Thulium
Thulium is a chemical element with symbol Tm and atomic number 69. It is the thirteenth and antepenultimate (third-last) element in the lanthanide series. Like the other lanthanides, the most common oxidation state is +3, seen in its oxide, halides and other compounds.
Identity
- Atomic mass
- 168.934 u
- Category
- Lanthanides
- Group
- 3
- Period
- 6
- Block
- f
- Oxidation States
- +2, +3
- Phase (25 °C)
- Solid
- Appearance
- silvery gray
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f13
- Short configuration
- [Xe] 4f13 6s2
- Electrons per shell
- 2, 8, 18, 31, 8, 2
- Electronegativity (Pauling)
- 1.25
- Electron affinity
- 99 kJ/mol
- 1st ionization energy
- 597 kJ/mol
- Ionization energies
- 597 · 1160 · 2285 · 4120 kJ/mol
Physical properties
- Melting point
- 1544.8 °C (1818 K)
- Boiling point
- 1949.8 °C (2223 K)
- Density
- 9.32 g/cm³
- Molar heat capacity
- 27.0 J/(mol·K)
Chemical behavior of thulium
Thulium has thirteen outer electrons ([Xe] 4f¹³ 6s²) and acts as +3, with a rare +2 in the iodide TmI₂. Its electronegativity is low (1.25) and it forms highly ionic bonds. It is soft, malleable and the scarcest lanthanide after promethium: it oxidizes slowly in dry air. Its Tm³⁺ ions emit at 1,900–2,000 nm, ideal for surgical lasers.
Reactivity
4 Tm + 3 O₂ → 2 Tm₂O₃
In dry air it tarnishes slowly with a greenish oxide; in moist air the corrosion progresses. When heated, it burns to the oxide.
2 Tm + 6 H₂O → 2 Tm(OH)₃ + 3 H₂
It reacts with water releasing hydrogen, vigorously when hot. The laser garnets that contain it are insoluble and stable.
2 Tm + 6 HCl → 2 TmCl₃ + 3 H₂
It dissolves in acids giving greenish thulium(III) salts. The chloride is the raw material for doped fibers and garnets.
Tm + I₂ → TmI₂
With iodine it gives thulium(II) iodide, a strong reducing agent used in synthesis and as a precursor of mixed-valence materials.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Thulium oxide | Tm₂O₃ | 2-micron lasers and fibers |
| Thulium-doped fiber | SiO₂:Tm | Urological surgical laser |
| Thulium(II) iodide | TmI₂ | Reducing agent in organic synthesis |
| Thulium chloride | TmCl₃ | Production of the metal and lasers |
| Sealed thulium-170 | ¹⁷⁰Tm | Portable industrial X-rays |
Electron shell distribution
View in the atom builderIsotopes
All natural thulium is thulium-169 (¹⁶⁹Tm), with 100 neutrons: it is monoisotopic and stable. Thulium-170 (¹⁷⁰Tm, 128 days) emits beta and X-rays and powers portable radiography equipment. Thulium-167 (¹⁶⁷Tm, 9.2 days) is researched as a bone and tumor tracer.
Occurrence and production
It is the scarcest stable lanthanide, at about 0.5 ppm, in monazite, bastnasite and xenotime. It does not form minerals of its own. It is obtained by acid dissolution and very fine chromatographic separation, and the metal by electrolysis or reduction with calcium in tantalum crucibles.
History and origin of the name
Per Teodor Cleve separated it in 1879 from erbium and named it thulium, after Thule, the mythical name of the far north: the edge of the world for an almost invisible element. Its purity was achieved by Urbain and James in the 20th century. Two-micron lasers took it out of surgical obscurity.
Applications and relevance of thulium
Uses of the element
As a sealed source of Tm-170 it produces portable X-rays for welds and field diagnosis without electricity. As a dopant in fibers it generates the 1,940 nm thulium laser, which vaporizes prostate tissue with minimal bleeding.
Uses of its compounds
The oxide dopes garnets and fibers for surgery, spectroscopy and LIDAR. The iodide reduces molecules in pharmaceuticals and the chloride feeds the manufacture of high-power laser ceramics.
Health, safety and environment
Biological role
It has no biological function and is considered of low toxicity in the usual forms. Medical isotopes are handled only in hospitals with shielding; in the laser the light acts, not the free element.
Safety
Fine powder can ignite in air and requires extraction and an inert atmosphere. Tm-170 sources travel shielded with dosimetry; lasers require IR glasses and marked enclosures.
Environment
Its scarcity forces much rock to be moved per gram, with reagent consumption. In return, a thulium laser replaces open surgery and a portable source avoids moving heavy equipment. Fiber recycling recovers high-value thulium.
Frequently asked questions about Thulium
What is thulium used for in portable X-ray equipment?
Some thulium isotopes, especially thulium-170, emit radiation that can be used as an X-ray source in portable equipment. These compact sources are useful in field medical diagnosis, industrial inspection and weld control where installing a conventional team is not practical. Thulium does not generate electricity to produce X-rays, but acts as an encapsulated radioactive source under strict safety controls.
Is thulium dangerous?
Thulium has no known biological function and is considered of low toxicity in the usual forms and exposures studied. However, the metal in fine powder can ignite on contact with air, and powders or compounds can irritate the respiratory tract, skin and eyes. Radioactive isotopes additionally require radiological protection, shielding and dosimetry. In final products, such as lasers or medical equipment, user exposure is minimal.
What does thulium have to do with the petrochemical industry?
The isotope thulium-170 has been used in industrial radiography to inspect welds, pipes, tanks and metal components. By emitting penetrating radiation, it allows the detection of cracks, pores or internal defects without destroying the piece. This non-destructive testing technique is valuable in the petrochemical industry, where the integrity of pipelines and pressure vessels is critical for safety and preventive maintenance.
What is the difference between thulium and holmium in medical lasers?
Both are lanthanides used in surgical lasers, but they emit at different wavelengths. The holmium laser, Ho:YAG, operates at around 2,100 nanometers and is widely used in urology to fragment stones and treat the prostate. Thulium emits in a band close to 1,900–2,000 nanometers, with even greater absorption by water, which allows a very localized interaction with the tissue. The choice depends on the procedure and the type of tissue.
What are thulium-doped lasers used for?
Thulium-doped lasers emit infrared light in a band strongly absorbed by the water in the tissues. This allows cutting, vaporizing or coagulating tissue with precision and with relatively limited thermal penetration. They are used in minimally invasive surgery, especially in urology, and also in spectroscopic research and precision industrial applications. Their advantage is combining a stable emission with fine control of the laser beam.
Why is thulium so scarce?
Thulium is one of the least abundant lanthanides in the Earth's crust and appears in very low concentrations within rare-earth minerals such as monazite, bastnasite and xenotime. Its production requires separating it from chemically very similar elements by means of solvent extraction and ion exchange. This low natural abundance, together with complex purification processes, explains why it is expensive and why its applications are concentrated in specialized technologies.
What is thulium-167 being researched for?
Thulium-167 is a radioactive isotope with a half-life of about 9.2 days that has been studied for medical research, including tumor and bone studies. Its emission can be used in imaging techniques and experimental tracers to follow biological processes or assess lesions. These are applications of research and nuclear medicine that are highly regulated, not a domestic or everyday use of the element.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.