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silvery gray
General properties
Name, symbol, number thulium, Tm, 69
Pronunciation / ˈ θj l i ə m / THEW-lee-əm
Element category lanthanide
Group, period, block n/a, 6, f
Standard atomic weight 168.93421
Electron configuration [Xe] 4f13 6s2
2, 8, 18, 31, 8, 2
Discovery Per Teodor Cleve (1879)
First isolation Per Teodor Cleve (1879)
Physical properties
Phase solid
Density (near r.t.) 9.32 g·cm−3
Liquid density at m.p. 8.56 g·cm−3
Melting point 1818 K, 1545 °C, 2813 °F
Boiling point 2223 K, 1950 °C, 3542 °F
Heat of fusion 16.84 kJ·mol−1
Heat of vaporization 247 kJ·mol−1
Molar heat capacity 27.03 J·mol−1·K−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 1117 1235 1381 1570 (1821) (2217)
Atomic properties
Oxidation states 2, 3, 4 ( basic oxide)
Electronegativity 1.25 (Pauling scale)
Ionization energies 1st: 596.7 kJ·mol−1
2nd: 1160 kJ·mol−1
3rd: 2285 kJ·mol−1
Atomic radius 176 pm
Covalent radius 190±10 pm
Crystal structure hexagonal close-packed
Thulium has a hexagonal close packed crystal structure
Magnetic ordering paramagnetic at 300 K
Electrical resistivity ( r.t.) (poly) 676 nΩ·m
Thermal conductivity 16.9 W·m−1·K−1
Thermal expansion ( r.t.) (poly)
13.3 µm/(m·K)
Young's modulus 74.0 GPa
Shear modulus 30.5 GPa
Bulk modulus 44.5 GPa
Poisson ratio 0.213
Vickers hardness 520 MPa
Brinell hardness 471 MPa
CAS registry number 7440-30-4
Most stable isotopes
Main article: Isotopes of thulium
iso NA half-life DM DE ( MeV) DP
167Tm syn 9.25 d ε 0.748 167Er
168Tm syn 93.1 d ε 1.679 168Er
169Tm 100% 169Tm is stable with 100 neutrons
170Tm syn 128.6 d β 0.968 170Yb
171Tm syn 1.92 y β 0.096 171Yb

Thulium is a chemical element that has the symbol Tm and atomic number 69. Thulium is the second least abundant of the lanthanides (promethium is only found in trace quantities on Earth). It is an easily workable metal with a bright silvery-gray luster. Despite its high price and rarity, thulium is used as the radiation source in portable X-ray devices and in solid-state lasers.


Physical properties

Pure thulium metal has a bright, silvery luster. It is reasonably stable in air, but should be protected from moisture. The metal is soft, malleable, and ductile. Thulium is ferromagnetic below 32 K, antiferromagnetic between 32 and 56 K and paramagnetic above 56 K. Liquid thulium is very volatile.

Chemical properties

Thulium metal tarnishes slowly in air and burns readily at 150 °C to form thulium(III) oxide:

4 Tm + 3 O2 → 2 Tm2O3

Thulium is quite electropositive and reacts slowly with cold water and quite quickly with hot water to form thulium hydroxide:

2 Tm (s) + 6 H2O (l) → 2 Tm(OH)3 (aq) + 3 H2 (g)

Thulium reacts with all the halogens. Reactions are slow at room temperature, but are vigorous above 200 °C:

2 Tm (s) + 3 F2 (g) → 2 TmF3 (s) (white)
2 Tm (s) + 3 Cl2 (g) → 2 TmCl3 (s) (yellow)
2 Tm (s) + 3 Br2 (g) → 2 TmBr3 (s) (white)
2 Tm (s) + 3 I2 (g) → 2 TmI3 (s) (yellow)

Thulium dissolves readily in dilute sulfuric acid to form solutions containing the pale green Tm(III) ions, which exist as [Tm(OH2)9]3+ complexes:

2 Tm (s) + 3 H2SO4 (aq) → 2 Tm3+ (aq) + 3 SO2−
(aq) + 3 H2 (g)

Thulium reacts with various metallic and non-metallic elements forming a range of binary compounds, including TmN, TmS, TmC2, Tm2C3, TmH2, TmH3, TmSi2, TmGe3, TmB4, TmB6 and TmB12. In those compounds, thulium exhibits valence states +2, +3 and +4, however, the +3 state is most common and only this state has been observed in Tm solutions.


