Thulium is a chemical element with symbol Tm and atomic number 69. Its valence shell electron configuration is 4f13 6s2, which makes it the second-largest lanthanide. (Lu, which is often considered a lanthanide, is now classed as a Group 3 transition metal, because of its 5d electron.) Because its Z is high, thulium's electrons are relatively tightly bound, making Tm one of the less reactive lanthanides.
Thulium is a rare metal by any standard. Its solar system mole fraction relative to Si is 3.78E-08. around 3% as abundant as Ce, and 20% as abundant as gold. Its inherent scarcity comes mainly from having an odd Z. Odd-Z elements have one or two stable elements, while even-Z elements often have five or more. It is also scarce because of its high A - large nuclides are built in a step-by-step fashion which demands that abundance go down when A rises. The element is also rare in earth's crust, but in a way which speaks to the element's chemistry. Its mass crustal mass fraction is 4.1E-07, which makes it easily the rarest lanthanide, and even rarer than Lu. Cerium is 150 times as abundant and even Ho is three times as common. Tm is not only inherently rare, but is more rare than its solar abundance indicates. That implies Tm is relatively inert for a lanthanide, a point hammered home by the fact that Tm is around 150 times as abundant as Au (which is as inert as metals get).
Thirty-five isotopes of Tm have been observed, along with 26 isomers. The total number of predicted isotopes is 84. Thulium's only stable isotope is 169Tm. All heavier isotopes decay by beta emission. Their half-lives are relatively short, with only 171Tm (1.92 yr) and 170Tm (129 d) exceeding 3 days. All thulium's isotopes lighter than 169Tm decay by emitting positive charge. Positive beta decay (positron emission or electron capture) has been observed in all lighter isotopes down to 146Tm, although proton emission dominates at 146Tm and below. Isotopes in the band 156Tm to 153Tm also decay by alpha emission due to structural instability at neutron counts slightly above 82.