Thulium (Tm)
lanthanideSolid
Standard Atomic Weight
168.93422 uElectron configuration
[Xe] 6s2 4f13Melting point
1544.85 °CBoiling point
1949.85 °CDensity
9320 kg/m³Oxidation states
0, +1, +2, +3Electronegativity (Pauling)
1.25Ionization energy (1st)
6.184402 eVDiscovery year
1879Atomic radius
175 pmDetails
Thulium is a lanthanide rare-earth metal and the least abundant stable lanthanide in Earth’s crust. It is chemically typical of the series, forming mainly Tm³⁺ compounds with a high affinity for oxygen and halogens. The element is not scarce enough to be unattainable, but it is dispersed and difficult to separate from neighboring lanthanides. Its technological importance is concentrated in isotope sources, specialty lasers, and optical materials rather than bulk structural use.
Thulium can be isolated by reduction of the oxide with lanthanum metal or by calcium reduction of a closed container. The element is silver-gray, soft, malleable, and ductile, and can be cut with a knife. Twenty five isotopes are known, with atomic masses ranging from 152 to 176. Natural thulium, which is 100% 169Tm, is stable.
The name derives from Thule, the earliest name for the northernmost part of the civilized world—Scandinavia. It was discovered in 1879 by the Swedish chemist Per Theodor Cleve in a sample of erbium mineral. Thulium was first isolated by the American chemist Charles James in 1911.
Thulium was discovered by Per Theodor Cleve, a Swedish chemist, in 1879. Cleve used the same method Carl Gustaf Mosander used to discover lanthanum, erbium and terbium, he looked for impurities in the oxides of other rare earth elements. He started with erbia, the oxide of erbium (Er2O3), and removed all of the known contaminants. After further processing, he obtained two new materials, one brown and the other green. Cleve named the brown material holmia and the green material thulia. Holmia is the oxide of the element holmium and thulia is the oxide of the element thulium. Today, thulium is primarily obtained through an ion exchange process from monazite sand ((Ce, La, Th, Nd, Y)PO4), a material rich in rare earth elements that can contain as much as 0.007% thulium.
Named after Thule, the earliest name for Scandinavia. Discovered in 1879 by Cleve. Thulium occurs in small quantities along with other rare earths in a number of minerals. It is obtained commercially from monazite, which contains about 0.007% of the element. Thulium is the least abundant of the rare earth elements, but with new sources recently discovered, it is now considered to be about as rare as silver,gold, or cadmium.
Pure thulium is a soft, malleable, silvery metal. It slowly tarnishes in air as oxide forms on the surface and reacts more readily when heated. Like many lanthanides, it can be cut with a knife and is normally stored to limit oxidation.
Thulium has specialized uses where its nuclear or optical properties are useful. Neutron-activated ¹⁷⁰Tm produces ¹⁷⁰Tm, a portable gamma and X-ray source formerly used in industrial radiography and still of technical interest where compact sealed sources are suitable. Thulium-doped yttrium aluminium garnet, commonly written Y₃Al₅O₁₂:Tm, and other host crystals are used in solid-state lasers near the two-micrometre infrared region. Thulium-doped silica fibers are also used in fiber lasers and amplifiers. Small amounts may be used in research alloys and magnetic studies, but there is no large-volume elemental application.
Thulium is the least abundant of the naturally occurring rare earth elements. Metallic thulium is relatively expensive and has only recently become available. It currently has no commercial applications, although one of its isotopes, thulium-169, could be used as a radiation source for portable X-ray machines.
Thulium forms no commercially important compounds. Some of thulium's compounds include: thulium oxide (Tm2O3), thulium fluoride (TmF3) and thulium iodide (TmI3).
Because of the relatively high price of the metal, thulium has not yet found many practical applications. 169Tm bombarded in a nuclear reactor can be used as a radiation source in portable X-ray equipment. 171Tm is potentially useful as an energy source. Natural thulium also has possible use in ferrites (ceramic magnetic materials) used in microwave equipment, and can be used for doping fiber lasers. As with other lanthanides, thulium has a low-to-moderate acute toxic rating. It should be handled with care.
Isotopes in Industry
170Tm (with a half-life of about 130 days) is used in the petrochemical industry for industrial radiography to test welds in pipes and tanks [486] Industrial Applications of Sealed Radiation Sources and Alternative Non Nuclear Technologies, Final Report, 68-D-00-210, p. 37. Trinity Engineering Associates Ohio (2002)..
