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Thorium,  90Th
General properties
Name, symbol Thorium, Th
Appearance Silvery-grey, often black tarnish
Thorium in the periodic table


Atomic number 90
Standard atomic weight (Ar) 232.0377(4)
Element category Actinides
Group, block n/a; f-block
Period period 7
Electron configuration [Rn] 6d2 7s2
per shell
2, 8, 18, 32, 18, 10, 2
Physical properties
Phase Solid
Melting point 2023 K ​(1750 °C, ​3182 °F)
Boiling point 5061 K ​(4788 °C, ​8650 °F)
Density near r.t. 11.7 g/cm3
Heat of fusion 13.81 kJ/mol
Heat of 514 kJ/mol
Molar heat capacity 26.23 J/(mol·K)
Atomic properties
Oxidation states 4, 3, 2, 1 ​(a weakly basic oxide)
Electronegativity Pauling scale: 1.3
energies 1st: 587 kJ/mol
2nd: 1110 kJ/mol
3rd: 1930 kJ/mol
Atomic radius empirical: 179.8 pm
Covalent radius 206±6 pm
Crystal structure ​Face-centred cubic (fcc)
Speed of sound thin rod 2490 m/s (at 20 °C)
Thermal expansion 11 µm/(m·K) (at 25 °C)
Thermal conductivity 54 W/(m·K)
Electrical resistivity 157 Ω·m (at 0 °C)
Magnetic ordering Paramagnetic
Young's modulus 79 GPa
Shear modulus 31 GPa
Bulk modulus 54 GPa
Poisson ratio 0.27
Mohs hardness 3.0
Vickers hardness 295–685 MPa
Brinell hardness 390–1500 MPa
CAS Registry Number 7440-29-1
Naming After Thor, the Norse god of thunder
Discovery Jöns Jakob Berzelius (1829)
· references

Thorium is a chemical element with the symbol Th and atomic number 90.



Thorium is a highly reactive metal. At room temperature, thorium is slowly attacked by water but does not readily dissolve in most common acids, the exception being hydrochloric acid. It will dissolve in concentrated nitric acid containing a small amount of catalytic fluoride or fluorosilicate ions. At high temperatures, it is easily attacked by oxygen, hydrogen, nitrogen, the halogens, and sulfur. Finely divided thorium is highly pyrophoric. Thorium burns with a bright white flame in the air.


Thorium is a soft, paramagnetic, bright silvery radioactive actinide metal. Pure thorium is soft, very ductile. Because thorium melts at 1750 °C and boils at 4788 °C, it has one of the widest liquid range of all metals.


Thorium was available in the past as thorium dioxide in gas mantles. Certain heat resistant ceramics contain thoria. Small amounts of thorium are added in TIG welding rods. Old gas mantles can contain amounts of thorium dioxide.

The acquisition of pure metal is problematic, as there are almost no companies that will sell very small samples to individuals. Element collector Max Whitby explained in a YT video that there are extremely few companies that will sell small samples of thorium, even chemical lab suppliers. He also discovered that shipping it outside requires a heavy-duty license for radioactive materials, as he discovered when he was visited by authorities.


Metallic thorium can be extracted by reducing thorium dioxide with calcium in an inert atmosphere:

ThO2 + 2 Ca → 2 CaO + Th

Dissolving TIG welding rods in acids or alkali is difficult, as tungsten is very resistant to most reagents, and bulk metal is even more inert. However, one can successfully dissolve them in concentrated H2O2, which forms tungstic acid and ThO2.


  • Thorium halides
  • Make thorium dioxide
  • Firestarters



Thorium is highly reactive and pyrophoric. It is only mildly radioactive (232Th has a half-life of 14.05 billion years and is alpha radioactive), the radiation is too weak to penetrate the skin. Soluble thorium compounds are toxic and should be handled with proper protection.

Bulk thorium is difficult to ignite, but finely divided metal is highly pyrophoric.


As thorium will slowly react with the air, it should be stored under oil or under an inert atmosphere, in a container. Sealing it is not necessary. Being weak radioactive, thorium does not require radiation shielding (the storage container walls will effectively absorb the weak alpha radiation).


Thorium and its compounds, are toxic, and it is recommended to avoid dumping the compounds in the environment and instead should be taken to waste disposal facilities.

If the thorium was extracted from its natural ore that was taken from an area where the said ore can be easily collected, it's not entirely a bad idea to dispose of the ore wastes as well as thorium compounds residues in the said area, as the said area is already naturally contaminated and radioactive and the amount of ore waste is (almost) the same as the one you originally picked it up.


Relevant Sciencemadness threads