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(Thor, Scandinavian god of war) Discovered by Berzelius in 1828. Much of the internal
heat the earth produces has been attributed to thorium and uranium. Because of its atomic
weight, valence, etc., it is now considered to be the second member of the actinide series
Thorium occurs in thorite and in thorianite. Large deposits of thorium minerals have
been reported in New England and elsewhere, but these have not yet been exploited. Thorium
is now thought to be about three times as abundant as uranium and about as abundant as
lead or molybdenum. Thorium is recovered commercially from the mineral monazite, which
contains from 3 to 9% ThO2
along with rare-earth minerals.
The metal is a source of nuclear power. There is probably more energy available for use
from thorium in the minerals of the earth's crust than from both uranium and fossil fuels.
Any sizable demand from thorium as a nuclear fuel is still several years in the future.
Work has been done in developing thorium cycle converter-reactor systems. Several
prototypes, including the HTGR (high-temperature gas-cooled reactor) and MSRE (molten salt
converter reactor experiment), have operated. While the HTGR reactors are efficient, they
are not expected to become important commercially for many years because of certain
Several methods are available for producing thorium metal; it can be obtained by
reducing thorium oxide with calcium, by electrolysis of anhydrous thorium chloride in a
fused mixture of sodium and potassium chlorides, by calcium reduction of thorium
tetrachloride mixed with anhydrous zinc chloride, and by reduction of thorium
tetrachloride with an alkali metal. Thorium was originally assigned a position in Group IV
of the periodic table.
When pure, thorium is a silvery-white metal which is air-stable and retains its luster
for several months. When contaminated with the oxide, thorium slowly tarnishes in air,
becoming gray and finally black. The physical properties of thorium are greatly influenced
by the degree of contamination with the oxide. The purest specimens often contain several
tenths of a percent of the oxide. High-purity thorium has been made. Pure thorium is soft,
very ductile, and can be cold-rolled, swaged, and drawn. Thorium is dimorphic, changing at
1400C from a cubic to a body-centered cubic structure. Thorium oxide has a melting point
of 3300C, which is 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 handled carefully.
When heated in air, thorium turnings ignite and burn brilliantly with a white light.
The principal use of thorium has been in the preparation of the Welsbach mantle, used
for portable gas lights. These mantles, consisting of thorium oxide with about 1% cerium
oxide and other ingredients, glow with a dazzling light when heated in a gas flame.
Thorium is an important alloying element in magnesium, imparting high strength and creep
resistance at elevated temperatures. Because thorium has a low work-function and high
electron emission, it is used to coat tungsten wire used in electronic equipment. The
oxide is also used to control the grain size of tungsten used for electric lamps; it is
also used for high-temperature laboratory crucibles. Glasses containing thorium oxide have
a high refractive index and low dispersion. Consequently, they find application in high
quality lenses for cameras and scientific instruments. Thorium oxide has also found use as
a catalyst in the conversion of ammonia to nitric acid, in petroleum cracking, and in
producing sulfuric acid.
Twenty five isotopes of thorium are known with atomic masses ranging from 212 to 236.
All are unstable. 232Th occurs naturally and has a half-life of 1.4 x 1010 years. It is an alpha emitter. 232Th goes
through six alpha and four beta decay steps before becoming the stable isotope 208Pb.
232Th is sufficiently radioactive to expose a photographic plate in a few hours. Thorium
disintegrates with the production of "thoron" (220Rn), which is an alpha emitter
and presents a radiation hazard. Good ventilation of areas where thorium is stored or
handled is therefore essential.
Thorium metal (99.9%) costs about $150/oz.
Sources: CRC Handbook of Chemistry
and Physics and the American Chemical Society.
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