Thulium
Thulium is a soft, silvery lanthanide identified in 1879, and the least abundant of the naturally occurring rare-earth elements found on Earth. Its name comes from Thule, an old name for a distant northern land, fitting for an element first found in Scandinavia.
- Group · Period
- — · 6
- At room temp
- solid
- Melts at
- 1818 K
- Density
- 9.32 g/cm³
- Discovered
- 1879
Uses
Because it’s the scarcest naturally occurring lanthanide, thulium tends to appear in smaller, more specialized applications rather than bulk industrial ones. A radioactive isotope, thulium-170, has been used as the radiation source in portable X-ray devices, valued in field settings where a bulky, powered X-ray machine isn’t practical. Thulium is also used in some solid-state lasers that emit light at a wavelength considered safer for the human eye than many other lasers, a property that’s useful for military rangefinding equipment and for certain delicate surgical procedures.
History
Thulium was identified in 1879 by Swedish chemist Per Teodor Cleve, who found it as one of two new elements — alongside holmium — hiding within what chemists had thought was pure erbium oxide. Separating thulium out required Cleve to carefully work through several rounds of fractional purification, a common challenge when teasing individual lanthanides apart from mineral mixtures. He named it after Thule, an old name ancient geographers used for a distant northern land, a fitting reference given the Scandinavian origins of so much early rare-earth research.
Fun facts
- Thulium is the least abundant of the naturally occurring lanthanides in Earth's crust.
- A radioactive form of thulium has been used to power small, portable X-ray devices that don't need an external power source.
- Thulium is used in some solid-state lasers that operate at wavelengths considered safer for the human eye, useful for rangefinding and medical procedures.
Frequently asked questions
Is thulium really the rarest rare-earth element?
Among the lanthanides that occur naturally and have stable, long-lived isotopes, thulium is generally considered the least abundant in Earth's crust. Promethium is rarer still, but that's because it has no stable isotopes at all and exists on Earth only in tiny, fleeting amounts, so it's usually treated as a special case rather than compared directly with thulium.
What is thulium used for if it's so rare?
Its scarcity and cost mean thulium is used in fairly specialized, low-volume applications rather than bulk industrial ones. A radioactive isotope, thulium-170, has been used in portable X-ray sources for situations without reliable power. Thulium is also used in certain lasers, including ones that work at an eye-safer wavelength useful for military rangefinding and some medical procedures.
Why is thulium named after Thule?
Thule was an ancient name used by Greek and Roman geographers for a distant land somewhere in the far north, often associated with Scandinavia. Swedish chemist Per Teodor Cleve, who identified thulium in 1879, chose the name as a nod to its Scandinavian discovery, following a common 19th-century habit of tying new element names to their place of origin.
Compounds
2 notable compounds containing Tm
- TmCl3halide
Thulium(III) chloride
A yellow, hygroscopic thulium salt normally handled as its hydrate, used as a common laboratory starting material for other thulium compounds.
Used for: Laboratory precursor for thulium metal and thulium compounds
- Tm2O3oxide
Thulium(III) oxide
A pale green rare-earth oxide and the standard commercial form of thulium, one of the least abundant naturally occurring lanthanides.
Used for: Starting material for thulium-doped solid-state and fiber lasers
Isotopes
38 known isotopes
Swipe to see all columns →
| Isotope | Natural abundance | Half-life | Decay mode | Mass excess | Binding energy / nucleon | Decay width |
|---|---|---|---|---|---|---|
| Tm-169 stable | 100% | Stable | — | -61,274.65 keV | 8,114.47 keV | — |
Show all 38 isotopes
| Tm-144 | — | 1.9 us | Proton emission (0%) | -22,159 keV | 7,850 keV | 2.40 × 10-10 eV |
