Thorium, 90Th
Small (3 cm) ampule with a tiny (5 mm) square of metal in it
Pronunciationm/ (THOR-ee-əm)
Appearancesilvery, often with black tarnish
Standard atomic weight Ar, std(Th)232.0377(4)[1]
Thorium in the periodic table
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon


Atomic number (Z)90
Groupgroup n/a
Periodperiod 7
Element category  Actinide
Electron configuration[Rn] 6d2 7s2
Electrons per shell2, 8, 18, 32, 18, 10, 2
Physical properties
Phase at STPsolid
Melting point2023 K ​(1750 °C, ​3182 °F)
Boiling point5061 K ​(4788 °C, ​8650 °F)
Density (near r.t.)11.7 g/cm3
Heat of fusion13.81 kJ/mol
Heat of vaporisation514 kJ/mol
Molar heat capacity26.230 J/(mol·K)
Vapour pressure
P (Pa)1101001 k10 k100 k
at T (K)263329073248368342595055
Atomic properties
Oxidation states+1, +2, +3, +4 (a weakly basic oxide)
ElectronegativityPauling scale: 1.3
Ionisation energies
  • 1st: 587 kJ/mol
  • 2nd: 1110 kJ/mol
  • 3rd: 1930 kJ/mol
Atomic radiusempirical: 179.8 pm
Covalent radius206±6 pm
Color lines in a spectral range
Spectral lines of thorium
Other properties
Natural occurrenceprimordial
Crystal structureface-centred cubic (fcc)
Facecentredcubic crystal structure for thorium
Speed of sound thin rod2490 m/s (at 20 °C)
Thermal expansion11.0 µm/(m·K) (at 25 °C)
Thermal conductivity54.0 W/(m·K)
Electrical resistivity157 nΩ·m (at 0 °C)
Magnetic orderingparamagnetic[2]
Magnetic susceptibility132.0·10−6 cm3/mol (293 K)[3]
Young's modulus79 GPa
Shear modulus31 GPa
Bulk modulus54 GPa
Poisson ratio0.27
Mohs hardness3.0
Vickers hardness295–685 MPa
Brinell hardness390–1500 MPa
CAS Number7440-29-1
Namingafter Thor, the Norse god of thunder
DiscoveryJöns Jakob Berzelius (1829)
Main isotopes of thorium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
227Thtrace18.68 dα223Ra
228Thtrace1.9116 yα224Ra
229Thtrace7917 yα225Ra
230Th0.02%75400 yα226Ra
231Thtrace25.5 hβ231Pa
232Th99.98%1.405×1010 yα228Ra
234Thtrace24.1 dβ234Pa
| references

Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state; it is quite reactive and can ignite in air when finely divided.

All known thorium isotopes are unstable. The most stable isotope, 232Th, has a half-life of 14.05 billion years, or about the age of the universe; it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable 208Pb. On Earth, thorium, bismuth, and uranium are the only three radioactive elements that still occur naturally in large quantities as primordial elements.[a] It is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.

Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. Its first applications were developed in the late 19th century. Thorium's radioactivity was widely acknowledged during the first decades of the 20th century. In the second half of the century, thorium was replaced in many uses due to concerns about its radioactivity.

Thorium is still being used as an alloying element in TIG welding electrodes but is slowly being replaced in the field with different compositions. It was also material in high-end optics and scientific instrumentation, used in some broadcast vacuum tubes, and as the light source in gas mantles, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built.

Bulk properties

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. Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn.[4] At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).[4]

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.[5] Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.[6]

Thorium is nearly half as dense as uranium and plutonium and is harder than either of them.[6] It becomes superconductive below 1.4 K.[4] 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.[6][7] (The f-electron count for thorium is a non-integer due to a 5f–6d overlap.)[7] 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.[b] Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points.[c]

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.[4] 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.[4] 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.[4]

Thorium can form alloys with many other metals. Addition of small proportions of thorium improves the mechanical strength of magnesium, and thorium-aluminum alloys have been considered as a way to store thorium in proposed future 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.[4]