Thorium (Th)

Thorium is a chemical element with the atomic number 90 in the periodic table. The average concentration of this substance in the upper crust of Earth amounts to 10.5 ppm, while thorium concentration in the middle layer (the mantle) is around 6.5 ppm. The core (lowest layer) of the Earth’s crust contains 1.2 ppm on average. 

Being a member of the actinide family of periodic table elements, this naturally occurring radioactive metal has four valence electrons. Its most important use is in nuclear power plants as a nuclear fuel. Due to its radioactivity, thorium is classified as a carcinogen substance.

Chemical and Physical Properties of Thorium

Atomic number90
Atomic weight (mass)232.04 g.mol-1
Group (number)Actinides
Period7 (f-block)
ColorA silvery-white lustrous metal
Physical stateSolid at 20°C
Half-lifeFrom 1.7(+1.7-0.6) microseconds to 1.405(6)×1010 years
Electronegativity according to Pauling1.3
Melting point1750°C, 3182°F, 2023 K
Boiling point4785°C, 8645°F, 5058 K
Van der Waals radius0.182 nm
Ionic radius0.110 nm (+4)
Most characteristic isotope230Th, 232Th
Electronic shell[Rn] 6d27s2
The energy of the first ionization1107.6 kJ.mol-1
The energy of the second ionization1962.4 kJ.mol-1
The energy of the third ionization2774 kJ.mol-1
Discovery dateIn 1829 by Jöns Jacob Berzelius

Classified in the periodic table under the symbol Th, atomic number 90, atomic mass of 232.04 g.mol-1, and electron configuration [Rn] 6d27s2, thorium is a silvery-white, soft and ductile, naturally occurring radioactive metal. 

reaches its boiling point at 4785°C, 8645°F, 5058 K, while the melting point is achieved at 1750°C, 3182°F, 2023 K. This member of the actinide family of elements in the periodic table has an electronegativity of 1.3 according to Pauling, whereas the atomic radius according to van der Waals is 0.182 nm.

Thorium has a dimorphic structure. It changes from face-centered cubic to body-centered cubic at above 1360oC. With a melting point of 3300°C, thorium oxide melts at the highest temperature among all oxides. Only tungsten, tantalum carbide, and a few other compounds have a higher melting point than element 90.

How Was Thorium Discovered?

After receiving a mineral sample from Hans Esmark, obtained from an island close to Brevik, Norway, the Swedish chemist Jöns Jacob Berzelius (1779 – 1848) embarked on studying the unusual black substance that he labeled as thorite

After successfully identifying iron, manganese, lead, tin, and uranium in the mineral, Berzelius observed another unfamiliar substance. In 1828, the scientist concluded that there is 57.91% of an oxide of the assumed new element contained in the black thorite mineral. Berzelius named this reactive substance thorium

Attempting to isolate the elemental form of the new element, Berzelius triggered a chemical reaction. First, he produced thorium chloride by mixing thorium oxide with carbon. In the next step, the discoverer heated it in a stream of chlorine gas. After this, Berzelius reacted thorium chloride with potassium and got thorium and potassium chloride as a result. 

In 1898, both the German organic chemist and chemical crystallographer Gerhard Schmidt (1919–1971) and the first female scientist to win the Nobel prize Marie Curie (1867–1934) independently discovered the radioactive properties of thorium. 

How Did Thorium Get Its Name?

Thorium is named after Thor, the Scandinavian god of thunder, lightning, and war. According to the legend, the Norse God was known for his quick and volatile outbursts of anger, always ready to fight. This relates to the high chemical reactivity and the volatile reactions of element 90. 

Where Can You Find Thorium?

Element 90 is a naturally occurring radioactive metal. Natural thorium can be found in the soil, rocks, fossil fuels, water, plants, and animals. Thorite and thorianite are minerals in which this radioactive metal mostly occurs. It can also be found in thorium silicate, monazite, etc. The metal allotrope of this chemical element is also found in minerals such as titanite, betafite, gadolinite, and zircon.

