Thulium is a silvery-gray lanthanide metal. It is soft, malleable, and forms a stable +3 oxidation state; a +2 state also occurs in some compounds.
The ground-state electron configuration of thulium is [Xe] 4f13 6s2. In the common +3 oxidation state (Tm3+), it loses the two 6s electrons and one 4f electron, resulting in a 4f12 configuration.
Thulium primarily forms the +3 oxidation state (Tm3+), found in compounds such as Tm2O3 and TmCl3. It can also exist in the +2 state (Tm2+) in certain halides and chalcogenides like TmI2 and TmS, though this state is less stable and more reactive.
Major applications of thulium include:
Thulium-doped lasers (especially Tm:YAG lasers) emit infrared light at ~2.0 µm wavelength, which is strongly absorbed by water and biological tissues. This makes them ideal for precise surgical applications with minimal thermal damage.
Thulium slowly oxidizes in air, forming a protective oxide layer of Tm2O3. It reacts slowly with water, producing thulium hydroxide and hydrogen gas:
\(\mathrm{2\,Tm(s) + 6\,H_2O(l) \rightarrow 2\,Tm(OH)_3(s) + 3\,H_2(g)}\)
Thulium is paramagnetic at room temperature due to unpaired 4f electrons. Below 32 K, it exhibits antiferromagnetic ordering. Its unique magnetic behavior makes it useful in cryogenic magnetic refrigeration research.
Thulium occurs in small quantities in rare-earth minerals such as monazite and bastnäsite. It is separated from other lanthanides using solvent extraction or ion-exchange chromatography, as the lanthanides have very similar chemical properties.
Important compounds include:
Natural thulium consists of a single stable isotope, 169Tm, which is not radioactive. It is considered to have low toxicity, but fine powders should be handled carefully to avoid inhalation or contact irritation. The isotope 170Tm is radioactive and used in industrial X-ray devices.
When thulium reacts with oxygen, it forms thulium(III) oxide:
\(\mathrm{4\,Tm(s) + 3\,O_2(g) \rightarrow 2\,Tm_2O_3(s)}\)
This oxide is stable and exhibits a pale green color, characteristic of trivalent thulium compounds.