Skip to main content Skip to Search Box

Definition: thulium from Collins English Dictionary


1 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]

Summary Article: thulium
From The Columbia Encyclopedia

(thō'lēӘm) [from Thule, an ancient name for Scandinavia], metallic chemical element; symbol Tm; at. no. 69; at. wt. 168.93421; m.p. about 1,545 degrees Celsius; b.p. 1,947 degrees Celsius; sp. gr. 9.3; valence +3. Thulium is a soft, malleable, ductile, lustrous silver-white metal. It is one of the rare-earth metals of the lanthanide series in Group 3 of the periodic table. It does not tarnish rapidly in dry air but should be protected from moisture. It forms compounds with oxygen and the halogens, most of which are light green. The oxide, Tm2O3, is called thulia. Thulium is the least abundant of the rare-earth metals. It is found in the minerals gadolinite and euxenite and in monazite, the chief commercial source. The metal can be obtained by chemical reduction of its compounds. The pure metal and compounds have few commercial uses, but they are often used without purification in combination with the other rare-earth metals and compounds, e.g., in lighter flints and carbon electrodes for arc lighting. Although the only naturally occurring isotope (thulium-169) is stable, there are 15 unstable isotopes. Thulium-170 (half-life about 129 days), prepared by irradiating thulium-169 in a nuclear reactor, emits X rays; it is used in portable X-ray sources. Thulium was discovered in 1879 by P. T. Cleve.

The Columbia Encyclopedia, © Columbia University Press 2018

Related Articles

Full text Article thulium
Philip's Encyclopedia

(symbol Tm) Lustrous, silver-white, metallic element of the lanthanide series (rare-earth metals). Its chief ore is monazite but thulium is as...

Full text Article thulium
The Hutchinson Unabridged Encyclopedia with Atlas and Weather Guide

Soft, silver-white, malleable and ductile, metallic element of the lanthanide series, atomic number 69, relative atomic mass 168.94. It is the least

Full text Article terbium
The Columbia Encyclopedia

(tûr'bēӘm) [from Ytterby, a village in Sweden], metallic chemical element; symbol Tb; at. no. 65; at. wt. 158.92535; m.p. 1,356 degrees Celsius; b.p

See more from Credo