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What is Thulium

Thulium (pronounced as Thoo-lee-em) [2] is a soft, silver-gray rare earth metal [3] belonging to the Lanthanides group and represented by the symbol Tm [1]. While its most abundant, naturally occurring isotope is 169Tm, it has 34 radioactive isotopes, out of which the most stable one is 171Tm with a half-life period of 1.92 years [4].

Thulium Symbol

Where is it Found

Thulium is found in small quantities in monazite, a mineral that has approximately 20 parts per million (ppm) of the metal in the earth’s crust [1]. It is then commercially extracted through ion-exchange and solvent-extraction processes [1]. The element can as well be isolated through calcium reduction of anhydrous fluoride or by reduction of oxide with lanthanum [1]. While the top 3 thulium producing nations are China, Russia, and Malaysia, the top 3 thulium reserve holders are China, CIS Countries, and Russia [1].


Origin of its Name: It is named after Thule, the ancient name that stood for the present-day Scandinavia [1, 2].

Who discovered it: The Swedish chemist Per Teodor Cleve is credited for the discovery of thulium [1].

When and How was it discovered

Men started finding the lanthanoids or rare earth elements in 1794 when yttrium was found by Johan Gadolin [1]. Later, the chemists analyzed the composition of yttrium and observed that it was contaminated with chemically similar elements [1]. As a result, two new elements terbium and erbium were isolated in 1843 [1].

Per Teodor Cleve started working on erbium in 1874 and was able to successfully isolate thulium in its oxide form in 1879 at the Uppsala University, Sweden [1, 5]. In 1911, the exact atomic weight of thulium was determined by Theodore William Richards, an American chemist who carried out 15,000 thulium bromate-recrystallizations to produce the pure form of thulium [1].


Thulium Identification

Atomic number 69 [1]
CAS number 7440-30-4 [1]
Position in the periodic table Group Period Block
  Lanthanides [1] 6 [1] f [1]

Location of Thulium in the Periodic Table

Properties and Characteristics of Thulium

General Properties

Relative atomic mass 168.934 [1]

Physical Properties

Color Bright silvery-gray [1, 5]
Melting point/freezing point 1545 °C, 2813 °F [1]
Boiling point 1950 °C, 3542 °F [1]
Density 9.32 g cm-3 [1]
State of matter at room temperature (solid/liquid/gas) Solid [1, 5]
– Brinell 471 MPa [6]
– Mohs 1.77 [6]
– Vickers 520 MPa [6]
Electrical conductivity 1.4X106 S/m [6]
Thermal (heat) conductivity 17 W/(m K) [6]
Specific heat 160 J kg-1 K-1 [1]
Bulk modulus 44.5 GPa [1]
Shear modulus 30.5 GPa [1]
Young’s modulus 74 GPa [1]
Vapor pressure
– Temperature (K) 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400
– Pressure (Pa) 6.03X 10-10 5.94X 10-5 5.61X 10-2 5.22 130

Chemical Properties

Oxidation state/Oxidation number +2, +3, (+4) [1]
Isotopes Isotope Mass Abundance Half-life Mode of decay
  169Tm 168.934 100

Atomic Data of Thulium (Element 69)

Valence electrons 3 [6]
Quantum numbers 2F7/2 [6]
Electron configuration (noble gas configuration) [Xe] 4f136s2 [1]
Atomic structure
– Number of electrons 69 [5]
– Number of neutrons 100 [5]
– Number of protons 69 [5]
Radius of Atom
– Atomic radius 2.27 Å [1]
– Covalent radius 1.77 Å [1]
Electronegativity (Pauling-scale) 1.25 [1]
Electron affinity 99.283 kJ mol-1 [1]
Ionization energy (kJ mol-1) 1st 2nd 3rd 4th 5th 6th 7th 8th
  596.695 1162.65 2284.77 4119.9

Thulium Electron Configuration (Bohr Model)

What is Thulium used for

  • Thulium-170 (170Tm), a radioactive isotope formed by bombarding thulium in a nuclear reactor, can be used as a portable source of radiation in medical x-ray machines [1, 7].
  • The element 69 is used as a dopant in fiber lasers for surgical applications [1, 3].
  • Since it emits a bluish glow when exposed to ultra-violet light, thulium is used in Euro banknotes for detecting counterfeit currency [5].
  • It can also be used with other lanthanides for producing alloys [5].

Possible Harmful Effects or Toxicity

Being a non-toxic element, it does not cause any health hazards [1].

Thulium Laser

Interesting Facts

  • Because of its softness and malleability, thulium can be easily cut with a knife [3].
  • The element is sometimes represented by an image of a snow-covered mountain to indicate the origin of its name, suggesting a region far off to the north [1].

Cost of Thulium

While one gram of thulium costs about $50 [8], a hundred gram of the element approximately costs $500-$700 [5, 8].



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