From Wikiversity
Jump to navigation Jump to search
Thorium sample (99.9 % = 3N) is a thin sheet under argon in a glass ampoule, ca. 0.1 g. Credit: Alchemist-hp.
Thorium spectrum is 400 nm - 700 nm. Credit: McZusatz.{{free media}}

Thorium is chemical element number 90 in the periodic table.


This is a modification of the original image to fix the half-life of Pb-212. Credit: BatesIsBack.

The decay chain on the right depicts that chain for thorium.

Thorium nuclei are susceptible to alpha decay because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.[1] The alpha decay of 232Th initiates the 4n decay chain which includes isotopes with a mass number divisible by 4 (hence the name; it is also called the thorium series after its progenitor). This chain of consecutive alpha and beta decays begins with the decay of 232Th to 228Ra and terminates at 208Pb.[2] Any sample of thorium or its compounds contains traces of these daughters, which are isotopes of thallium, lead, bismuth, polonium, radon, radium, and actinium.[2] Natural thorium samples can be chemically purified to extract useful daughter nuclides, such as 212Pb, which is used in nuclear medicine for cancer therapy.[3][4] 232Th also very occasionally undergoes spontaneous fission rather than alpha decay, and has left evidence of doing so in its minerals (as trapped xenon gas formed as a fission product), but the partial half-life of this process is very large at over 1021 years and alpha decay predominates.[5][6]

Theoretical thorium[edit]

Def. "a chemical element (symbol Th) with atomic number 90"[7] is called thorium.


This diagram shows a face-centered cubic lattice. Credit: Daniel Mayer and DrBob.

Thorium is a moderately soft, paramagnetic, bright silvery, radioactive, actinide metal. In the periodic table, it lies to the right of actinium, to the left of protactinium, and below cerium.

At room temperature and pressure, thorium crystallizes into a face-centered cubic lattice, where one thorium atom occupies each location of a black sphere in the diagram on the left.

At high temperature over 1360 °C thorium crystallizes into a body-centred cubic lattice.

At high pressure around 100 GPa thorium crystallizes into a body-centred tetragonal lattice.[8]

Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn.[8]

Thorium metal has a bulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same as tin's (58.2 GPa). Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is 160–169 GPa.[9]

Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.[10]

Thorium is nearly half as dense as uranium and plutonium and is harder than either of them.[10]

It becomes superconductive below 1.4 K.[8]

Thorium's melting point of 1750 °C is above both those of actinium (1227 °C) and protactinium (1568 °C). At the start of period 7, from francium to thorium, the melting points of the elements increase (as in other periods), because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding.[10][11] (The f-electron count for thorium is a non-integer due to a 5f–6d overlap.)[11] Among the actinides up to californium, which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter.

While einsteinium has been measured to have a lower density, this measurement was done on small, microgram-mass samples, and is likely because of the rapid self-destruction of the crystal structure caused by einsteinium's extreme radioactivity.[12]

Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points, behind osmium, tantalum, tungsten, and rhenium;[8] higher boiling points are speculated to be found in the 6d transition metals, but they have not been produced in large enough quantities to test this prediction.[13]}}


The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually thorium dioxide (ThO2); even the purest thorium specimens usually contain about a tenth of a percent of the dioxide.[8] Experimental measurements of its density give values between 11.5 and 11.66 g/cm3: these are slightly lower than the theoretically expected value of 11.7 g/cm3 calculated from thorium's lattice parameters, perhaps due to microscopic voids forming in the metal when it is cast.[8] These values lie between those of its neighbours actinium (10.1 g/cm3) and protactinium (15.4 g/cm3), part of a trend across the early actinides.[8]


Thorium can form alloys with many other metals. Addition of small proportions of thorium improves the mechanical strength of magnesium, and thorium-aluminium alloys have been considered as a way to store thorium in thorium nuclear reactors. Thorium forms eutectic mixtures with chromium and uranium, and it is completely miscible in both solid and liquid states with its lighter congener cerium.[8]


Four-fifths of the thorium present at Earth's formation has survived to the present.[2][14][15] 232Th is the only isotope of thorium occurring in quantity in nature.[2] Its stability is attributed to its closed nuclear shell with 142 neutrons.[16][17] Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377(4). It is one of only three radioactive elements (along with protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.[18]

