Thorium Atomic Number

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  1. Thorium Element Atomic Number
  2. Thorium Atomic Number And Mass

Thorium (Th) is a silvery-white colored radioactive metal that has the atomic number 90 in the periodic table. It is an Actinoid Metal with the symbol Th. Thorium is named after the norse god of thunder, Thor.

About Thorium


Thorium is a relatively common radioactive element in the actinide family. In pure form, it is a soft silvery-white metal with properties very similar to lead, and can be shaped rather easily under proper conditions. The abundance of thorium within the earth’s crust is much greater (15 ppm) than other radioactive elements, particularly uranium. This makes thorium an attractive candidate for lowering the cost of nuclear power sources in the future. Thorium is typically isolated by converting a sample of monazite or thorite to thorium dioxide, and then heating the compound with calcium; which, if desired (e.g. breeder reactors), can then be transformed into various synthetic isotopes of uranium through neutron bombardment. Only one thorium nuclear reactor was ever built (1979) and due to numerous economic and technical problems, shut down after only a decade.

The native characteristics of thorium lend itself to a number of compounds with industrial and consumer applications. In fact, thorium is the only radioactive element besides uranium with non-radioactive applications. Central to these applications is its ability to maintain strength at high temperature. In some magnesiumalloys, thorium is used in aircraft engines and rockets. Oxidized thorium is used in arc welders. Compounds such as thorium dioxide and thorium nitrate are used in various lights and lamps as a brilliant white light source when heated with a gas flame. Thoriated tungsten elements are contained in filaments of magnetrons, most commonly exemplified in microwave ovens and radar systems. Furthermore, thorium dioxide is utilized in many ceramics and glasswork applications. Thorium dioxide is known to increase refractive index in glass, thus has many applications in optical lenses for cameras and scientific apparatuses. Thorium fluoride is used as an optical coating, and the list goes on. Although exposure to small amounts of thorium in applications such as those listed above is considered safe, thorium-based products have begun to fall out of favor (2000s) due to its perceived biological danger.

The element was discovered in 1828 by Swedish chemist Jons Jakob Berzelius, at a time when the concept of radioactivity was not yet known. It was first recognized as a radioactive element as a result of English chemist Gerhard C. Schmidt and Marie Curie’s radioactivity research in 1898. There are 27 known isotopes of thorium, all radioactive. The isotope with the longest half-life is 232Th, at a staggering 14 billion years, which explains this primordial radioactive element’s relative abundance on Earth. This particular isotope decays into 228Ra through alpha decay; other elemental decay products of thorium isotopes include radon and actinium.

  1. Thorium is a chemical element with symbol Th and atomic number 90.A radioactive actinide metal, thorium is one of only three radioactive elements that still occurs in quantity in nature as a primordial element (the other two being bismuth and uranium).
  2. Complete and detailed technical data about the element Thorium in the Periodic Table. Atomic Number: 90: Atomic Weight: 232.0377: Density: 11.724 g/cm 3.


Thorium Atomic Number
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Thorium is a lanthanide (rare earth) material with potential nuclear power applications. It is presently used as a tungsten coating in electronic parts due to its high emission factor. In its fluoride and oxide forms, Thorium is used in advanced optic applications for its high refractive index. It is also used in several other high temperature glass applications, such as in the mantle of lamps and to produce crystal growth crucibles and ampules. Thorium is available as metal and compounds with purities from 99% to 99.999% (ACS grade to ultra-high purity). Oxides are available in powder and dense pellet form for such uses as optical coating and thin film applications. Oxides tend to be insoluble. Fluorides are another insoluble form for uses in which oxygen is undesirable such as metallurgy, chemical and physical vapor deposition and in some optical coatings. Thorium is also available in soluble forms including chlorides, nitrates and acetates. These compounds can be manufactured as solutions at specified stoichiometries.

Thorium Properties

Thorium is a Block F, Group 3, Period 7 element. The number of electrons in each of Thorium's shells is 2, 8, 18, 32, 18, 10, 2 and its electron configuration is [Rn] 6d2 7s2. The thorium atom has a radius of and its Van der Waals radius is In its elemental form, CAS 7440-29-1, thorium has a silvery, sometimes black-tarnished, appearance. Thorium was first discovered by Jons Berzelius in 1829. It is found in small amounts in most rocks and soils. The name Thorium originates from the Scandinavian god Thor, the Norse god of war and thunder.

