Thulium (Tm)


  • Symbol: Tm
  • Atomic Number: 69
  • Atomic Weight: 168.93422
  • Element Classification: Lanthanide
  • Discovered By: Per Teodor Cleve
  • Discovery Date: 1879
  • Name Origin: Named after Thule, a mythical northern country, often associated with Scandinavia
  • Density(g/cc): 9.32
  • Melting Point: 1545°C
  • Boiling Point: 1950°C
  • Appearance: Silvery-gray, soft, malleable, and ductile metal
  • Atomic Radius(pm): 176


Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879. While examining the residues left after removing other rare earth elements from erbia (erbium oxide), Cleve identified two new elements, thulium and holmium. Thulium was named after Thule, an ancient name for a mythical place in the far north, often identified with Scandinavia, reflecting the region where the element was discovered.

Relation to Other Elements

Thulium is a member of the lanthanide series, a group of fifteen chemically similar elements. It is one of the least abundant rare earth metals and exhibits characteristics typical of lanthanides, such as forming trivalent (+3) ions and having properties useful in various technological applications. Thulium’s physical and chemical properties are closely related to those of its lanthanide neighbors, particularly ytterbium and erbium.

Natural Occurrence

Thulium is not found free in nature but occurs in small amounts in minerals such as monazite and bastnasite, alongside other rare earth elements. Due to its scarcity, thulium is one of the least abundant of the naturally occurring lanthanides. The extraction and purification of thulium from these minerals require complex separation processes due to the similar chemical properties of the lanthanide elements.


Thulium has several specialized but limited applications due to its rarity:

  • Medical Imaging and Radiation Therapy: Thulium-170, a radioactive isotope, is used in portable X-ray devices and in some types of cancer treatment, leveraging its radiation-emitting properties.
  • Lasers: Thulium-doped lasers find applications in medical surgery and dentistry, as well as in industrial settings for cutting and welding materials. These lasers operate in the near-infrared range, suitable for precise tissue ablation.
  • Research: Due to its scarcity, thulium is primarily used in research settings to study the properties of lanthanides and their potential applications in new technologies.
  • Nuclear Reactors: Thulium can be used as a radiation source in nuclear reactors and in research involving neutron absorption studies.

Despite its limited abundance, the discovery of thulium has contributed to the understanding of rare earth elements and their chemistry. Thulium’s applications, particularly in medical and industrial lasers, demonstrate the value of even the rarest elements in advancing technology and improving human health.

Erbium (Er)

Ytterbium (Yb)