Atomic Number: 90
Group: None
Atomic Weight: 232.0381
Period: 7
CAS Number: 7440-29-1


Noble Gas
Rare Earth Element
Platinum Group Metal
No Stable Isotopes
Solid (Predicted)

Description • Uses/Function

Discovered by Berzelius in 1828. Thorium occurs in thorite (ThSiO4) and in thorianite (ThO2 + UO2). Large deposits of thorium minerals havebeen reported in New England and elsewhere, but these have not yet been exploited. Thorium is now thought to be about three times as abundant asuranium and about as abundant as lead or molybdenum. The metal is a source of nuclear power. There is probably more energy available for use fromthorium in the minerals of the earth’s crust than from both uranium and fossil fuels. Any sizable demand for thorium as a nuclear fuel is still severalyears in the future. Work has been done in developing thorium cycle converter-reactor systems. Several prototypes, including the HTGR (hightemperaturegas-cooled reactor) and MSRE (molten salt converter reactor experiment), have operated. While the HTGR reactors are efficient, theyare not expected to become important commercially for many years because of certain operating difficulties. Thorium is recovered commercially fromthe mineral monazite, which contains from 3 to 9% ThO2 along with rare-earth minerals. Much of the internal heat the earth produces has been attributedto thorium and uranium. Several methods are available for producing thorium metal: it can be obtained by reducing thorium oxide with calcium, byelectrolysis of anhydrous thorium chloride in a fused mixture of sodium and potassium chlorides, by calcium reduction of thorium tetrachloride mixedwith anhydrous zinc chloride, and by reduction of thorium tetrachloride with an alkali metal. Thorium was originally assigned a position in Group IVof the periodic table. Because of its atomic weight, valence, etc., it is now considered to be the second member of the actinide series of elements. Whenpure, thorium is a silvery-white metal which is air-stable and retains its luster for several months. When contaminated with the oxide, thorium slowlytarnishes in air, becoming gray and finally black. The physical properties of thorium are greatly influenced by the degree of contamination with theoxide. 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 oxidehas 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. Powderedthorium metal is often pyrophoric and should be carefully handled. When heated in air, thorium turnings ignite and burn brilliantly with a white light.The principal use of thorium has been in the preparation of the Welsbach mantle, used for portable gas lights. These mantles, consisting of thoriumoxide with about 1% cerium oxide and other ingredients, glow with a dazzling light when heated in a gas flame. Thorium is an important alloyingelement in magnesium, imparting high strength and creep resistance at elevated temperatures. Because thorium has a low work-function and highelectron 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 forelectric lamps; it is also used for high-temperature laboratory crucibles. Glasses containing thorium oxide have a high refractive index and lowdispersion. Consequently, they find application in high quality lenses for cameras and scientific instruments. Thorium oxide has also found use as acatalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in producing sulfuric acid. Twenty seven isotopes of thorium are knownwith atomic masses ranging from 212 to 237. All are unstable. Thorium-232 occurs naturally and has a half-life of 1.4 X 10^10 years. It is an alpha emitter. Thorium-232goes through six alpha and four beta decay steps before becoming the stable isotope lead-208. thorium-232 is sufficiently radioactive to expose a photographicplate in a few hours. Thorium disintegrates with the production of “thoron” (radon-220), which is an alpha emitter and presents a radiation hazard. Goodventilation of areas where thorium is stored or handled is therefore essential. Thorium metal (99.8%) costs about $15/g. 1

Physical Properties

Melting Point:2*  1750 °C = 2023.15 K = 3182 °F
Boiling Point:2* 4788 °C = 5061.15 K = 8650.4 °F
Sublimation Point:2 
Triple Point:2 
Critical Point:2 
Density:3  11.7 g/cm3

* - at 1 atm

Electron Configuration

Electron Configuration:  *[Rn] 7s2 5f2
Block: f
Highest Occupied Energy Level: 7
Valence Electrons: 2

Quantum Numbers:

n = 5
ℓ = 3
m = -2
ms = +½


Electronegativity (Pauling scale):4 1.3
Electropositivity (Pauling scale): 2.7
Work Function:5 3.71 eV = 5.94342E-19 J

Ionization Potential   eV 6  kJ/mol  
1 6.3067    608.5
Ionization Potential   eV 6  kJ/mol  
2 11.5    1109.6
Ionization Potential   eV 6  kJ/mol  
3 20    1929.7
4 28.8    2778.8


Specific Heat: 0.113 J/g°C 7 = 26.220 J/mol°C = 0.027 cal/g°C = 6.267 cal/mol°C
Thermal Conductivity: 54 (W/m)/K, 27°C 8
Heat of Fusion: 16.1 kJ/mol 9 = 69.4 J/g
Heat of Vaporization: 514.4 kJ/mol 10 = 2216.9 J/g
State of Matter Enthalpy of Formation (ΔHf°)11 Entropy (S°)11 Gibbs Free Energy (ΔGf°)11
(kcal/mol) (kJ/mol) (cal/K) (J/K) (kcal/mol) (kJ/mol)
(s) 0 0 13.6 56.9024 0 0