Atomic Number:90
Atomic Weight:232
Element Category:Actinide
Group, Period, Block:n/a, 7, f
Color:silvery white
Other Names:N/A
Melting Point:1842 °C, 3348 °F, 2115 K
Boiling Point:4788 °C, 8650 °F, 5061 K
Density:11.7 g·cm3
Liquid Density @ Melting Point:N/A
Density @ 20°C:11.7 g/cm3
Density of Solid:11724 kg·m3
Specific Heat:0.13 kJ/kg/K
Superconductivity Temperature:1.38 [or -271.77 °C (-457.19 °F)] K
Triple Point:N/A
Critical Point:N/A
Heat of Fusion (kJ·mol-1):19.2
Heat of Vaporization (kJ·mol-1):513.67
Heat of Atomization (kJ·mol-1):598.65
Thermal Conductivity:54.0 W·m-1·K-1
Thermal Expansion:(25 °C) 11.0 µm·m-1·K-1
Electrical Resistivity:(0 °C) 147 nΩ·m
Tensile Strength:144 MPa
Molar Heat Capacity:26.230 J·mol-1·K-1
Young's Modulus:79 GPa
Shear Modulus:31 GPa
Bulk Modulus:54 GPa
Poisson Ratio:0.27
Mohs Hardness:3
Vickers Hardness:350 MPa
Brinell Hardness:400 MPa
Speed of Sound:(20 °C) 2490 m·s-1
Pauling Electronegativity:1.3
Sanderson Electronegativity:N/A
Allred Rochow Electronegativity:1.11
Mulliken-Jaffe Electronegativity:N/A
Allen Electronegativity:N/A
Pauling Electropositivity:2.7
Reflectivity (%):N/A
Refractive Index:N/A
Electron Configuration:[Rn] 6d2 7s2
Atomic Radius:179 pm
Atomic Radius,
non-bonded (Å):
Covalent Radius:206±6 pm
Covalent Radius (Å):1.9
Van der Waals Radius:237 pm
Oxidation States:4, 3, 2, 1 (weakly basic oxide)
Crystal Structure:face-centered cubic
Magnetic Ordering:paramagnetic
Electron Affinity (kJ·mol-1)Unknown
1st Ionization Energy:587 kJ·mol-1
2nd Ionization Energy:1110 kJ·mol-1
3rd Ionization Energy:1930 kJ·mol-1
CAS Number:7440-29-1
EC Number:231-139-7
MDL Number:N/A
Beilstein Number:N/A
SMILES Identifier:[Th]
InChI Identifier:InChI=1S/Th
PubChem CID:23960
ChemSpider ID:22399
Earth - Total:51.2 ppb
Mercury - Total:39.4 ppb
Venus - Total:53.7 ppb
Earth - Seawater (Oceans), ppb by weight:0.00004
Earth - Seawater (Oceans), ppb by atoms:1.1E-06
Earth - Crust (Crustal Rocks), ppb by weight:6000
Earth - Crust (Crustal Rocks), ppb by atoms:540
Sun - Total, ppb by weight:0.3
Sun - Total, ppb by atoms:0.002
Stream, ppb by weight:0.1
Stream, ppb by atoms:0.0004
Meterorite (Carbonaceous), ppb by weight:40
Meterorite (Carbonaceous), ppb by atoms:3
Typical Human Body, ppb by weight:N/A
Typical Human Body, ppb by atom:N/A
Universe, ppb by weight:0.4
Universe, ppb by atom:0.002
Discovered By:Jöns Jakob Berzelius
Discovery Date:1829
First Isolation:N/A

Health, Safety & Transportation Information for Thorium

Thorium is radioactive and can collect in bones which may cause bone cancer several years after exposure. Breathing in substantial amounts of thorium may be lethal. Safety data for Thorium and its compounds can vary widely depending on the form. For potential hazard information, toxicity, and road, sea and air transportation limitations, such as DOT Hazard Class, DOT Number, EU Number, NFPA Health rating and RTECS Class, please see the specific material or compound referenced in the Products tab. The below information applies to elemental (metallic) Thorium.