Isotopes in Medicine
167Tm (with a half-life of 9.2 days) is useful for tumor and bone studies [487] F. Tárkányi, A. Hermanne, S. Takács, B. Király, I. Spahn, A. V. Ignatyuk. Appl. Radiat. Isot.68, 250 (2010).. Stable 169Tm can be bombarded in a nuclear reactor to create 170Tm, via the 169Tm (n, γ) 170Tm reaction, which emits X-rays and has been used in portable X-ray equipment as a radiation source [488] D. Granero, J. Pérez-Calatayud, F. Ballester, A. J. Bos, J. Venselaar. Radiat. Prot. Dosimetry118, 11 (2006).. 170Tm has been used in high-dose-rate (HDR) brachytherapy [489] F. Ballester, D. Granero, J. Perez-Calatayud, J. L. Venselaar, M. J. Rivard. Med. Phys.37, 1629 (2010). and for use in radiosynovectomy of medium sized joints (Fig. IUPAC.69.1) [490] A. Polyak, T. Das, S. Chakraborty, R. Kiraly, G. Dabasi, R. P. Joba, C. Jakab, J. Thuroczy, Z. Postenyi, V. Haasz, G. Janoki, G. A. Janoki, M. R. A. Pillai, L. Balogh. Cancer Biother. Radiopharm.29, 330 (2014)..
Thulium chemistry is dominated by the +3 oxidation state. Thulium(III) oxide, Tm₂O₃, is the common oxide and is a useful starting material for preparing other salts and optical ceramics. Thulium(III) chloride, TmCl₃, and thulium(III) fluoride, TmF₃, are representative halides; the fluoride is sparingly soluble and useful in separations and fluoride materials. A +2 state is known in some strongly reducing solid compounds, but it is much less stable than Tm³⁺ in ordinary aqueous chemistry. In solution, Tm³⁺ salts are typically pale green to nearly colorless depending on concentration and ligand field.
See more information at the Thulium compound page.
Metallic thulium has low acute chemical toxicity compared with many heavy metals, but fine powder is a fire hazard and can react with air or moisture. Soluble thulium salts should be handled as toxic laboratory chemicals because rare-earth ions can affect biological processes at elevated exposure. Radioactive ¹⁷⁰Tm presents an external radiation hazard and an internal hazard if contamination occurs; sealed sources require isotope-specific controls.
Thulium occurs naturally only as a minor constituent of rare-earth minerals, especially minerals that contain the heavy lanthanides. Weathering releases it mainly as Tm³⁺, which tends to bind to clays, oxides, phosphates, and organic matter rather than remain highly mobile. It has no known biological role. Environmental concentrations are usually very low, so ecological effects are mainly a concern near rare-earth mining, processing, or disposal sites.
Thulium is obtained as a by-product of rare-earth extraction rather than mined for its own sake. Sources include ion-adsorption clays and minerals such as monazite and xenotime that contain small fractions of heavy rare earths. Separation relies on solvent extraction, ion exchange, and conversion through oxide or salt intermediates, with cost driven more by separation complexity than by absolute crustal scarcity. Demand is small and specialized, so supply is tied to broader rare-earth processing rather than an independent commodity chain. Recycling is limited because applications use small amounts in dispersed devices or sealed sources.
Found with other rare earths in the minerals gadolinite, euxenite, xenotime, and monazite. Monazite is often 50% rare earth by weight and 0.007% thulium.
Thulium is a rare cosmic element produced mainly by neutron-capture processes in evolved stars and stellar explosions. Its odd atomic number and position among the heavy lanthanides make it much less abundant than lighter rock-forming elements and less abundant than several neighboring rare earths. In planets and meteorites it follows refractory lithophile behavior and partitions with other rare-earth elements rather than forming native metal.
- Thulium is the least abundant lanthanide that has a stable isotope.
- Natural thulium is essentially all ¹⁶⁹Tm.
- ¹⁷⁰Tm can be made by neutron irradiation of ¹⁶⁹Tm.
- Thulium was named after Thule, a classical name associated with the far north.
- Despite its rarity among lanthanides, thulium is more abundant in Earth’s crust than silver.
