noun Chemistry.
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.

1875–80; < Neo-Latin; see Thule, -ium Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
thulium (ˈθjuːlɪəm)
a malleable ductile silvery-grey element occurring principally in monazite. The radioisotope thulium-170 is used as an electron source in portable X-ray units. Symbol: Tm; atomic no: 69; atomic wt: 168.93421; valency: 3; relative density: 9.321; melting pt: 1545°C; boiling pt: 1950°C
[C19: New Latin, from Thule + -ium]

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American Heritage
Medical Dictionary

thulium thu·li·um (thōō'lē-əm, thyōō'-)
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.

The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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American Heritage
Science Dictionary
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.
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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica


((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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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Electronic shielding by closed shells in thulium compounds.
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