thulium

Thulium is a chemical element; it has symbol Tm and atomic number 69. It is the thirteenth element in the lanthanide series of metals. It is the second-least abundant lanthanide in the Earth's crust, after radioactively unstable promethium. It is an easily workable metal with a bright silvery-gray luster. It is fairly soft and slowly tarnishes in air. Despite its high price and rarity, thulium is used as a dopant in solid-state lasers. It has no significant biological role and is not particularly toxic. Artificial radioactive isotopes of thulium are used as radiation sources in some portable X-ray devices.

In 1879, the Swedish chemist Per Teodor Cleve separated two previously unknown components, which he called holmia and thulia, from the rare-earth mineral erbia; these were the oxides of holmium and thulium, respectively. His example of thulium oxide contained impurities of ytterbium oxide. A relatively pure sample of thulium oxide was first obtained in 1911. The metal itself was first obtained in 1936 by Wilhelm Klemm and Heinrich Bommer.