Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is light silver and tarnishes olive gray when it is exposed to air, forming thorium dioxide; it is moderately soft and 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 and uranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as primordial elements. Thorium 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 1828 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. Thorium is also used in strengthening magnesium, coating tungsten wire in electrical equipment, controlling the grain size of tungsten in electric lamps, high-temperature crucibles, and glasses including camera and scientific instrument lenses. Other uses for thorium include heat-resistant ceramics, aircraft engines, and in light bulbs. Ocean science has utilised 231Pa/230Th isotope ratios to understand the ancient ocean.

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Atomic properties

Standard atomic weight232.037 ±4·10-4
Atomic mass232.037 u

Atomic radii

Radius (empirical)180 pm
Covalent radius206 ±6 pm

Atomic shell

Electron configurationRn 6d2 7s2
Ionization energy(1st) 6.3067 eV
(2nd) 12.1 eV
(3rd) 18.32 eV
(4th) 28.648 eV
(5th) 58 eV
Shell model

Physical properties

Density11.72 g·cm−3
Molar volume1.98·10-5 m3·mol−1
Speed of sound2,490 m·s−1 (293.1 K)


Melting point2,023 K
Boiling point5,061 K
Liquid range3,038 K
Transition temperature1.4 K


Melting enthalpy16 kJ·mol-1
Enthalpy of vaporization530 kJ·mol-1
Binding energy598 kJ·mol-1

Heat and conductivity

Thermal conductivity54 W·m-1·K-1
Expansion coefficient1.1·10-5 K-1


Mohs hardness3
Brinell hardness400 NM·m-2
Vickers hardness350 NM·m-2

Elastic properties

Young’s modulus79 GPa
Shear modulus31 GPa
Bulk modulus54 GPa
Poisson’s ratio0.27

Electrical properties

Electrical conductivity6.67·106 S·m-1
Resistance1.5·10-7 Ωm


Magnetic susceptibility1.32·10-4 cm3·mol−1 (293 K)

Chemical properties

Basicityslightly basic
Oxidation state4, 3, 2


Pauling scale1.3
Allred-Rochow scale1.11
Nagle scale1.03

Other properties

Natural occurrenceDecay product
Crystal structureFace-centered cubic
Goldschmidt Classificationlithophile
Superconductorwith transition tempperature (solid body, normal pressure)
Radioactivityslightly radioactive
Price/kg287 USD

Natural abundances

0.4 ppb ≈ 4.02·109 M☉
0.3 ppb ≈ 5.96·108 Mt
40 ppb ≈ 40 mg
Earth’s crust
6,000 ppb ≈ 166,000 Mt
4·10-5 ppb ≈ 54.8 t
Flowing water
0.1 ppb ≈ 1.6 kt