Z(p) N(n)  
isotopic mass (u)
half-life[n 1] decay
mode(s)[19][n 2]
isotope(s)[n 3]
(mole fraction)
range of natural
(mole fraction)
excitation energy
209Th 90 119 209.01772(11) 7(5) ms
210Th 90 120 210.015075(27) 17(11) ms
[9(+17-4) ms]
α 206Ra 0+
β+ (rare) 210Ac
211Th 90 121 211.01493(8) 48(20) ms
[0.04(+3-1) s]
α 207Ra 5/2-#
β+ (rare) 211Ac
212Th 90 122 212.01298(2) 36(15) ms
[30(+20-10) ms]
α (99.7%) 208Ra 0+
β+ (.3%) 212Ac
213Th 90 123 213.01301(8) 140(25) ms α 209Ra 5/2-#
β+ (rare) 213Ac
214Th 90 124 214.011500(18) 100(25) ms α 210Ra 0+
215Th 90 125 215.011730(29) 1.2(2) s α 211Ra (1/2-)
216Th 90 126 216.011062(14) 26.8(3) ms α (99.99%) 212Ra 0+
β+ (.006%) 216Ac
216m1Th 2042(13) keV 137(4) µs (8+)
216m2Th 2637(20) keV 615(55) ns (11-)
217Th 90 127 217.013114(22) 240(5) µs α 213Ra (9/2+)
218Th 90 128 218.013284(14) 109(13) ns α 214Ra 0+
219Th 90 129 219.01554(5) 1.05(3) µs α 215Ra 9/2+#
β+ (10−7%) 219Ac
220Th 90 130 220.015748(24) 9.7(6) µs α 216Ra 0+
EC (2×10−7%) 220Ac
221Th 90 131 221.018184(10) 1.73(3) ms α 217Ra (7/2+)
222Th 90 132 222.018468(13) 2.237(13) ms α 218Ra 0+
EC (1.3×10−8%) 222Ac
223Th 90 133 223.020811(10) 0.60(2) s α 219Ra (5/2)+
224Th 90 134 224.021467(12) 1.05(2) s α 220Ra 0+
β+β+ (rare) 224Ra
225Th 90 135 225.023951(5) 8.72(4) min α (90%) 221Ra (3/2)+
EC (10%) 225Ac
226Th 90 136 226.024903(5) 30.57(10) min α 222Ra 0+
227Th Radioactinium 90 137 227.0277041(27) 18.68(9) d α 223Ra 1/2+ Trace[n 4]
228Th Radiothorium 90 138 228.0287411(24) 1.9116(16) a α 224Ra 0+ Trace[n 5]
CD (1.3×10−11%) 208Pb
229Th 90 139 229.031762(3) 7.34(16)×103 a α 225Ra 5/2+
229mTh 0.0076(5) keV 70(50) h IT 229Th 3/2+
230Th[n 6] Ionium 90 140 230.0331338(19) 7.538(30)×104 a α 226Ra 0+ Trace[n 7]
CD (5.6×10−11%) 206Hg
SF (5×10−11%) (Various)
231Th Uranium Y 90 141 231.0363043(19) 25.52(1) h β 231Pa 5/2+ Trace[n 4]
α (10−8%) 227Ra
232Th[n 8] Thorium 90 142 232.0380553(21) 1.405(6)×1010 a α 228Ra 0+ 1.0000
ββ (rare) 232U
SF (1.1×10−9%) (various)
CD (2.78×10−10%) 182Yb
233Th 90 143 233.0415818(21) 21.83(4) min β 233Pa 1/2+
234Th Uranium X1 90 144 234.043601(4) 24.10(3) d β 234mPa 0+ Trace[n 7]
235Th 90 145 235.04751(5) 7.2(1) min β 235Pa (1/2+)#
236Th 90 146 236.04987(21)# 37.5(2) min β 236Pa 0+
237Th 90 147 237.05389(39)# 4.8(5) min β 237Pa 5/2+#
238Th 90 148 238.0565(3)# 9.4(20) min β 238Pa 0+
  1. Bold for nuclides with half-lives longer than the age of the universe (nearly stable)
  2. Abbreviations:
    CD: Cluster decay
    EC: Electron capture
    IT: Isomeric transition
    SF: Spontaneous fission
  3. Bold for stable isotopes
  4. 4.0 4.1 Intermediate decay product of 235U
  5. Intermediate decay product of 232Th
  6. Used in Uranium-thorium dating
  7. 7.0 7.1 Intermediate decay product of 238U
  8. Primordial radionuclide