Safety Data
Material Safety Data SheetMSDS
Signal Word Danger
Hazard Statements H272-H301-H373-H315-H319-H335
Hazard CodesN/A
Risk Codes N/A
Safety Precautions N/A
RTECS Number XO6400000
Transport InformationUN 2912
WGK Germany 3
Globally Harmonized System of
Classification and Labelling (GHS)

Thorium Isotopes

Thorium (Th) has six naturally occuring isotopes. None of these are stable; however, 232Th is observationally stable with a half life of 14.05 billion years.

NuclideIsotopic MassHalf-LifeMode of DecayNuclear SpinMagnetic MomentBinding Energy (MeV)Natural Abundance
(% by atom)
209Th209.01772(11)7(5) ms [3.8(+69-15)]Unknown5/2-#N/A1578.15-
210Th210.015075(27)17(11) ms [9(+17-4) ms]a to 206Ra0+N/A1586.23-
211Th211.01493(8)48(20) ms [0.04(+3-1) s]a to 207Ra5/2-#N/A1594.31-
212Th212.01298(2)36(15) ms [30(+20-10) ms]a to 208Ra; ß+ to 212Ac0+N/A1602.38-
213Th213.01301(8)140(25) msa to 209Ra5/2-#N/A1610.46-
214Th214.011500(18)100(25) msa to 210Ra0+N/A1618.54-
215Th215.011730(29)1.2(2) sa to 211Ra(1/2-)N/A1626.62-
216Th216.011062(14)26.8(3) msa to 212Ra; ß+ to 216Ac0+N/A1634.7-
217Th217.013114(22)240(5) µsa to 213Ra(9/2+)N/A1642.78-
218Th218.013284(14)109(13) nsa to 214Ra0+N/A1650.86-
219Th219.01554(5)1.05(3) µsa to 215Ra9/2+#N/A1658.93-
220Th220.015748(24)9.7(6) µsa to 216Ra; EC to 220Ac0+N/A1667.01-
221Th221.018184(10)1.73(3) msa to 217Ra(7/2+)N/A1675.09-
222Th222.018468(13)2.237(13) msa to 218Ra; EC to 220Ac0+N/A1683.17-
223Th223.020811(10)0.60(2) sa to 219Ra(5/2)+N/A1681.93-
224Th224.021467(12)1.05(2) sa to 220Ra; 2ß+ to 225Ac0+N/A1690.01-
225Th225.023951(5)8.72(4) mina to 221Ra(3/2)+N/A1698.09-
226Th226.024903(5)30.57(10) mina to 222Ra0+N/A1706.17-
227Th227.0277041(27)18.68(9) da to 223Ra1/2+N/A1714.25-
228Th228.0287411(24)1.9116(16) ya to 224Ra; 20O0+N/A1722.33-
229Th229.031762(3)7.34(16)E+3 ya to 225Ra5/2+0.461721.09-
230Th230.0331338(19)7.538(30)E+4 ya to 226Ra; SF 0+N/A1729.17-
231Th231.0363043(19)25.52(1) ha to 227Ra; ß- to 231Pa5/2+N/A1737.25-
232Th232.0380553(21)1.405(6)E+10 ya to 228Ra; SF 0+N/A1745.33100
233Th233.0415818(21)21.83(4) minß- to 233Pa1/2+N/A1744.09-
234Th234.043601(4)24.10(3) dß- to 234Pa0+N/A1752.17-
235Th235.04751(5)7.2(1) minß- to 235Pa(1/2+)#N/A1760.25-
236Th236.04987(21)#37.5(2) minß- to 236Pa0+N/A1768.32-
237Th237.05389(39)#4.8(5) minß- to 237Pa5/2+#N/A1767.09-
238Th238.0565(3)#9.4(20) minß- to 238Pa0+N/A1775.16-


Atomic Number:90Atomic Radius:237 pm (Van der Waals)
Atomic Symbol:ThMelting Point:1750 °C
Atomic Weight:232Boiling Point:4788 °C
Electron Configuration:[Rn]7s26d2Oxidation States:4, 3, 2 , 1


Morten Esmark found a black mineral on Løvøya island, Norway and gave a sample to his father Jens Esmark, a noted mineralogist. The elder Esmark was not able to identify it and sent a sample to Swedish chemist Jöns Jakob Berzelius for examination in 1828. In 1829 Berzelius determined that it contained a new element, which he named thorium after Thor, the Norse god of thunder. The metal had no practical uses until 1885 when Carl Auer von Welsbach invented the gas mantle. Thorium was first observed to be radioactive in 1898, independently, by Polish-French physicist Marie Curie and German chemist Gerhard Carl Schmidt. Between 1900 and 1903, Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half-life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity. The crystal bar process (or 'iodide process') was discovered by Anton Eduard van Arkel and Jan Hendrik de Boer in 1925 to produce high-purity metallic thorium. Because of health concerns, the thorium in classic lantern mantles has been replaced by rare-earth elements that also produce intense light without the radioactivity.


