Thorium

Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. It 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.

Properties

Thorium is a naturally occurring, slightly radioactive metal. It is found in small amounts in most rocks and soils, where it is about three times more abundant than uranium. Soil commonly contains an average of around 6 parts per million (ppm) of thorium.

History

Thorium was discovered in 1828 by the Norwegian mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder.

Applications

Thorium is used as an alloying element in nonconsumable TIG welding electrodes, in heat-resistant ceramics, in glass elements for high quality lenses for cameras and scientific instruments, and in the aerospace industry.

Health effects

Exposure to an aerosol of thorium can lead to increased risk of cancers of the lung, pancreas, and blood. Disposal of thorium waste has been a problem, and contamination has caused health problems and deaths in the past.

See also

• Actinide
• Nuclear power
• Radioactive decay
• Uranium

References

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