Thulium is a chemical element with the symbol Tm and atomic number 69. It is the thirteenth and third-last element in the lanthanide series.
Phase at STP： solid
Melting point： 1818 K (1545 °C, 2813 °F)
Boiling point： 2223 K (1950 °C, 3542 °F)
Density (near r.t.)： 9.32 g/cm3
when liquid (at m.p.)： 8.56 g/cm3
Heat of fusion： 16.84 kJ/mol
Heat of vaporization： 191 kJ/mol
Molar heat capacity： 27.03 J/(mol·K)
Despite its high price and rarity, thulium is used as the radiation source in portable X-ray devices, and in some solid-state lasers. It has no significant biological role and is not particularly toxic.