very soft, bright silvery metal that keeps its shine in sealed containers but slowly tarnishes in open air, and it can be cut with an ordinary knife. Dissolved in dilute acid it gives Tm³⁺, whose salts tint solutions a faint, delicate green.
Where you meet it
✦ Uses
- Thulium-170 portable X-ray sources — isotope capsules that emit penetrating rays with no electricity at all, serving field clinics, army medics and pipeline inspectors far from any power line.
- Thulium fibre lasers near 2 micrometres, whose water-hungry beam dusts urinary stones and trims prostate tissue with remarkably little bleeding.
- Industrial thulium fibre lasers for welding transparent plastics and precision-cutting where ordinary 1-micrometre beams pass straight through.
- Phosphor doping, including X-ray intensifying screens where Tm³⁺ turns invisible rays into visible light.
◍ Everyday examples
- A mobile medical team X-raying patients in a riverine char village, far from the grid, can carry a thulium-170 capsule instead of a power-hungry machine.
- Urology departments advertising “no-scalpel” stone removal are increasingly reaching for thulium fibre lasers to do the job.
History & name
In 1879, while pulling erbium oxide apart at Uppsala, the Swedish chemist Per Teodor Cleve isolated two new earths beside erbium itself and named one thulia — for Thule, the semi-mythical island the ancient Greeks and Romans placed at the far northern edge of the world, usually read as Scandinavia. Even among rare earths thulium proved exceptionally shy: it took until 1911, when the American chemist Charles James ground through an estimated fifteen thousand bromate fractional crystallisations, to obtain a nearly pure sample.