Thorium is so frequently occurring in nature, that the mining locations rich in this substance can be found all over the Globe, in all continents. However, the largest quantities thorium reserves originate from the mines in Australia, the United States, Russia, Canada, and India. 

Extraction of thorium as a byproduct of rare-earth elements (REE), as well as isolation of this chemical element from the monazite ore is the most feasible source of thorium production. For commercial purposes, thorium is also obtained by the methods of electrolysis, extraction, and decomposition with sodium hydroxide. 

The world’s first thorium molten salt reactor (TMSR) experiment, after the initial experiment at the Oak Ridge National Laboratory (ORNL) during the 1960s, was conducted by the scientists at the Nuclear Research and Consultancy Group (NRG) in Petten, Netherlands. The Salt Irradiation Experiment, or SALIENT, has been prepared in collaboration with the European Commission Laboratory Joint Research Center-ITU, Karlsruhe. Currently, only China, India and Indonesia are included in this project

List of Thorium Minerals

The list of minerals from which thorium can be isolated also contains the items:

  • Aeschynite-(Y)
  • Althupite
  • Aspedamite
  • Auerlite
  • Cheralite
  • Cleveite
  • Ekanite
  • Euxenite
  • Grayite
  • Huttonite
  • Ichnusaite
  • Monazite
  • Nuragheite
  • Polycrase
  • Steacyite
  • Thorianite
  • Thorite
  • Yttrialite
  • Zirkelite

Thorium in Everyday Life

Thorium’s physical and chemical properties can be used in a variety of ways which are beneficial for our everyday life:

  • As more quantities of thorium are being made available, element 90 is researched as a uranium substitute in nuclear reactors for the production of fuel that generates nuclear energy;
  • Thorium used to be applied in the manufacturing of carbon arc lamps, as well as in mantles of gas lights for that emit intense white light;
  • Thorium dioxide (ThO2) is used as a control mechanism for small amounts of plutonium and tungsten applied in the production process of the metal spirals in electric lamps;
  • When added to glass, thorium improves its refractive index and decreases dispersion. The thorium-enriched glass is used in the manufacturing of camera lenses and scientific equipment. Over time, this type of glass gets a slightly yellow tint, but can be cleared again by exposure to high levels of UV light. The health risks of using lenses made with thorium dioxide are minimal;
  • Element 90 is often used in radiometric dating of fossils, seabeds and mountain ranges;
  • The gaseous form of element 90 (thoria) is used in arc welding for improvement of its strength and stability;
  • In the Mg-Th alloy, tungsten is used along with magnesium metal to increase both creep resistance and strength of the parts used in aircraft engines and rockets;
  • Crucibles, scientific instruments, and heat-resistant ceramic owe their resistance to high temperatures to thorium dioxide as one of the main components;
  • This chemical element is also one of the catalyst agents that participates in the production of sulphuric acid, as well as in the conversion of ammonia to nitric acid in petroleum cracking;
  • Uranium-233 isotopes can be used in nuclear weapons or for making a nuke;
  • The filaments of magnetron tubes which are used to generate microwave frequencies contain traces of thorium;
  • Thorium is also used in gas mantles that produce light in gas lamps;
  • Until the 1950s, thorium dioxide was used in medical radiology as a contrast agent (under the label Thorotrast) for making diagnostic X-ray images. It has been discontinued after the studies have related thorium exposure of the patients to the increased risk of liver tumors;
  • Thorium fluoride gives the antireflection properties of the optical coatings.

How Dangerous Is Thorium?

Thorium metal dust possesses high pyrophoricity that increases the risk of fire and explosion. It’s able to ignite spontaneously when exposed to air and burns brilliantly with a white light.

Thorium Toxicity

Exposure to thorium may occur in several ways:

  • Inhalation; 
  • Intravenous injection; 
  • Ingestion (via contaminated water or food);
  • Absorption through the skin. 

