thulium thu·li·um (thōō'lē-əm, thyōō'-)
A rare-earth element having an x-ray emitting isotope that is used in small portable medical x-ray units. Atomic number 69; atomic weight 168.934; melting point 1,545°C; boiling point 1,946°C; specific gravity 9.3; valence 3.
A soft, silver-gray metallic element of the lanthanide series. One of its artificial radioactive isotopes is used as a radiation source in small, portable x-ray machines. Atomic number 69; atomic weight 168.934; melting point 1,545°C; boiling point 1,727°C; specific gravity 9.3; valence 2, 3. See Periodic Table.