Twenty-seven thorium radioisotopes have been characterized, with a range in atomic weight from 210 to 236. All are unstable with the most stable being 232Th with a half-life of 14.05 billion years. Thorium-232 represents all but a trace of naturally occurring thorium. It is an alpha emitter and goes through six alpha and four beta decay steps before becoming the stable isotope 208Pb. 232Th is sufficiently radioactive to expose a photographic plate in a few hours. Other isotopes of thorium are short-lived intermediates in the decay chains of higher elements, and only found in trace amounts. The longer-lived of these trace isotopes include: 230Th with a half-life of 75,380 years which is a daughter product of 238U decay; 229Th with a half-life of 7340 years and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than thirty days and the majority of these have half-lives less than ten minutes. Much of the internal heat the earth produces has been attributed to thorium and uranium.


Thorium-232 is a primordial nuclide, having existed in its current form for over 4.5 billion years, a half-life is comparable to the age of the Universe and thus predating the formation of the Earth. Thorium was forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovas. Thorium is found in small amounts in most rocks and soils. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals including thorite (ThSiO4), thorianite (ThO2 + UO2) and monazite. Thorianite is a rare mineral and may contain up to about 12% thorium oxide. Monazite contains 2.5% thorium, allanite has 0.1 to 2% thorium and zircon can have up to 0.4% thorium.[66] Thorium-containing minerals occur on all continents. Thorium is now thought to be about three times as abundant as uranium and about as abundant as lead or molybdenum. Thorium is recovered commercially from the mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals.


When pure, thorium is a silvery-white metal that is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with the oxide. The purest specimens often contain several tenths of a percent of the oxide. High-purity thorium has been made. Pure thorium is soft, very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at 1400°C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point of 3300°C, which is 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 handled carefully. When heated in air, thorium turnings ignite and burn brilliantly with a white light.


The principal historic use of thorium has been in the preparation of the Welsbach mantle, used for portable gaslights. These mantles, consisting of thorium oxide with about 1% cerium oxide and other ingredients, glow with a dazzling light when heated in a gas flame. Thorium is an important alloying element in magnesium, imparting high strength and creep resistance at elevated temperatures. Because thorium has a low work-function and high electron emission, it is used to coat tungsten wire used in electronic equipment. The oxide is also used to control the grain size of tungsten used for electric lamps; it is also used for high-temperature laboratory crucibles. Glasses containing thorium oxide have a high refractive index and low dispersion. Consequently, they find application in high quality lenses for cameras and scientific instruments. Thorium oxide has also found use as a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid. Thorium metal is a source of nuclear power. There is probably more energy available for use from thorium in the minerals of the earth's crust than from both uranium and fossil fuels. Any sizable demand from thorium as a nuclear fuel is still several years in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, they are not expected to become important commercially for many years because of certain operating difficulties.


Thorium Element Atomic Number

Several methods are available for producing thorium metal; it can be obtained by reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal.


Powdered thorium metal is pyrophoric and will often ignite spontaneously in air. Thorium disintegrates with the production of 'thoron' (220Rn), which is an alpha emitter and presents a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential. Secondary decay products of thorium include radium and actinium. Because of this, there are concerns about the safety of thorium mantles and some nuclear safety agencies state precautions about their use.

Further reading

  • Mathias S. Wickleder, Blandine Fourest, and Peter K. Dorhout, (2006) Thorium, Chapter 3 in The Chemistry of the Actinide and Transactinide Elements, Third Edition, L. R. Morss, J. Fuger, and N. M. Edelstein, Eds., Springer Publishers.

Thorium Atomic Number And Mass

Updated by Dr. David Hobart July 2013.