Periodic Table


Atomic number: 90
Atomic weight: 232.0381
Symbol: Th
Group number: (actinide)
Electronic configuration: [Rn].6d2.7s2


Thorium is a source of nuclear power. There is probably more untapped energy available for use from thorium in the minerals of the earth's crust than from combined uranium and fossil fuel sources. Much of the internal heat the earth has been attributed to thorium and uranium.

When pure, thorium is a silvery white metal which is air-stable and retains its lustre for several months. When contaminated with the oxide, thorium slowly tarnishes in air, becoming grey and finally black. Thorium oxide has a melting point of 3300°C, the highest of all oxides. Only a few elements, such as tungsten, and a few compounds, such as tantalum carbide, have higher melting points.

Thorium is slowly attacked by water, but does not dissolve readily in most common acids, except hydrochloric. Powdered thorium metal is often pyrophoric and should be carefully handled.When heated in air, thorium turnings ignite and burn brilliantly with a white light.

Thorium is a radioactive rare earth metal, which slowly tarnishes to grey. It is, soft and ductile with 2 allotropic forms. When heated in air it turns bright and gives off white light. Thorium is named for Thor, the Scandinavian god of war. It is found in thorite and thorianite in New England (USA) and other sites.

General information

Discoveror: Jons Berzelius
Date discovered: 1829
Discovered at: Sweden
Meaning of name: Named after "Thor", the mythological Scandinavian god of war

Physical data

Standard state: solid at 298 K
Colour: silvery white
Density of solid at ambient temperature/kg m-3: 11724
Molar volume/cm3: 19.80

Radii /pm

Atomic: no data
Covalent (single bond):
Pauling radius for the ion [Th]-: no data

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius223.5no data121.5no data


Both values are quoted on the Pauling scale.

Pauling: 1.3
Allred Rochow: 1.11

Crystal Structure

structure: ccp (cubic close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 2115
boiling point: 5093

Enthalpies /kJ mol-1

fusion: 15.8
vaporization: 529

single bond enthalpies:
Th-F Th-Cl Th-Br Th-I Th-Th
no data no data no data no data 289

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/Th.html