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Thulium,  69Tm
A piece of thulium metal from IrC's collection.
General properties
Name, symbol Thulium, Tm
Appearance Silvery-gray
Thulium in the periodic table


Atomic number 69
Standard atomic weight (Ar) 168.93422(2)
Group, block , f-block
Period period 6
Electron configuration [Xe] 4f13 6s2
per shell
2, 8, 18, 31, 8, 2
Physical properties
Phase Solid
Melting point 1818 K ​(1545 °C, ​​2813 °F)
Boiling point 2223 K ​(1950 °C, ​​3542 °F)
Density near r.t. 9.32 g/cm3
when liquid, at  8.56 g/cm3
Heat of fusion 16.84 kJ/mol
Heat of 191 kJ/mol
Molar heat capacity 27.03 J/(mol·K)
Atomic properties
Oxidation states 2, 3 ​(a basic oxide)
Electronegativity Pauling scale: 1.25
energies 1st: 596.7 kJ/mol
2nd: 1160 kJ/mol
3rd: 2285 kJ/mol
Atomic radius empirical: 176 pm
Covalent radius 190±10 pm
Crystal structure ​​hexagonal close-packed (hcp)
Thermal expansion 13.3 µm/(m·K) (poly)
Thermal conductivity 16.9 W/(m·K)
Electrical resistivity 676 Ω·m (at 20 °C) (poly)
Magnetic ordering paramagnetic (300 K)
Young's modulus 74 GPa
Shear modulus 30.5 GPa
Bulk modulus 44.5 GPa
Poisson ratio 0.213
Vickers hardness 470–650 MPa
Brinell hardness 470–900 MPa
CAS Registry Number 7440-30-4
Naming after Thule, a mythical region in Scandinavia
Discovery and first isolation Per Teodor Cleve (1879)
· references

Thulium is a lanthanide with the symbol Tm and the atomic number 69. Thulium is the second-least abundant of the lanthanides, after radioactively unstable promethium which is only found in trace quantities on Earth.

It is often considered by many chemists a useless element, because it's rare and expensive, and the other more available lanthanides have similar physical and chemical properties.



Thulium metal is a reducing agent and is reactive towards acids and oxidizers. Most ionic thulium compounds are soluble except for the fluoride and the nitrate.

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

4 Tm + 3 O2 → 2 Tm2O3

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

2 Tm + 6 H2O → 2 Tm(OH)3 + 3 H2

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

2 Tm + 3 X2 → 2 TmX3


Thulium metal has a bright, silvery luster. It slowly tarnishes in air and reacts with water. The metal is soft enough to be cut with a knife. Solutions containing thulium(II) ions often present a wine red coloration. Thulium(III) ions may fluoresce blue under shortwave UV light, though this has not been confirmed.


Thulium is more common than iodine on Earth, but it is hard to find and extremely expensive. One source for thulium, as well as other rare earth metals, is Metallium. It is sold in 1 gram, 5 gram and 25 gram sizes, as well as rods, ampoules, and coins. Metallium also takes custom orders. Thulium and its compounds may be occasionally found on eBay. Because the Tm(III) ion color is hard to distinguish from other ions, other lanthanides or metals in general may be sold as thulium. This is a common problem with lanthanides, and online sellers.

Occasionally you can find sellers on eBay, like this one.


  • Blue fluorescence (used in Euro banknotes)
  • Make ferrite ceramic materials
  • Special solid-state lasers



Thulium has not been observed to have a biological role, but small amounts of soluble thulium salts stimulate metabolism. Soluble thulium salts are mildly toxic, but insoluble thulium salts are completely nontoxic.


Thullium should be stored in closed bottles, and kept away from moisture and any corrosive vapors, such as acids.


Best to try to recycle it, as it's a rare and expensive material.


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