Thulium is a chemical element that has the symbol Tm and atomic number 69. It is the thirteenth and antepenultimate element in the lanthanide series. Like the other lanthanides, the most common oxidation state is +3, seen in its oxide, halides and other compounds. In aqueous solution, like compounds of other late lanthanides, soluble thulium compounds form complexes with nine water molecules.
Thulium is the second least abundant of the lanthanides after promethium, which is only found in trace quantities on Earth. 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 the radiation source in portable X-ray devices and in solid-state lasers. It has no significant biological role and is not particularly toxic.