  1. Thorium can be fissioned and fusioned.

See also[edit]


  1. Beiser, A. (2003). Nuclear Transformations, In: Concepts of Modern Physics (PDF) (6 ed.). McGraw-Hill Education. pp. 432–434. ISBN 978-0-07-244848-1.
  2. 2.0 2.1 2.2 2.3 Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. (2003). "The NUBASE evaluation of nuclear and decay properties". Nuclear Physics A 729: 3–128. doi:10.1016/j.nuclphysa.2003.11.001. 
  3. "AREVA Med launches production of lead-212 at new facility". Areva. 2013. Retrieved 1 January 2017.
  4. Mineral Yearbook 2012. United States Geological Survey. Retrieved 30 September 2017. 
  5. Wickleder, Fourest & Dorhout 2006, pp. 53–55.
  6. Bonetti, R.; Chiesa, C.; Guglielmetti, A.; Matheoud, R.; Poli, G.; Mikheev, V. L.; Tretyakova, S. P. (1995). "First observation of spontaneous fission and search for cluster decay of 232Th". Physical Review C 51 (5): 2530. doi:10.1103/PhysRevC.51.2530. 
  7. Emperorbma (9 July 2015). "thorium, In: Wiktionary". San Francisco, California: Wikimedia Foundation, Inc. Retrieved 2015-07-16.
  8. 8.0 8.1 8.2 8.3 8.4 8.5 8.6 8.7 Wickleder, Fourest & Dorhout 2006, pp. 61–63.
  9. Gale, W. F.; Totemeier, T. C. (2003). Smithells Metals Reference Book. Butterworth-Heinemann. pp. 15–2–15–3. ISBN 978-0-08-048096-1.
  10. 10.0 10.1 10.2 Tretyakov, Yu. D., ed. (2007). Non-organic chemistry in three volumes. Chemistry of transition elements. 3. Academy. ISBN 978-5-7695-2533-9.
  11. 11.0 11.1 Johansson, B.; Abuja, R.; Eriksson, O.; Wills, J. M. (1995). "Anomalous fcc crystal structure of thorium metal.". Physical Review Letters 75 (2). doi:10.1103/PhysRevLett.75.280. 
  12. Haire, R. G.; Baybarz, R. D. (1979). "Studies of einsteinium metal". Le Journal de Physique 40: C4–101. doi:10.1051/jphyscol:1979431. 
  13. Fricke, Burkhard (1975). "Superheavy elements: a prediction of their chemical and physical properties". Recent Impact of Physics on Inorganic Chemistry. Structure and Bonding 21: 89–144. doi:10.1007/BFb0116498. ISBN 978-3-540-07109-9. Retrieved 4 October 2013. 
  14. De Laeter, J. R.; Böhlke, J. K.; De Bièvre, P.; Hidaka, H.; Peiser, H. S.; Rosman, K. J. R.; Taylor, P. D. P. (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry 75 (6): 683–800. doi:10.1351/pac200375060683. 
  15. International Union of Pure and Applied Chemistry (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry 78 (11): 2051–2066. doi:10.1351/pac200678112051. Retrieved 27 July 2017. 
  16. Nagy, S. (2009). Radiochemistry and Nuclear Chemistry. 2. EOLSS Publications. p. 374. ISBN 978-1-84826-127-3.
  17. Griffin, H. C. (2010). A. Vértes, S. Nagy, Z. Klencsár, R. G. Lovas, F. Rösch, ed. Natural Radioactive Decay Chains, In: Handbook of Nuclear Chemistry. Springer Science+Business Media. p. 668. ISBN 978-1-4419-0719-6.CS1 maint: Multiple names: editors list (link)
  18. Meija, J.; Coplen, T.B.; Berglund, M.; Brand, W.A.; De Bièvre, P.; Gröning, M.; Holden, N.E.; Irrgeher, J. et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry 88 (3): 265-91. doi:10.1515/pac-2015-0305. ISSN 0033-4545. 

External links[edit]