Thorium Element Facts – Th or Atomic Number 90

Thorium Facts
Thin sheet of pure thorium metal under argon (Alchemist-hp)

Thorium is a silvery-white, slightly radioactive metal with the symbol Th and atomic number 90. It is an element the actinide series and f-block of the periodic table. Thorium takes its name for Thor, the Norse god of thunder, reflecting its powerful properties and mineral origin. While you don’t encounter thorium often in daily life, it holds promise as an important nuclear fuel and has uses in material science and metallurgy.

Discovery, Naming, and Isolation

Thorium was discovered in 1828 by the Swedish chemist Jöns Jakob Berzelius. Berzelius isolated thorium from a mineral called thorite, which had been forwarded to him from Norwegian mineralogist Morten Thrane Esmark. The isolation process involved treating thorite with acid, producing thorium dioxide. Further refinement was needed to isolate pure thorium metal, which wasn’t accomplished until later using advanced chemical reduction techniques. Berzelius named the element after Thor, the Norse god of thunder. The name reflects the element’s power and the Norwegian origin of the mineral.

Appearance and Properties

Pure thorium is a bright silver-colored metal, but it darkens to olive gray or black in air from the accumulation of thorium dioxide on its surface. This oxide layer protects the metal from further corrosion. It is a soft, ductile, and malleable metal. Thorium is only about half as dense as uranium or plutonium, but is harder than either metal and considerably less radioactive. At room temperature, the metal is paramagnetic, but it becomes superconductive below 1.4 K. Thorium has the highest melting point of the actinides.

Under normal conditions, thorium metal is in the face-centered cubic crystal system. However, it transitions to a body-centered cubic at temperatures over over 1360 °C and a body-centered tetragonal crystal form around 100 GPa of pressure.

Element Group

Thorium is part of the actinide series, a group of 15 elements in the f-block of the periodic table. All of the actinides are radioactive metals that occur in a variety of minerals.

Electron Levels of a Thorium Atom


In nature, thorium is almost exclusively the isotope 232Th, which is a stable isotope with a half-life of 14 billion years (about as long as the age of the universe). Scientists believe the radioactive decay of this isotope makes the single largest contribution to internal heating of the Earth.

A total of 32 radioisotopes are known, ranging from mass number 207 to 238. These other isotopes of the element have shorter half-lives. 230Th has a half-life of 75,000 years and accounts for around 0.04% of the natural element.

Abundance and Sources

The isotope 232Th is primordial, meaning it existed prior to the formation of the Earth. The element forms through the r-process, which mainly occurs in supernovae and neutron star collisions. Thorium is relatively abundant in the Earth’s crust, with an average concentration of about 10.5 ppm. It is more common than tin and uranium. Major thorium sources include the minerals monazite, thorite, and thorianite.


Thorium comes from minerals like monazite through a series of chemical processes:

  1. Crushing and grinding: The first step is crushing the mineral into a fine powder.
  2. Separation: Magnetic and electrostatic separation methods concentrate the thorium.
  3. Acid treatment: Sulfuric acid treatment of the concentrated ore yields thorium sulfate.
  4. Alkaline Digestion: Exposure to 30-45% hot sodium hydroxide solution precipitates out thorium and rare earths as hydroxides, while uranium forms sodium salts.
  5. Solvent extraction: Organic solvents separate thorium from other elements.
  6. Reduction: Thorium is finally reduced to its metallic form using reducing agents like calcium or magnesium.

Thorium Uses

Historically, thorium was very popular in gas mantles that produce bright light upon heating. Yttrium, which is not radioactive, largely replaces thorium for this use.

However, the element still has several important applications, mainly as the compound thoria (thorium oxide):

  • Nuclear fuel: Thorium is an alternative to uranium in nuclear reactors due to its abundance and lower waste production. At least one nuclear weapon design also uses thorium.
  • Alloying agent: Thorium addition to magnesium alloys improves high-temperature strength.
  • Optics: Thorium compounds improve the refractive index and decrease dispersion in high-quality optical lenses.
  • Catalysts: Thorium dioxide is useful in chemical reactions as a catalyst due to its high melting point and stability.
  • High-temperature ceramics: Thorium is a component of high-temperature ceramics and refractories.
  • Tungsten Filaments: While incandescent light bulbs have largely been phased out, coating the tungsten filament with thorium greatly improves its performance. Thorium dioxide also improves arc stability in gas tungsten arc welding.

Oxidation States

Thorium primarily exhibits the +4 oxidation state, which is the most stable and common. The +3 and +2 states also occur, but are less stable. The +1 and -1 states are also possible.

Biological Role, Health Effects, and Toxicity

Thorium has no known biological role in humans or other organisms. Because it releases alpha particles, inhalation or ingestion of thorium dust increases the risk of cancer as well as lung and liver damage. Skin blocks the radiation, so the element and its compounds are relatively safe to handle. The decay products of thorium, however, include highly dangerous radium and radon.

Table of Key Thorium Facts

Atomic Number90
Atomic Weight232.038 u
Electron Configuration[Rn] 6d2 7s2
Number of Electrons per Shell2, 8, 18, 32, 18, 10, 2
State of Matter at Room TemperatureSolid
Melting Point1750°C (3182°F)
Boiling Point4788°C (8650°F)
Density11.7 g/cm³
Heat of Fusion13.81 kJ/mol
Heat of Vaporization514 kJ/mol
Molar Heat Capacity26.23 J/(mol·K)
Oxidation States-1, +1, +2, +3, +4 (main)
Electronegativity1.3 (Pauling scale)
First Ionization Energy587 kJ/mol
Second Ionization Energy1110 kJ/mol
Third Ionization Energy1930 kJ/mol
Atomic Radius179 pm
Covalent Radius206 pm
Crystal StructureFace-centered cubic (FCC)
Thermal Conductivity54 W/(m·K)
Electrical Resistivity157 nΩ·m
Magnetic OrderingParamagnetic
Young’s Modulus79 GPa
Shear Modulus31 GPa
Mohs Hardness3.0


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