Tm

Thulium (symbol Tm) is a chemical element with atomic number 69, belonging to the lanthanide series. It is a soft, silvery-gray metal that tarnishes slowly in air to form a protective oxide layer. In most compounds, thulium shows the +3 oxidation state.

Thulium occurs only in trace amounts in the Earth's crust and is one of the rare earth elements. It is primarily recovered from mineral concentrates such as xenotime and monazite, from which it is separated from other lanthanides by ion-exchange and solvent-extraction processes and then purified by reduction or electrolysis to metal.

There is a single naturally occurring isotope, 169Tm, which is considered stable. Several radioactive isotopes of thulium have been synthesized in laboratories and occur in trace quantities for research, but they are not found in nature.

Thulium has applications in optics and photonics. It is used as a dopant in solid-state lasers (for example, Tm:YAG and other Tm-doped crystals) and in thulium-doped fiber lasers that emit around 2 micrometers, a wavelength useful for materials processing, sensing, and certain medical procedures. Research uses of thulium also include spectroscopy, development of specialized optical devices, and fundamental studies of lanthanide chemistry.

Historically, thulium was discovered in 1879 by Per Teodor Cleve in Sweden, and it was named after Ytterby, the mine locality associated with several rare earth elements. Safety-wise, thulium and its compounds are treated as hazardous and should be handled with appropriate precautions to avoid inhalation or ingestion.