| Tm-145 | — | 3.17 us | Proton emission (100%) | -27,583 keV | 7,889 keV | 1.44 × 10-10 eV |
| Tm-146 | — | 68 ms | Proton emission, Electron capture / beta-plus decay | -31,055 keV | 7,914 keV | 6.71 × 10-15 eV |
| Tm-147 | — | 0.58 s | Electron capture / beta-plus decay (85%), Proton emission (15%) | -35,974.41 keV | 7,948.82 keV | 7.87 × 10-16 eV |
| Tm-148 | — | 0.7 s | Electron capture / beta-plus decay (100%) | -38,765.03 keV | 7,968.5 keV | 6.52 × 10-16 eV |
| Tm-149 | — | 0.9 s | Electron capture / beta-plus decay (100%), ECP (0.2%) | -43,940 keV | 8,004 keV | 5.07 × 10-16 eV |
| Tm-150 | — | 2.20 s | Electron capture / beta-plus decay (100%), ECP (1.2%) | -46,491 keV | 8,021 keV | 2.07 × 10-16 eV |
| Tm-151 | — | 4.17 s | Electron capture / beta-plus decay (100%) | -50,771.61 keV | 8,050.06 keV | 1.09 × 10-16 eV |
| Tm-152 | — | 8.0 s | Electron capture / beta-plus decay (100%) | -51,720.28 keV | 8,056.44 keV | 5.70 × 10-17 eV |
| Tm-153 | — | 1.48 s | Alpha decay (91%), Electron capture / beta-plus decay (9%) | -53,972.61 keV | 8,071.26 keV | 3.08 × 10-16 eV |
| Tm-154 | — | 8.1 s | Alpha decay (54%), Electron capture / beta-plus decay (46%) | -54,427.14 keV | 8,074.21 keV | 5.63 × 10-17 eV |
| Tm-155 | — | 21.6 s | Electron capture / beta-plus decay (99.17%), Alpha decay (0.83%) | -56,625.92 keV | 8,088.38 keV | 2.11 × 10-17 eV |
| Tm-156 | — | 83.8 s | Electron capture / beta-plus decay (99.936%), Alpha decay (0.064%) | -56,834.42 keV | 8,089.6 keV | 5.44 × 10-18 eV |
| Tm-157 | — | 3.63 m | Electron capture / beta-plus decay (100%) | -58,709.28 keV | 8,101.43 keV | 2.09 × 10-18 eV |
| Tm-158 | — | 3.98 m | Electron capture / beta-plus decay (100%) | -58,703.2 keV | 8,101.2 keV | 1.91 × 10-18 eV |
| Tm-159 | — | 9.13 m | Electron capture / beta-plus decay (100%) | -60,570.4 keV | 8,112.75 keV | 8.33 × 10-19 eV |
| Tm-160 | — | 9.4 m | Electron capture / beta-plus decay (100%) | -60,301.04 keV | 8,110.81 keV | 8.09 × 10-19 eV |
| Tm-161 | — | 30.2 m | Electron capture / beta-plus decay (100%) | -61,898.72 keV | 8,120.49 keV | 2.52 × 10-19 eV |
| Tm-162 | — | 21.70 m | Electron capture / beta-plus decay (100%) | -61,477.48 keV | 8,117.59 keV | 3.50 × 10-19 eV |
| Tm-163 | — | 1.810 h | Electron capture / beta-plus decay (100%) | -62,728.41 keV | 8,124.98 keV | 7.00 × 10-20 eV |
| Tm-164 | — | 1.95 m | Electron capture / beta-plus decay (100%), Beta-plus decay (39%) | -61,908.94 keV | 8,119.65 keV | 3.90 × 10-18 eV |
| Tm-165 | — | 30.06 h | Electron capture / beta-plus decay (100%) | -62,930.02 keV | 8,125.55 keV | 4.22 × 10-21 eV |
| Tm-166 | — | 7.70 h | Electron capture / beta-plus decay (100%) | -61,886.48 keV | 8,118.94 keV | 1.65 × 10-20 eV |
| Tm-167 | — | 9.25 d | Electron capture (100%) | -62,543.12 keV | 8,122.58 keV | 5.71 × 10-22 eV |
| Tm-168 | — | 93.1 d | Electron capture / beta-plus decay (99.99%), Beta-minus decay (0.01%) | -61,312.38 keV | 8,114.95 keV | 5.67 × 10-23 eV |
| Tm-170 | — | 128.6 d | Beta-minus decay (99.869%), Electron capture (0.131%) | -59,795.31 keV | 8,105.51 keV | 4.11 × 10-23 eV |
| Tm-171 | — | 1.92 Y | Beta-minus decay (100%) | -59,210.27 keV | 8,101.89 keV | 7.53 × 10-24 eV |
| Tm-172 | — | 63.6 h | Beta-minus decay (100%) | -57,373.55 keV | 8,091.04 keV | 1.99 × 10-21 eV |
| Tm-173 | — | 8.24 h | Beta-minus decay (100%) | -56,256.07 keV | 8,084.46 keV | 1.54 × 10-20 eV |
| Tm-174 | — | 5.4 m | Beta-minus decay (100%) | -53,864.52 keV | 8,070.64 keV | 1.41 × 10-18 eV |
| Tm-175 | — | 15.2 m | Beta-minus decay (100%) | -52,310.56 keV | 8,061.77 keV | 5.00 × 10-19 eV |
| Tm-176 | — | 1.85 m | Beta-minus decay (100%) | -49,371.32 keV | 8,045.12 keV | 4.11 × 10-18 eV |
| Tm-177 | — | 95 s | Beta-minus decay (100%) | -47,570 keV | 8,035 keV | 4.80 × 10-18 eV |
| Tm-178 | — | 300 ns | Beta-minus decay | -44,240 keV | 8,017 keV | 1.52 × 10-9 eV |
| Tm-179 | — | 160 ns | Beta-minus decay | -41,900 keV | 8,004 keV | 2.85 × 10-9 eV |
| Tm-180 | — | 300 ns | Beta-minus decay (100%) | -38,170 keV | 7,983 keV | 1.52 × 10-9 eV |
| Tm-181 | — | 160 ns | Beta-minus decay, Beta-minus, neutron emission | -35,440 keV | 7,969 keV | 2.85 × 10-9 eV |