Main Page | See live article | Alphabetical index


Name, Symbol, NumberThorium, Th, 90
Chemical series Transition metals
Group, Period, Block_ , 7 , f
Density, Hardness 11724 kg/m3, 3.0
Appearance silvery white
Atomic Properties
Atomic weight 232.0381 amu
Atomic radius (calc.) 180 (n/a) pm
Covalent radius n/a pm
van der Waals radius n/a pm
Electron configuration [Rn]6d6d27s2
e- 's per energy level 2, 8,18,32,18,10, 2
Oxidation states (Oxide) 4 (weak base)
Crystal structure Cubic face centered
Physical Properties
State of matter solid (__)
Melting point 2028 K (3191 °F)
Boiling point 5061 K (8650 °F)
Molar volume 19.80 ×1010-3 m3/mol
Heat of vaporization 514.4 kJ/mol
Heat of fusion 16.1 kJ/mol
Vapor pressure n/a Pa at 2028 K
Velocity of sound 2490 m/s at 293.15 K
Electronegativity 1.3 (Pauling scale)
Specific heat capacity 120 J/(kg*K)
Electrical conductivity 6.53 106/m ohm
Thermal conductivity 54 W/(m*K)
1st ionization potential 587 kJ/mol
2nd ionization potential 1110 kJ/mol
3rd ionization potential 1930 kJ/mol
4th ionization potential 2780 kJ/mol
Most Stable Isotopes
isoNAhalf-life DMDE MeVDP
228Th{syn.}1.9116 years α5.520224Ra
229Th{syn.}7340 years α5.168225Ra
230Th{syn.}75380 years α4.770226Ra
232Th1001.405 E10 yearsα4.083228Ra
SI units & STP are used except where noted.
Thorium is a chemical element in the periodic table that has the symbol Th and atomic number 90.

Table of contents
1 Notable Characteristics
2 Applications
3 History
4 Biological Role
5 Occurrence
6 Compounds
7 Isotopes
8 Precautions
9 External Links

Notable Characteristics

Thorium is a naturally occurring, slightly radioactive metal. When pure, thorium is a silvery white metal that retains its lustre for several months. However, when it is contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and eventually black. Thorium oxide (ThO2), also called thoria, has one of the highest boiling points of all oxides (3300°C). When heated in air, thorium metal turnings ignite and burn brilliantly with a white light.


Applications of thorium:


Thorium was discovered in 1828 by the Swedish chemist
Jöns Jacob Berzelius, who named it after Thor, the Norse god of war. The metal had virtually no uses until the invention of the lantern mantle in 1885.

Biological Role

This element has no known biological role.


Thorium is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium, and is about as common as lead. Soil commonly contains an average of around 6 parts per million (ppm) of thorium. Thorium occurs in several minerals, the most common being the rare earth-thorium-phosphate mineral, monazite, which contains up to about 12% thorium oxide. There are substantial deposits in several countries. Thorium-232 decays very slowly (its half-life is about three times the age of the earth) but other thorium isotopes occur in its and in uranium decay chains. Most of these are short-lived and hence much more radioactive than Th-232, though on a mass basis they are negligible.

Thorium as a nuclear fuel

Thorium, as well as uranium, can be used as fuel in a nuclear reactor. Although not fissile itself, thorium-232 (Th-232) will absorb slow neutrons to produce uranium-233 (U-233), which is fissile. Hence, like uranium-238 (U-238), it is fertile.

In one significant respect U-233 is better than uranium-235 and plutonium-239, because of its higher neutron yield per neutron absorbed. Given a start with some other fissile material (U-235 or Pu-239), a breeding cycle similar to but more efficient than that with U-238 and plutonium (in slow-neutron reactors) can be set up. The Th-232 absorbs a neutron to become Th-233 which normally decays to protactinium-233 and then U-233. The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle.

Problems include the high cost of fuel fabrication due partly to the high radioactivity of U-233 which is always contaminated with traces of U-232; the similar problems in recycling thorium due to highly radioactive Th-228, some weapons proliferation risk of U-233; and the technical problems (not yet satisfactorily solved) in reprocessing. Much development work is still required before the thorium fuel cycle can be commercialised, and the effort required seems unlikely while (or where) abundant uranium is available.

Nevertheless, the thorium fuel cycle, with its potential for breeding fuel without the need for fast neutron reactors, holds considerable potential long-term. Thorium is significantly more abundant than uranium, so it is a key factor in the sustainability of nuclear energy.

India has particularly large reserves of thorium, and so have planned their nuclear power program to eventually use it exclusively, phasing out uranium as an input material. This ambitious plan uses both fast and thermal breeder reactors.



Naturally occurring thorium is composed of 1 isotope: 232-Th. 25 radioisotopes have been characterized with the most {abundant and/or stable} being 232-Th with a half-life of 14.05 billion years, 230-Th with a half-life of 75,380 years, 229-Th with a half-life of 7340 years, and 228-Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lifes that are less than 30 days and the majority of these have half lifes that are less than 10 minutes. This element also has 1 meta state.

The isotopes of thorium range in atomic weight from 212 amu (212-Th) to 236 amu (236-Th).


Powdered thorium metal is often pyrophoric and should be handled carefully. Thorium disintegrates with the eventual production of "thoron", an isotope of radon (220-Rn). Radon gas is a radiation hazard. Good ventilation of areas where thorium is stored or handled is therefore essential.

Exposure to thorium in the air can lead to increased risk of cancers of the lung, pancreas and blood. Exposure to thorium internally leads to increased risk of liver diseases.

External Links

See: Periodic table, nuclear reactor