Naturally occurring thulium is composed of one stable isotope, 169Tm (100% natural abundance). Thirty one radioisotopes have been characterized, with the most stable being 171Tm with a half-life of 1.92 years, 170Tm with a half-life of 128.6 days, 168Tm with a half-life of 93.1 days, and 167Tm with a half-life of 9.25 days. All of the remaining radioactive isotopes have half-lives that are less than 64 hours, and the majority of these have half-lives that are less than 2 minutes. This element also has 14 meta states, with the most stable being 164mTm (t½ 5.1 minutes), 160mTm (t½ 74.5 seconds) and 155mTm (t½ 45 seconds).

The isotopes of thulium range in atomic weight from 145.966 u (146Tm) to 176.949 u (177Tm). The primary decay mode before the most abundant stable isotope, 169Tm, is electron capture, and the primary mode after is beta emission. The primary decay products before 169Tm are element 68 (erbium) isotopes, and the primary products after are element 70 (ytterbium) isotopes.


Thulium was discovered by Swedish chemist Per Teodor Cleve in 1879 by looking for impurities in the oxides of other rare earth elements (this was the same method Carl Gustaf Mosander earlier used to discover some other rare earth elements). Cleve started by removing all of the known contaminants of erbia (Er2O3). Upon additional processing, he obtained two new substances; one brown and one green. The brown substance was the oxide of the element holmium and was named holmia by Cleve, and the green substance was the oxide of an unknown element. Cleve named the oxide thulia and its element thulium after Thule, Scandinavia.

Thulium was so rare that none of the early workers had enough of it to purify sufficiently to actually see the green colour; they had to be content with spectroscopically observing the strengthening of the two characteristic absorption bands, as erbium was progressively removed. The first researcher to obtain nearly pure thulium was Charles James, a British expatriate working on a large scale at New Hampshire College in Durham. In 1911 he reported his results, having used his discovered method of bromate fractional crystallization to do the purification. He famously needed 15,000 "operations" to establish that the material was homogeneous.

High-purity thulium oxide was first offered commercially in the late 1950s, as a result of the adoption of ion-exchange separation technology. Lindsay Chemical Division of American Potash & Chemical Corporation offered it in grades of 99% and 99.9% purity. The price per kilogram has oscillated between US$4,600 and $13,300 in the period from 1959 to 1998 for 99.9% purity, and it was second highest for lanthanides behind lutetium.

Occurrence and production

Monazit - Madagaskar.jpg

The element is never found in nature in pure form, but it is found in small quantities in minerals with other rare earths. Its abundance in the Earth crust is 0.5 mg/kg. Thulium is principally extracted from monazite (~0.007% thulium) ores found in river sands, through ion-exchange. Newer ion-exchange and solvent-extraction techniques have led to easier separation of the rare earths, which has yielded much lower costs for thulium production. The principal sources today are the ion adsorption clays of southern China. In these, where about two-thirds of the total rare-earth content is yttrium, thulium is about 0.5% (or about tied with lutetium for rarity). The metal can be isolated through reduction of its oxide with lanthanum metal or by calcium reduction in a closed container. None of thulium's natural compounds are commercially important.


Despite being rare and expensive, thulium has a few applications.


Holmium-chromium-thulium triple-doped YAG (Ho:Cr:Tm:YAG, or Ho,Cr,Tm:YAG) is an active laser medium material with high efficiency. It lases at 2097 nm and is widely used in military applications, medicine, and meteorology. Single-element thulium-doped YAG (Tm:YAG) lasers operate between 1930 and 2040 nm. The wavelength of thulium-based lasers is very efficient for superficial ablation of tissue, with minimal coagulation depth in air or in water. This makes thulium lasers attractive for laser-based surgery.


Thulium has been used in high temperature superconductors similarly to yttrium. Thulium potentially has use in ferrites, ceramic magnetic materials that are used in microwave equipment. Thulium is also similar to scandium in that it is used in arc lighting for its unusual spectrum, in this case, its green emission lines, which are not covered by other elements.

Biological role and precautions

Thulium has no known biological role, although it has been noted that it stimulates metabolism. Soluble thulium salts are regarded as slightly toxic if taken in large amounts, but the insoluble salts are non-toxic. Thulium is not taken up by plant roots to any extent and thus does not get into the human food chain. Vegetables typically contain only one milligram of thulium per tonne (dry weight).

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