[thoo-lee-uh
m] 
| a rare-earth metallic element found in the minerals euxenite, gadolinite, etc. Symbol: Tm; atomic weight: 168.934; atomic number: 69; specific gravity: 9.32. |
| thu·li·um (thōō'lē-əm, thyōō'-) n. Symbol Tm A bright, silvery rare-earth element obtained commercially from monazite, 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 1,727°C; specific gravity 9.3; valence 2, 3. See Table at element. [After Thule1.] |
thulium thu·li·um (th&oomacr;'lē-əm, thy&oomacr;'-)
n.
Symbol Tm
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.
thulium (th 'lē-əm) Pronunciation Key
Symbol Tm 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. |
thulium
((Tm), chemical element, rare-earth metal of the lanthanoid series of the periodic table. One of the rarest of the rare-earth elements, though more abundant than silver, thulium has few commercial uses. Natural thulium is wholly composed of the stable isotope thulium-169. Bombarded by neutrons, it becomes radioactive thulium-170 (128-day half-life), which ejects soft gamma radiation (0.084 million electron volts) resembling an X ray. It is useful in small portable X-ray units suitable for photographing bony tissues and for examining thin-walled machine parts and has been used by archaeologists to examine markings and symbols on ancient metallic artifacts.
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