Name, Symbol, Number Thulium, Tm, 69
Chemical series Lanthanides
Group, Period, Block NA, 6, f
Density, Hardness 9321 kg/m3, no data
Appearance silvery gray
Missing image

Atomic properties
Atomic weight 168.93421 u
Atomic radius (calc.) 175 (222) pm
Covalent radius ND pm
van der Waals radius ND pm
Electron configuration [Xe]6s24f13
e- 's per energy level 2,8,18,31,8,2
Oxidation states (Oxide) 3 (basic)
Crystal structure Hexagonal
Physical properties
State of matter solid (__)
Melting point 1818 K (2813 ?F)
Boiling point 2220 K (3537 ?F)
Molar volume 19.1 ×10-6 m3/mol
Heat of vaporization 191 kJ/mol
Heat of fusion 16.84 kJ/mol
Vapor pressure ND Pa at __ K
Velocity of sound ND m/s at 293.15 K
Electronegativity 1.25 (Pauling scale)
Specific heat capacity 160 J/(kg*K)
Electrical conductivity 1.5 106/m ohm
Thermal conductivity 16.8 W/(m*K)
1st ionization potential 596.7 kJ/mol
2nd ionization potential 1160 kJ/mol
3rd ionization potential 2285 kJ/mol
4th ionization potential 4120 kJ/mol
Most stable isotopes
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% Thulium is stable with 100 neutrons
170Tm {syn.} 128.6 d β- 0.968 170Yb
171Tm {syn.} 1.92 y β- 0.096 171Yb
SI units & STP are used except where noted.

Thulium is a chemical element that has the symbol Tm and atomic number 69 in the periodic table. A lanthanide element, thulium is the least abundant of the rare earths and its metal is easy to work, has a bright silvery-gray luster and can be cut by a knife. It also has some corrosion resistance in dry air and good ductility. Naturally occurring thulium is made entirely of the stable isotope Tm-169.

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Thulium has been used to create lasers but high production costs have prevented other commercial uses from being developed. Other uses/potential uses:

  • When stable thulium (Tm-169) is bombarded in a nuclear reactor it can later serve as a radiation source in portable X-ray devices.
  • The unstable Tm-171 could possibly be used as an energy source.
  • Tm-169 has potential use in ceramic magnetic materials called ferrites, which are used in microwave equipment.


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 earths elements). Cleve started by removing all of the known contaminants of erbia (Er2O3) and upon additional processing, obtained two new substances; one brown and one green. The brown substance turned out to be 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, an ancient Roman name for a mythical country in the far north, perhaps Scandinavia.


The element is never found in nature in pure form, but it is found in small quantities in minerals with other rare earths. It 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 metal can be isolated through reduction of its oxide with lanthanum metal or by calcium reduction in a closed container. None of thulium's compounds is commercially important.


Naturally occurring thulium is composed of 1 stable isotope, Tm-169 (100% natural abundance). 31 radioisotopes have been characterized, with the most stable being Tm-171 with a half-life of 1.92 years, Tm-170 with a half-life of 128.6 days, Tm-168 with a half-life of 93.1 days, and Tm-167 with a half-life of 9.25 days. All of the remaining radioactive isotopes have half-lifes that are less than 64 hours, and the majority of these have half lifes that are less than 2 minutes. This element also has 14 meta states, with the most stable being Tm-164m (t? 5.1 minutes), Tm-160m (t? 74.5 seconds) and Tm-155m (t? 45 seconds).

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


Thulium has a low-to-moderate acute toxic rating and should be handled with care. Metallic thulium in dust form presents a fire and explosion hazard.

See also Ytterby.


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