Images
Properties
Physical
- Atomic radius (empirical)
- 175 pm Compare Atomic radius (empirical) of all elements →
- Covalent radius
- 190 pm Compare Covalent radius of all elements →
- Van der Waals radius
- 227 pm Compare Van der Waals radius of all elements →
- Density
- 9320 kg/m³ Compare Density of all elements →
- Molar volume
- 0.0181 L/mol
- Phase at STP
- Solid Compare Phase at STP of all elements →
- Melting point
- 1544.85 °C Compare Melting point of all elements →
- Boiling point
- 1949.85 °C Compare Boiling point of all elements →
- Specific heat capacity
- 0.16 J/(g·K) Compare Specific heat capacity of all elements →
- Molar heat capacity
- 27.03 J/(mol·K) Compare Molar heat capacity of all elements →
- Crystal structure
- Hexagonal close-packed Compare Crystal structure of all elements →
Chemical
- Electronegativity (Pauling)
- 1.25 Compare Electronegativity (Pauling) of all elements →
- Electron affinity
- 1.029 eV
- Ionization energy (1st)
- 6.184402 eV Compare Ionization energy (1st) of all elements →
- Ionization energy (2nd)
- 12.065042 eV Compare Ionization energy (2nd) of all elements →
- Ionization energy (3rd)
- 23.660081 eV Compare Ionization energy (3rd) of all elements →
- Ionization energy (4th)
- 42.410146 eV Compare Ionization energy (4th) of all elements →
- Ionization energy (5th)
- 65.400225 eV Compare Ionization energy (5th) of all elements →
- Oxidation states
- 0, +1, +2, +3 Compare Oxidation states of all elements →
- Valence electrons
- 3 Compare Valence electrons of all elements →
- Electron configuration
- [Xe] 6s2 4f13
Thermodynamic
- Heat of fusion
- 0.12437166 eV Compare Heat of fusion of all elements →
- Heat of vaporization
- 1.979582 eV Compare Heat of vaporization of all elements →
- Heat of sublimation
- 2.404519 eV
- Heat of atomization
- 2.404519 eV
- Atomization enthalpy
- 2.406592 eV
Nuclear
- Protons
- 69 Compare Protons of all elements →
- Neutrons
- 100 Compare Neutrons of all elements →
- Known isotopes
- 39 Compare Known isotopes of all elements →
- Stable isotopes
- 1 Compare Stable isotopes of all elements →
- Most stable isotope
- Tm-169
- Discovery year
- 1879
Abundance
- Abundance (Earth's crust)
- 0.52 mg/kg Compare Abundance (Earth's crust) of all elements →
- Abundance (ocean)
- 1.7 × 10−7 mg/L Compare Abundance (ocean) of all elements →
Crystal Structure
- Lattice constant a
- 354 pm
Electronic Structure
- Electrons per shell
- 2, 8, 18, 31, 8, 2 Compare Electrons per shell of all elements →
Identifiers
- CAS number
- 7440-30-4 Compare CAS number of all elements →
- Term symbol
- 2F°7/2
- InChI
- InChI=1S/Tm
- InChI Key
- FRNOGLGSGLTDKL-UHFFFAOYSA-N
Electron Configuration Measured
Tm: 4f¹³ 6s²[Xe] 4f¹³ 6s²1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d¹⁰ 5s² 5p⁶ 4f¹³ 6s²Atomic model
Isotopes change neutron count, mass, and stability — not the electron configuration of a neutral atom.
Schematic atomic model, not to scale.
Atomic Fingerprint
Emission / Absorption Spectrum
Isotope Distribution
| Mass number | Atomic mass (u) | Natural abundance | Half-life |
|---|---|---|---|
| 169 Stable | 168.9342179 ± 0.0000022 | 100.0000% | Stable |
Phase / State
Reason: 1519.8 °C below melting point (1544.85 °C)
Schematic, not to scale
Phase transition points
Transition energies
Energy required to melt 1 mol at melting point
Energy required to vaporize 1 mol at boiling point
Energy required to sublime 1 mol at sublimation point
Density
At standard conditions
At standard conditions
Atomic Spectra
Showing 10 of 69. Sorted by ion charge (ascending).
Lines Holdings ?
| Ion | Charge | Total lines | Transition probabilities | Level designations |
|---|---|---|---|---|
| Tm I | 0 | 538 | 408 | 525 |
| Tm II | +1 | 267 | 13 | 13 |
| Tm III | +2 | 186 | 0 | 0 |
Levels Holdings ?
