Thulium (Tm) - Discovery, Occurrence, Production, Properties and Applications of Thulium

Topics Covered

Chemical Formula
Basic Information
Health Aspects
Key Properties

Chemical Formula



Thulium was discovered by Per Teodor Cleve, a Swedish chemist, in 1879. Cleve discovered two new materials that were brown and green in color. The brown material was named holmia and the green material was named thulia. Cleve later discovered that thulia was thulium oxide.

Thulium comes from ‘Thule’, an ancient name for Scandinavia. Initially thulium was not found in its pure form but was available as a compound of thulium combined with other elements. Pure thulium was later produced by Charles James, an American scientist, in 1910.

Basic Information

Name Thulium
Symbol Tm
Atomic number 69
Atomic weight 168.93421 (2)
Standard state Solid at 298 K
CAS Registry ID 7440-30-4
Group name Lanthanoid
Period in periodic table 6 (lanthanoid)
Block in periodic table f-block
Color Silvery white
Classification Metallic
Melting point 1818 K (1545°C or 2813°F)
Boiling point 2223 K (1950°C or 3542°F)
Density 9.32 g/cm3
Phase at room temperature Solid


Thulium is mostly found in monazite. The Earth’s crust also has large quantities of this element amounting to about 0.2 to 1 part per million. Thulium compounds also mix with other rare earth compounds in minerals such as gadolinite and euxenite.

Thulium - Periodic Table of Videos


Thulium has 32 isotopes whose half-lives are known, with mass numbers 146 to 177. Thulium-169 is the only naturally occurring isotope of thulium.


Pure thulium is produced by reducing TmF3 with calcium metal.

2TmF3 + 3Ca → 2Tm + 3CaF2

Key Properties

The key properties of thulium include:

  • It is a silvery metal that can be cut with a knife due it its soft texture.
  • It is easy to work with.
  • It is ductile and malleable.
  • It is stable in air.
  • It reacts rapidly with acids and slowly with water.


Thulium is useful in the following applications:

  • Lasers -Thulium lasers require less cooling and function very well at high temperatures and are used in satellites.
  • It is used in alloys along with other rare earth metals.
  • It is used to dope yttrium aluminum garnets (YAG) used in lasers.
  • It is used in euro banknotes because of its blue fluorescence under UV light that helps defeat counterfeiters.


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G.P. Thomas

Written by

G.P. Thomas

Gary graduated from the University of Manchester with a first-class honours degree in Geochemistry and a Masters in Earth Sciences. After working in the Australian mining industry, Gary decided to hang up his geology boots and turn his hand to writing. When he isn't developing topical and informative content, Gary can usually be found playing his beloved guitar, or watching Aston Villa FC snatch defeat from the jaws of victory.


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