VIAS Encyclopedia provides a collection of tables and definitions commonly needed in science and engineering.


Atomic Number 69
Atomic Symbol Tm
Atomic Weight 168.9340 amu
Electron Configuration [Xe] 4f13 6s2
Melting Point 1545.0 C
Boiling Point 1947 C
Density 9.330 g/cm3

Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements. Cleve started by removing all of the known contaminants of erbia (Er2O3) and upon additional processing, obtained two new substances: one brown and one green. The brown substance turned out to be the oxide of the element holmium and was named holmia by Cleve and the green substance was the oxide of an unknown element. Cleve named the oxide thulia and its element thulium after Thule, an ancient Roman name for a mythical country in the far north, perhaps Scandinavia.

It is obtained commercially from monazite, which contains about 0.007% of the element. Thulium is the least abundant of the rare earth elements, but with new sources recently discovered, it is now considered to be about as rare as silver, gold, or cadmium.


Thulium can be isolated by reduction of the oxide with lanthanum metal or by calcium reduction of a closed container. The element is silver-gray, soft, malleable, and ductile, and can be cut with a knife. Twenty five isotopes are known, with atomic masses ranging from 152 to 176. Natural thulium, which is 100% 169Tm, is stable. 


Thulium has been used to create lasers but high production costs have prevented other commercial uses from being developed. 169Tm bombarded in a nuclear reactor can be used as a radiation source in portable X-ray equipment. 171Tm is potentially useful as an energy source. Natural thulium also has possible use in ferrites (ceramic magnetic materials) used in microwave equipment, and can be used for doping fiber lasers. As with other lanthanides, thulium has a low-to-moderate acute toxic rating. It should be handled with care.

This text is partially based on public educational material provided by Los Alamos National Laboratory.

Last Update: 2004-11-19