Nuclide Mass 12 Half-Life 12 Nuclear Spin 12 Binding Energy
209Th 209.01772(11) 7(5) ms [3.8(+69-15)] 5/2-# 1,607.18 MeV
210Th 210.015075(27) 17(11) ms [9(+17-4) ms] 0+ 1,615.25 MeV
211Th 211.01493(8) 48(20) ms [0.04(+3-1) s] 5/2-# 1,623.32 MeV
212Th 212.01298(2) 36(15) ms [30(+20-10) ms] 0+ 1,631.39 MeV
213Th 213.01301(8) 140(25) ms 5/2-# 1,639.46 MeV
214Th 214.011500(18) 100(25) ms 0+ 1,647.53 MeV
215Th 215.011730(29) 1.2(2) s (1/2-) 1,655.60 MeV
216Th 216.011062(14) 26.8(3) ms 0+ 1,663.68 MeV
217Th 217.013114(22) 240(5) μs (9/2+) 1,671.75 MeV
218Th 218.013284(14) 109(13) ns 0+ 1,679.82 MeV
219Th 219.01554(5) 1.05(3) μs 9/2+# 1,687.89 MeV
220Th 220.015748(24) 9.7(6) μs 0+ 1,695.96 MeV
221Th 221.018184(10) 1.73(3) ms (7/2+) 1,704.03 MeV
222Th 222.018468(13) 2.237(13) ms 0+ 1,712.10 MeV
223Th 223.020811(10) 0.60(2) s (5/2)+ 1,710.86 MeV
224Th 224.021467(12) 1.05(2) s 0+ 1,718.93 MeV
225Th 225.023951(5) 8.72(4) min (3/2)+ 1,727.00 MeV
226Th 226.024903(5) 30.57(10) min 0+ 1,735.07 MeV
227Th 227.0277041(27) 18.68(9) d 1/2+ 1,743.15 MeV
228Th 228.0287411(24) 1.9116(16) a 0+ 1,751.22 MeV
229Th 229.031762(3) 7.34(16)E+3 a 5/2+ 1,749.97 MeV
230Th 230.0331338(19) 7.538(30)E+4 a 0+ 1,758.04 MeV
231Th 231.0363043(19) 25.52(1) h 5/2+ 1,766.12 MeV
232Th 232.0380553(21) 1.405(6)E+10 a 0+ 1,774.19 MeV
233Th 233.0415818(21) 21.83(4) min 1/2+ 1,772.94 MeV
234Th 234.043601(4) 24.10(3) d 0+ 1,781.01 MeV
235Th 235.04751(5) 7.2(1) min (1/2+)# 1,789.09 MeV
236Th 236.04987(21)# 37.5(2) min 0+ 1,797.16 MeV
237Th 237.05389(39)# 4.8(5) min 5/2+# 1,795.91 MeV
238Th 238.0565(3)# 9.4(20) min 0+ 1,803.99 MeV
Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses. 12


Earth - Seawater: 0.000001 mg/L 13
Earth -  Crust:  9.6 mg/kg = 0.00096% 13
Earth -  Total:  51.2 ppb 14
Mercury -  Total:  39.4 ppb 14
Venus -  Total:  53.7 ppb 14
Chondrites - Total: 0.027 (relative to 106 atoms of Si) 15


Safety Information

Material Safety Data Sheet - ACI Alloys, Inc.

For More Information

External Links:


(1) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:31-4:32.
(2) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:132.
(3) - Lide, David R. CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2002; p 4:39-4:96.
(4) - Dean, John A. Lange's Handbook of Chemistry, 11th ed.; McGraw-Hill Book Company: New York, NY, 1973; p 4:8-4:149.
(5) - Speight, James. Lange's Handbook of Chemistry, 16th ed.; McGraw-Hill Professional: Boston, MA, 2004; p 1:132.
(6) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 10:178 - 10:180.
(7) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:133.
(8) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:193, 12:219-220.
(9) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:123-6:137.
(10) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:107-6:122.
(11) - Dean, John A. Lange's Handbook of Chemistry, 12th ed.; McGraw-Hill Book Company: New York, NY, 1979; p 9:4-9:94.
(12) - Atomic Mass Data Center. (accessed July 14, 2009).
(13) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 14:17.
(14) - Morgan, John W. and Anders, Edward, Proc. Natl. Acad. Sci. USA 77, 6973-6977 (1980)
(15) - Brownlow, Arthur. Geochemistry; Prentice-Hall, Inc.: Englewood Cliffs, NJ, 1979, pp 15-16.