Initially, the affected individual may experience symptoms such as:

  • Eye irritation;
  • Skin irritation;
  • Nausea and vomiting;
  • Headaches;
  • Dermatitis;
  • Bronchospasms;
  • Severe bouts of cough;
  • ARTI (Acute Respiratory Tract Infection);
  • Blood disorders.

Prolonged exposure to high levels of thorium can be lethal. In the human body, element 90 typically absorbs in the bones, as well as in the soft tissues and organs. As a consequence, liver, bone, and pancreatitis cancers may develop in individuals exposed to high levels of this carcinogen substance. 

Environmental Effects of Thorium

Despite being one of the most frequently occurring chemical elements in nature, this carcinogen substance is not hazardous to the health of the biological, geological, and aquatic systems in the environment. This is mostly due to the fact that exposure to high levels of this radioactive substance may occur only near the thorium-mining areas and the factories that work with thorium or nuclear waste. 

Also, thorium radioactive waste takes more than 500 years to biodegrade. During this period, it poses an environmental threat due to its radioactivity. 

Isotopes of Thorium

Element 90 has 31 observed forms. Among them, seven are naturally occurring isotopes. Since thorium is a radioactive substance, all its isotopes are unstable. While most of the thorium radioisotopes have a half-life of several microseconds to several minutes, the 232Th isotope has a half-life of 1.405(6)×1010 years. 

The Thorium Cycle

Thorium reactors are based on the thorium fuel cycle that uses the thorium-232 isotope as a fertile material. In the thorium cycle of reactions, the thorium-232 isotope can be transformed by thermal neutrons to a fissionable uranium-233 isotope. 

The uranium-233 isotope is fissile on its own, i.e. the fission of this form of thorium can provide neutrons for a new thorium cycle. Th-232 isotopes produce Th-233 which undergoes a beta-decay mode (in 22 minutes of half-life) to protactinium-233. The protactinium isotope further decays to uranium-233 by undergoing a beta decay. This parallels the uranium fuel cycle in fast breeder reactors. 

The described chain reaction sequence can be observed from the following nuclear reactions:

232Th + n ⇒ 233Th

ß decay       ß decay

233Th ⇒    233Pa ⇒     233U

It has been observed that when the uranium-235 content burns down to nearly 0.3%, the highly radioactive residue of the fuel contains radioactive isotopes of iodine, plutonium, americium, and technetium. During the Cold War, the United States had reportedly produced about 2 tonnes of uranium-233 from thorium in plutonium production reactors. 


[n 1]



ZNIsotopic mass (Da)

[n 2][n 3]


[n 4]



[n 5]



[n 6]

Spin and


[n 7][n 8]

Natural abundance (mole fraction)
Excitation energyNormal proportionRange of variation
208Th[7] 90118208.01791(4)1.7(+1.7-0.6) msα204Ra0+  
209Th[8] 90119209.01772(11)7(5) ms


210Th 90120210.015075(27)17(11) ms

[9(+17−4) ms]

β+ (rare)210Ac
211Th 90121211.01493(8)48(20) ms

[0.04(+3−1) s]

β+ (rare)211Ac
212Th 90122212.01298(2)36(15) ms

[30(+20-10) ms]

α (99.7%)208Ra0+  
β+ (.3%)212Ac
213Th 90123213.01301(8)140(25) msα209Ra5/2−#  
β+ (rare)213Ac
214Th 90124214.011500(18)100(25) msα210Ra0+  
215Th 90125215.011730(29)1.2(2) sα211Ra(1/2−)  
216Th 90126216.011062(14)26.8(3) msα (99.99%)212Ra0+  
β+ (.006%)216Ac
217Th 90127217.013114(22)240(5) μsα213Ra(9/2+)  
218Th 90128218.013284(14)109(13) nsα214Ra0+  
219Th 90129219.01554(5)1.05(3) μsα215Ra9/2+#  
β+ (10−7%)219Ac
220Th 90130220.015748(24)9.7(6) μsα216Ra0+  
EC (2×10−7%)220Ac
221Th 90131221.018184(10)1.73(3) msα217Ra(7/2+)  
222Th 90132222.018468(13)2.237(13) msα218Ra0+  
EC (1.3×10−8%)222Ac
223Th 90133223.020811(10)0.60(2) sα219Ra(5/2)+  
224Th 90134224.021467(12)1.05(2) sα220Ra0+  
β+β+ (rare)224Ra


