Thorium (Th)

Isotopes of Thorium

Isotope Atomic Mass Half-life Mode of Decay Nuclear Spin Nuclear Magnetic Moment
Th-227  227.027699  18.72 days  α to Ra-223 3/2  No data available 
Th-228  228.028731  1.913 years  α to Ra-224;
No data available 
Th-229  229.031754  7900 years  α to Ra-225 5/2  0.46
Th-230  230.033126  75,400 years  α to Ra-226; SF No data available 
Th-231  231.036296  1.063 days  α to Ra227;
β- to Pa-231
5/2  No data available 
Th-232  232.0380508  1.40 x 1010 years  α to Ra-228; SF No data available 
Th-233  233.041576  22.30 minutes  β- to Pa-233 1/2  No data available 
Th-234  234.036596  24.10 days  β- to Pa-234 No data available 


Thorium was discovered in 1828 by the Norwegian mineralogist Morten Thrane Esmark and identified in 1829 by the Swedish chemist Jöns Jakob Berzelius. It is named after Thor, the mythological Scandinavian god of war.

Thorium is a grayish-white lustrous metal that is soft when pure and is quite ductile and malleable. It can be shaped by cold or hot rolling, swaging or drawing. It is dimorphic with face-centered cubic crystals, changing to a body-centered cubic structure at 1400 ºC. It is soluble in hydrochloric and sulfuric acids and aqua regia, slightly soluble in nitric acid, and insoluble in water. Thorium combines with practically all nonmetallic elements except the noble gases, forming binary compounds. It combines with nitrogen at elevated temperatures to form ThN and Th2N3. Thorium reacts with all halogens, forming tetrahalides. At elevated temperatures, thorium also forms inter-metallic compounds with iron, copper, aluminum, selenium, nickel, cobalt, manganese, bismuth and many other metals.

The principal use of thorium is as a nuclear fuel. Another major application is the Welsbach incandescent mantle used in portable gaslights. Alloyed with magnesium, thorium imparts high strength and creep resistance to the magnesium at elevated temperatures. Such alloys are used in vehicles and aerospace equipment. Thorium oxide-coated tungsten filaments are used in incandescent lamps, and rods are employed as electrodes in arc-melting. Other uses are in photoelectric cells, as a target in x-ray tubes, and as a reducing agent in metallurgy.

All thorium isotopes are radioactive. All of its intermediate decay products, including Radon-220, are radioactive and present radiation hazard. Exposure can cause cancer.

Properties of Thorium

Name Thorium 
Symbol Th 
Atomic number 90 
Atomic weight 232.03806 
Standard state Solid at 298 ºK 
CAS Registry ID 7440-29-1 
Group in periodic table N/A 
Group name Actinoid 
Period in periodic table 7 (Actinoid) 
Block in periodic table f-block 
Color Silvery white 
Classification Metallic 
Melting point 1750 °C
Boiling point 4820 °C
Vaporization point 4788 °C
Thermal conductivity 54 W/(m·K)
Electrical resistivity 15 x 10-8 Ω·m 
Electronegativity 1.3 
Specific heat 0.13 kJ/kg K
Heat of vaporization 530 kJ·mol-1
Heat of fusion 16 kJ·mol-1
Density of solid 11.72 g/cm3 
Electron configuration [Rn]6d27s2 
Atomic radius 1.80 Å 
Ionic radius Th4+: 1.05 Å (coordination number 8)
Most stable oxidation state  +4 

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