Thulium (Tm)

Stable Isotopes of Thulium 

Isotope Z(p) N(n) Atomic Mass Natural Abundance Nuclear Spin
Tm-169  69  100  168.934211 100.00%  1/2+ 


Thulium was discovered in 1879 by Per Theodore Cleve. Its name derives from Thule, an ancient name for Scandinavia.

Thulium is a silvery-white metal with a metallic luster and a hexagonal, close-packed structure. Its salts are colored green. It has an effective magnetic moment of 7.62 Bohr magneton. It is insoluble in water and dissolves in concentrated acids. It is relatively stable in air at ambient temperatures. Reactions with halogens are slow at ordinary temperatures, but vigorous above 200 ºC, forming halides. Thulium reacts with concentrated mineral acids, forming corresponding salts and liberating hydrogen.

The metal has very few commercial applications because of its high cost and low relative abundance. Thulium metal pellets containing natural isotope Thulium-169 and radioactive isotope Thulium-170 are used in portable x-ray equipment as medical and dental diagnostic tools. These pellets are also used to detect flaws in small, inaccessible parts of mechanical and electrical devices. Radioactive Thulium-171 is a beta emitter with a half-life of two years and is potentially useful as an energy source. Natural thulium is used in ceramic magnetic materials (ferrites) for microwave devices.

Properties of Thulium

Name Thulium 
Symbol Tm 
Atomic number 69 
Atomic weight 168.9342 
Standard state Solid at 298 ºK 
CAS Registry ID 7440-30-4 
Group in periodic table N/A 
Group name Lanthanoid 
Period in periodic table 6 (Lanthanoid) 
Block in periodic table f-block 
Color Silvery white 
Classification Metallic 
Melting point 1545 °C
Boiling point 1947 °C
Vaporization point 1947 °C
Thermal conductivity 16.9 W/(m·K) at 298.2 ºK
Electrical resistivity 79.0 µΩ·cm at 25 ºC 
Electronegativity 1.2
Specific heat 0.16 kJ/kg K
Heat of vaporization 250 kJ·mol-1
Heat of fusion 16.8 kJ·mol-1
Density of liquid 8.56 g/cm3 at 1545 °C
Density of solid 9.32 g/cm3 
Electron configuration [Xe]4f136s2 
Atomic radius 1.73 Å 
Ionic radius Tm3+: 1.09 Å (coordination number 7)
Oxidation states  +2, +3 
Most stable oxidation state  +3

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