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General Scientific Information

  • When pure, thorium is a silvery white metal which is air-stable and retains its lustre for several months.
  • When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black.
  • Thorium oxide has a melting point of 3300C, the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points.
  • Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric.
  • Powdered thorium metal is often pyrophoric and should be carefully handled.When heated in air, thorium turnings ignite and burn brilliantly with a white light.
  • Monazite - phosphate mineral [(Ce,La,Y,Th)PO4], found in the form of sand in the U.S., Madagascar, Brazile, India, Sri Lanka and Australia.  Monazite is an important source of Cerium, Thorium and other Rare-Earth Metals.
Atomic Structure
Atomic Number 90
Atomic Weight 232.0381
No. of Electrons Shells 7
Periodic Chart Group Actinide (Group IIIb)
Classification Rare Earth

Physical Properties
Symbol: Th
Melting Point: 1750.0 oC
Boiling Point: 4790.0 oC
Density @ 293 K: 11.72 g/cm3
Electrical resistivity 15E-8 Siemens
Velocity of Sound 2490 m/s
Appearance soft, ductile, lustrous, silvery-white
CAS Registry Number: 7440-29-1

Chemical Properties
Electron Valence State +4
Molar Volume 19.80 cm3
Crystal Structure @ 20 C face-centered cubic
Additional Information Thorium



The following represents the electronic configuration and its associated term symbol for the ground state neutral gaseous atom. The configuration associated with thorium in its compounds is not necessarily the same.

Ground state electron configuration [Rn].6d2.7s2
Term symbol 3F2

thorite A rare mineral, thorium silicate, ThSiO4, occurring in the form of a yellow or black crystal
thorianite A rare mineral, mainly thoria, ThO2, but also containing uranium, cerium and other rare-earth metals.  Thorianite is found as small, black cubic crystals and is a source of thorium.

State DfH
/kJ mol-1
/kJ mol-1
/J K-1 mol-1
/J K-1 mol-1
/kJ mol-1
Solid *0 0 *51.8 0.5 27.3  
Gas *602 6 561 *190.17 0.05 20.8  


These tables give a few thermodynamic data. Most values are those given in the NBS technical notes (reference 1) after conversion from the units used within those notes. Values labelled with an asterisk (*) are Committee on Data for Science and Technology (CODATA) agreed values for the thermodynamic properties of key chemical substances (reference 2). These values are published in a number of places including the WWW (reference 3)

  1. R.H. Schumm, D.D. Wagman, S. Bailey, W.H. Evans, and V.B. Parker in National Bureau of Standards (USA) Technical Notes 270-1 to 270-8, 1973.

  2. J.D. Cox, DD., Wagman, and V.A. Medvedev, CODATA Key Values for Thermodynamics, Hemisphere Publishing Corp., New York, USA, 1989.



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