Thulium Facts – Element Tm or Atomic Number 69

Thulium Facts

Thulium (Tm) is a rare earth element, part of the lanthanide series and f-block in the periodic table. Its atomic number is 69 and its element symbol is Tm. Most of the rare earths are not all that uncommon, but thulium truly is rare. The place you most likely encounter it is in portable x-ray machines, but it has other applications in lasers and as a fluorescent marker in euro banknotes.


Per Teodor Cleve
Per Teodor Cleve, circa 1885

Swedish chemist Per Teodor Cleve discovered thulium in 1879. Cleve named the element after “Thule,” an ancient name for Scandinavia, reflecting his Swedish heritage. Initially, the symbol for thulium was Tu (which was also a symbol for tungsten for a while), but it was eventually changed to Tm. The discovery was made through the analysis of the mineral erbia (erbium oxide), where Cleve successfully separated the oxides of thulium and also holmium from other rare earth elements.

However, it wasn’t until the 20th century that Charles James successfully isolated the pure element. James reportedly performed 15,000 fractional crystallizations to get his sample in 1911.

With the development of ion-exchange chromatography, rare earth isolation became easier. Even so, thulium remains expensive. In 2005, the price for 99% pure thulium metal was around $70/gram.

Appearance and Properties

Thulium is a silvery-gray metal that tarnishes and darkens after exposure to air. It is malleable and ductile and soft enough to cut with a knife. There are two main allotropes. The more stable form is hexagonal, but the tetragonal crystal structure also occurs.

The element displays interesting magnetic properties. It is paramagnetic above 56 K and at room temperature . Between 32 and 56 K, it is antiferromagnetic, and below 32 K it is ferromagnetic.

Thulium primarily exhibits a +3 oxidation state, which is common among lanthanides. In this state, thulium ions are stable and participate in various chemical reactions. The Tm3+ ions display a bright blue luminescence. The element forms only a single oxide (Tm2O3 or thulia), but a variety of binary compounds and hydrates.

Electron Levels of a Thulium Atom

Thulium Isotopes

Natural thulium consists of a mixture of isotopes, ranging from 144Tm to 183Tm. Thulium has only one stable isotope, 169Tm, which makes up nearly 100% of natural thulium.

The radioisotope with the longest half-life is thulium-169 (1.92 years). Thulium-170 is a notable isotope, due to its use in portable X-ray devices. However, its half-life of 128.6 days means that a radioactive source only has a practical lifetime of around a year.

Abundance and Sources

Thulium is one of the least abundant lanthanides in the Earth’s crust, with an average concentration of about 0.5 mg/kg. It occurs in soil at concentrations ranging between 0.4 to 0.8 parts per million. The element is similarly uncommon in the Solar System, occurring at levels of around 200 parts per trillion by mass.

Thulium does not occur in pure native form in nature. It exists in small amounts mixed with other rare earth elements in minerals such as monazite, gadolinite, xenotime, euxenite, and bastnäsite. The rare earth minerals come from mining operations in China, the United States, Australia, Brazil, India, Tanzania, and Greenland.

Thulium Uses

To some extent, the rarity and price of thulium limits its uses. But, it still has practical applications:

  • X-ray Devices: Thulium-170 is used in portable X-ray machines due to its ability to emit X-rays when excited. These machines find uses in medicine, dentistry, and for detecting defects in mechanical components.
  • Cancer Treatment: Research is ongoing into the use of thulium isotopes for cancer therapy.
  • Lasers: Thulium-doped lasers have various applications, including medical procedures, meteorology, military uses, and telecommunications.
  • High-Temperature Superconductors: Thulium is a component of some high-temperature superconducting materials.
  • Ferrites: Ceramic magnetic materials containing thulium are important in microwave equipment.
  • Arc Lighting: Thulium produces green emission lines, contributing an unusual color.
  • Anti-Counterfeiting Measures: The bright blue fluorescence of the element makes it a useful component of anti-counterfeiting ink. If you look at a euro banknote under a black light, the blue glow comes from thulium.
  • Dosimetry: Tm-doped calcium sulfate glows blue as an indicator of radiation exposure in dosimeters.
  • Powder-Generating Windows: One newer application uses the Tm2+ oxidation state for luminescent windows and other materials that generate electricity.

Biological Role, Health Effects, and Toxicity

Thulium has no known essential biological role in humans or other organisms. Because the element occurs in soil, it is also found within the human body, mainly in the bones, liver, and kidney.

Thulium and its compounds are generally of low to moderate toxicity. However, direct injection of the element into the bloodstream damages the liver and spleen. As with most metals, the powdered element ignites when exposed to an open flame.

Key Thulium Facts

Atomic Number69
Atomic Weight168.934
Electron Configuration[Xe] 4f13 6s2
Electrons per Shell2, 8, 18, 31, 8, 2
State at Room TemperatureSolid
Melting Point1545 °C (2813 °F)
Boiling Point1950 °C (3542 °F)
Density9.32 g/cm³
Heat of Fusion16.84 kJ/mol
Heat of Vaporization191 kJ/mol
Molar Heat Capacity27.03 J/(mol·K)
Oxidation States0, +1, +2. +3
Electronegativity1.25 (Pauling)
First Ionization Energy596.7 kJ/mol
Second Ionization Energy1160 kJ/mol
Third Ionization Energy2285 kJ/mol
Atomic Radius176 pm
Covalent Radius190 pm
Crystal StructureHexagonal close-packed (hcp)
Thermal Conductivity16.9 W/(m·K)
Electrical Resistivity0.68 µΩ·m
Magnetic OrderingParamagnetic (at 300 K)
Young’s Modulus74.0 GPa
Shear Modulus30.5 GPa
Mohs Hardness2.0-3.0


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