Thulium Thulium is a chemical element with symbol Tm and atomic number 69. It is the thirteenth and third-last element in the lanthanide series. Like the other lanthanides, the most common oxidation state is +3, seen in its oxide, halides and other compounds; because it occurs so late in the series, however, the +2 oxidation state is also stabilized by the nearly full 4f shell that results. Wikipedia https://en.wikipedia.org/wiki/Thulium Added to support Critical Zone Observatory (CZO) data use cases. ODM2 Variable Name Controlled Vocabulary ODM2 Working Group A vocabulary for describing the name of Variables.