69 Tm 168.93421

Tm - Thulium

Chemical Element Thulium Melting Point °C 1545
Chemical Symbol Tm Boiling Point °C 1947
Atomic Number 69 Density g/cm3 9.3
Atomic Weight 168.93421 Oxide Tm2O3


Thulium is a bright, silvery metal, and similarly to many of the rare earth metals, it is also ductile, malleable and soft enough to cut with a knife. It is a member of the lanthanoid group of elements and is the 61st most abundant element within the Earth’s crust. Thulium reacts with water and slowly tarnishes in air, but compared with the other rare earth metals, it is fairly corrosion resistant. Along with lutetium, thulium is the rarest rare earth metal, but is still 200 times more abundant than gold.


Thulium was discovered by the Swedish chemist, Per Teodor Cleve in 1879, and he named it after Thule, the ancient name for Scandinavia. Cleve used the method championed by Carl Gustaf Mosander, which was to search for impurities in the oxides of other rare earth elements. Starting with erbia (erbium oxide), he first removed all of the identifiable impurities and then continued processing until he obtained two new oxides, holmia and thulia, the respective oxides of holmium and thulium. A pure sample of thulium was not obtained until 1911 when the American chemist Theodore William Richards performed 15,000 recrystillisations of thulium bromate in order to obtain a pure sample.


Similarly to all of the rare earth metals, thulium is mainly mined in the Inner Mongolian mines in China, which accounts for 97% of the world’s rare earth metals. Small quantities of thulium are found in the rare earth minerals monazite and bastnasite. Monazite contains about 0.002% thulium and bastnasite contains about 0.0008% thulium. Other countries with deposits of thulium ores include the USA, Brazil, India, Australia, Greenland and Tanzania. Reserves of thulium are estimated to be around 100,000 tonnes and world production of thulium in the form of its oxide is about 50 tonnes per year.


Due to the difficulty in obtaining large quantities of thulium, it has few commercial applications. It is used in magnetic refrigeration to significantly reduce carbon dioxide emissions due to increased energy efficiency, in cathode ray tubes and in medical image stabilisation. One of its isotopes, thulium-169, could be used in sensitive X-ray phosphors for portable X-ray machines, but it is considered too expensive for commercial application.

  • Emsley, John. Nature’s Building Blocks, An A-Z Guide to the Elements, New Edition, Oxford University Press, 2011
  • Gray, Theodore. The Elements, A Visual Exploration of Every Known Atom in the Universe, Black Dog & Leventhal Publishers, Inc, NY, 2009
  • Stwertka, Albert. A Guide to the Elements, 3rd Edition, Oxford University Press, 2012

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