| Ion | Charge | Levels |
|---|---|---|
| Tm I | 0 | 631 |
| Tm II | +1 | 367 |
| Tm III | +2 | 128 |
| Tm IV | +3 | 8 |
| Tm V | +4 | 2 |
| Tm VI | +5 | 2 |
| Tm VII | +6 | 2 |
| Tm VIII | +7 | 2 |
| Tm IX | +8 | 2 |
| Tm X | +9 | 2 |
Ionic Radii
| Charge | Coordination | Spin | Radius |
|---|---|---|---|
| +2 | 6 | N/A | 103 pm |
| +2 | 7 | N/A | 109.00000000000001 pm |
| +3 | 6 | N/A | 88 pm |
| +3 | 8 | N/A | 99.4 pm |
| +3 | 9 | N/A | 105.2 pm |
Compounds
Isotopes (1)
| Mass number | Atomic mass (u) | Natural abundance | Half-life | Decay mode | |
|---|---|---|---|---|---|
| 169 Stable | 168.9342179 ± 0.0000022 | 100.0000% | Stable | stable |
Extended Properties
Covalent Radii (Extended)
- Covalent radius (Pyykkö)
- 164 pm
- Covalent radius (Pyykkö, double)
- 131 pm
Van der Waals Radii
- Alvarez
- 279 pm
- UFF
- 337.4 pm
- MM3
- 267 pm
Atomic & Metallic Radii
- Atomic radius (Rahm)
- 271 pm
Numbering Scales
- Mendeleev
- 37
- Pettifor
- 22
- Glawe
- 20
Electronegativity Scales
- Ghosh
- 0
- Miedema
- 3
- Gunnarsson–Lundqvist
- 6
- Robles–Bartolotti
- 5
Polarizability & Dispersion
- Dipole polarizability
- 144 a.u.
- Dipole polarizability (unc.)
- 15 a.u.
- C₆ (Gould–Bučko)
- 2020 Ha·Bohr6
Miedema Parameters
- Miedema molar volume
- 18.12 cm3/mol
- Miedema electron density
- 2
Supply Risk & Economics
- Production concentration
- 97
- Relative supply risk
- 10
- Reserve distribution
- 50
- Political stability (top producer)
- 24
- Political stability (top reserve)
- 24
Phase Transitions & Allotropes
| Melting point | 1818.15 K |
| Boiling point | 2223.15 K |
Oxidation State Categories
Advanced Reference Data
Screening Constants (13)
| n | Orbital | σ |
|---|---|---|
| 1 | s | 1.3437 |
| 2 | p | 4.3588 |
| 2 | s | 18.0522 |
| 3 | d | 13.6257 |
| 3 | p | 20.5239 |
| 3 | s | 21.0816 |
| 4 | d | 36.056 |
| 4 | f | 40.366 |
| 4 | p | 33.012 |
| 4 | s | 31.8624 |
Crystal Radii Detail (5)
| Charge | CN | Spin | rcrystal (pm) | Origin |
|---|---|---|---|---|
| 2 | VI | 117 | ||
| 2 | VII | 123 | ||
| 3 | VI | 102 | from r^3 vs V plots, | |
| 3 | VIII | 113.4 | from r^3 vs V plots, | |
| 3 | IX | 119.2 | from r^3 vs V plots, |
Isotope Decay Modes (58)
| Isotope | Mode | Intensity |
|---|---|---|
| 144 | p | — |
| 144 | B+ | — |
| 145 | p | 100% |
| 146 | p | 100% |
| 146 | B+ | — |
| 146 | B+p | — |
| 147 | B+ | 85% |
| 147 | p | 15% |
| 148 | B+ | 100% |
| 148 | B+p | — |
X‑ray Scattering Factors (514)
| Energy (eV) | f₁ | f₂ |
|---|---|---|
| 10 | — | 0.17918 |
| 10.1617 | — | 0.18356 |
| 10.3261 | — | 0.18805 |
| 10.4931 | — | 0.19265 |
| 10.6628 | — | 0.19737 |
| 10.8353 | — | 0.2022 |
| 11.0106 | — | 0.20714 |
| 11.1886 | — | 0.21221 |
| 11.3696 | — | 0.21741 |
| 11.5535 | — | 0.22273 |
Additional Data
Estimated Crustal Abundance
The estimated element abundance in the earth's crust.
5.2×10-1 milligrams per kilogram
References (1)
Estimated Oceanic Abundance
The estimated element abundance in the earth's oceans.
1.7×10-7 milligrams per liter
References (1)
References
(9)
Data deposited in or computed by PubChem
The half-life and atomic mass data was provided by the Atomic Mass Data Center at the International Atomic Energy Agency.
Element data are cited from the Atomic weights of the elements (an IUPAC Technical Report). The IUPAC periodic table of elements can be found at https://iupac.org/what-we-do/periodic-table-of-elements/. Additional information can be found within IUPAC publication doi:10.1515/pac-2015-0703 Copyright © 2020 International Union of Pure and Applied Chemistry.
The information are cited from Pure Appl. Chem. 2018; 90(12): 1833-2092, https://doi.org/10.1515/pac-2015-0703.
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The periodic table at the LANL (Los Alamos National Laboratory) contains basic element information together with the history, source, properties, use, handling and more. The provenance data may be found from the link under the source name.
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This section provides all form of data related to element Thulium.
The element property data was retrieved from publications.