From New World Encyclopedia

69 erbiumthuliumytterbium


periodic table
Name, Symbol, Number thulium, Tm, 69
Chemical series lanthanides
Group, Period, Block n/a, 6, f
Appearance silvery gray
Atomic mass 168.93421(2) g/mol
Electron configuration [Xe] 4f13 6s2
Electrons per shell 2, 8, 18, 31, 8, 2
Physical properties
Phase solid
Density (near r.t.) 9.32 g/cm³
Liquid density at m.p. 8.56 g/cm³
Melting point 1818 K
(1545 °C, 2813 °F)
Boiling point 2223 K
(1950 °C, 3542 °F)
Heat of fusion 16.84 kJ/mol
Heat of vaporization 247 kJ/mol
Heat capacity (25 °C) 27.03 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 1117 1235 1381 1570 (1821) (2217)
Atomic properties
Crystal structure hexagonal
Oxidation states 3
(basic oxide)
Electronegativity 1.25 (Pauling scale)
Ionization energies
1st: 596.7 kJ/mol
2nd: 1160 kJ/mol
3rd: 2285 kJ/mol
Atomic radius 175 pm
Atomic radius (calc.) 222 pm
Magnetic ordering no data
Electrical resistivity (r.t.) (poly) 676 nΩ·m
Thermal conductivity (300 K) 16.9 W/(m·K)
Thermal expansion (r.t.) (poly)
13.3 µm/(m·K)
Speed of sound (thin rod) (r.t.) 74.0 m/s
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
Notable 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% Tm is stable with 100 neutrons
170Tm syn 128.6 d β- 0.968 170Yb
171Tm syn 1.92 y β- 0.096 171Yb

Thulium (chemical symbol Tm, atomic number 69) is the least abundant of the rare earth metals.[1] It has a bright, silvery-gray luster.


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 percent 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 are commercially important.


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) 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, Scandinavia.

Notable characteristics

Thulium is an inner transition metal (or lanthanide) that lies in period six of the periodic table, between erbium and ytterbium. It is easy to work and can be cut by a knife. It is ductile and is somewhat resistant to corrosion in dry air.


Naturally occurring thulium is composed of a single stable isotope, Tm-169 (100 percent 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-lives that are less than 64 hours, and the majority of these have half lives that are less than two minutes. This element also has 14 meta states, with the most stable being Tm-164m (t½ 5.1 minutes), Tm-160m (t½ 74.5 seconds), and Tm-155m (t½ 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 been used to create lasers, but high production costs have prevented other commercial uses from being developed. Other applications, real and potential, include:

  • 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 isotope 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 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

  • Thulium compounds.


  1. The term "rare earth metals" (or "rare earth elements") is a trivial name applied to 16 chemical elements: scandium, yttrium, and 14 of the 15 lanthanides (excluding promethium), which occur naturally on Earth. Some definitions also include the actinides. The word "earth" is an obsolete term for oxide. The term "rare earth" is discouraged by the International Union of Pure and Applied Chemistry (IUPAC), as these elements are relatively abundant in the Earth's crust.

ISBN links support NWE through referral fees

  • Chang, Raymond. Chemistry. 9th ed. New York: McGraw-Hill Science/Engineering/Math, 2006. ISBN 0073221031
  • Cotton, F. Albert, and Geoffrey Wilkinson. Advanced Inorganic Chemistry. 4th ed. New York: Wiley, 1980. ISBN 0-471-02775-8
  • Greenwood, N.N. and A. Earnshaw. Chemistry of the Elements. 2nd ed. Oxford, U.K.; Burlington, MA: Butterworth-Heinemann, Elsevier Science, 1998. ISBN 0750633654 Online version Retrieved September 20, 2007.
  • Jones, Adrian P., Frances Wall, and C. Terry Williams, eds. Rare Earth Minerals: Chemistry, Origin and Ore Deposits. The Mineralogical Society Series. London, UK: Chapman and Hall, 1996. ISBN 0412610302
  • Stwertka, Albert. Guide to the Elements. Rev. ed. Oxford, UK: Oxford University Press, 1998. ISBN 0-19-508083-1
  • "Thulium" Los Alamos National Laboratory, Chemistry Division. Retrieved September 20, 2007.
  • "Thulium" It's Elemental. Jefferson Lab. Retrieved September 20, 2007.

External links

All links retrieved April 30, 2023.


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