225Th 90135225.023951(5)8.72(4) minα (90%)221Ra(3/2)+  
EC (10%)225Ac
226Th 90136226.024903(5)30.57(10) minα222Ra0+  
227ThRadioactinium90137227.0277041(27)18.68(9) dα223Ra1/2+Trace[n 9] 
228ThRadiothorium90138228.0287411(24)1.9116(16) yα224Ra0+Trace[n 10] 
CD (1.3×10−11%)208Pb


229Th 90139229.031762(3)7.34(16)×103 yα225Ra5/2+Trace[n 11] 
230Th[n 12]Ionium90140230.0331338(19)7.538(30)×104 yα226Ra0+0.0002(2)[n 13] 
CD (5.6×10−11%)206Hg


SF (5×10−11%)(Various)
231ThUranium Y90141231.0363043(19)25.52(1) hβ231Pa5/2+Trace[n 9] 
α (10−8%)227Ra
232Th[n 14]Thorium90142232.0380553(21)1.405(6)×1010 yα228Ra0+0.9998(2) 
ββ (rare)232U
SF (1.1×10−9%)(various)
CD (2.78×10−10%)182Yb



233Th 90143233.0415818(21)21.83(4) minβ233Pa1/2+  
234ThUranium X190144234.043601(4)24.10(3) dβ234mPa0+Trace[n 13] 
235Th 90145235.04751(5)7.2(1) minβ235Pa(1/2+)#  
236Th 90146236.04987(21)#37.5(2) minβ236Pa0+  
237Th 90147237.05389(39)#4.8(5) minβ237Pa5/2+#  
238Th 90148238.0565(3)#9.4(20) minβ238Pa0+  

Source: Wikipedia

List of Thorium Compounds 

There are four outer shell electrons of thorium. All valence electrons of this highly reactive and electropositive element are able to participate in a chemical compound. It readily reacts with oxygen, hydrogen, nitrogen, the halogen elements, and sulfur, when exposed to high temperatures. With phosphorus and carbon, thorium forms binary compounds. 

Out of the many compounds prepared with thorium, the following are most common:

  • Thorium dioxide
  • Thorium monoxide
  • Thorium oxalate
  • Thorium tetrafluoride
  • Thorium(IV) carbide
  • Thorium(IV) chloride
  • Thorium(IV) hydroxide
  • Thorium(IV) iodide
  • Thorium(IV) nitrate
  • Thorium(IV) orthosilicate
  • Thorium(IV) sulfide
  • Thorocene

5 Interesting Facts and Explanations

  1. Thorium and uranium are not only the most stable actinides but also the only members of the actinide group of the periodic table that can be safely studied in a regular laboratory.
  2. The substances that spontaneously ignite upon exposure to air at or below 54°C(129 °F) or shortly after being exposed to air are referred to as pyrophoric substances (from the Greek word ‘πυρφόρος / pyrophorus’, meaning ‘fire-bearing’).
  3. In 2011, the China Academy of Sciences launched an R&D program on LFTR. Liquid fluoride thorium reactors (LFTR) produce less waste during the production energy than the reactors powered by uranium. As a comparison, a traditional pressurised water reactor (PWR) would need to burn 250 tonnes of uranium to produce the same amount of energy. Also, no solid fuel rods (or chemical reprocessing) are needed because LFTRs use thorium in its natural state. 
  4. This process of isolating an element from its ore used by the discoverer of thorium was a very familiar one to Berzelius’ fellow chemists. Namely, Ørsted isolated aluminum by the same method in 1825, while in 1828 Wöhler and Bussy succeeded in isolating beryllium in the same way.
  5. Thorium is the second naturally occurring element that has been identified as a radioactive substance, after uranium.