Thorium is found in the mineral now called thorite. In 1828, Swedish chemist Jöns Jakob Berzelius received a black rock from mineralogist Jens Esmark, a rock found by Jens' son Morten. Berzelius concluded that the mineral rock contained a new element, which he named thorium after the god Thor.
French scientist Henri Becquerel in 1896 observed that uranium salts will darken a photographic plate wrapped in black paper. This was the first discovery of radioactivity. Widening the search, Marie Curie of France and Gerhard Carl Schmidt of Germany each determined that thorium exhibits radioactivity as well.
Occurrence and Production
Thorium is a radioactive element, but one of the most stable. Its half-life of 14 billion years exceeds the age of the earth, and so quantities of thorium, like uranium, from the formation of the planet can still be found in nature. The original quantities of other radioactive elements with shorter half-lives have decayed to negligible amounts. Thorium decays with spontaneous fission into a chain of elements that includes actinium, then radium, bismuth, and finally lead. Therefore, all of these can be found in some amount in any deposit of thorium.
Thorium is found in several minerals, including thorite (thorium silicate). An important source is monazite, which is a mixture of rare earth phosphates that also contains about 6-12% thorium phosphate. When monazite is processed to extract rare earths, the thorium is also isolated.
Properties and Uses
Thorium is a silver metal that is very soft and ductile. Its density is comparable to lead. It is quite reactive, tarnishing quickly in air but reacting slowly in water.
Thorium has an extremely high melting point (1750°C), and its oxide thorium dioxide (ThO2, thoria) melts at an even higher temperature. This is why thorium's first major application starting in 1891 was in gas mantles (right). Besides its tolerance for high temperature, thoria in a mantle gives off more white light than heat radiation compared to most substances. Thorium is still used in gas mantles for portable lamps and, while mildly radioactive, does not pose a significant health hazard in this use.
Thoria is still used in ceramics that must withstand extreme heat. It is also added to glass for use in high-quality lenses. Radiation decay changes such lenses over a long period of time, but the health risk is minimal. Other historical uses of thorium have been discontinued because of radiation concerns, however.
Thorium minerals are a useful tool for dating rocks and geologic deposits. An examination of the radiation decay products in the thorium will give an estimate of when the deposit was laid down. This is one of the best available techniques for dating very old rocks.
Thorium has been considered as a nuclear fuel and some test reactors have been built (reactor under construction in Haiyan, China, right). Unlike enriched uranium, thorium does not itself fission. It must be bombarded by neutrons to convert it to a fissionable uranium isotope. At that point, the fuel will fission and generate power as well as produce more neutrons to sustain the reaction.
The major theoretical advantage of thorium vs. uranium as a nuclear fuel is its abundance. There is much more thorium on Earth, a fact that may become significant if the use of nuclear power in general increases substantially.