Atomic Number: 90
Atomic Symbol: Th
Atomic Weight: 232
Melting Point: 3,182 F (1,750 C)
Boiling Point: 8,650 F (4,788 C)
Word origin: Thorium is named for Thor, the Norse god of thunder.
Discovery: Thorium was discovered as an element in 1928 by Swedish chemist Jons Jakob Berzelius. Berzelius received a sample of an unidentified black mineral from mineralogist Jens Esmark, whose son Morten Esmark had found it on Lovoya Island, Norway. Esmark suspected it contained an unknown substance. His mineral is now known as thorite.
Properties of thorium
Thorium is radioactive and decays at a fixed rate into a series of other elements. In its pure state, thorium is a silvery-white metal that is stable in air and retains its luster for several months. If contaminated with oxide, however, it tarnishes slowly in air. It turns gray and eventually black. [See Periodic Table of the Elements]
Thorium’s physical properties are strongly influenced by how much it is contaminated with oxide. Even the purest specimens of thorium often contain several tenths of a percent of oxide. Thorium oxide has a melting point of 3,300 C (5,972 F), the highest of all the oxides.
Pure thorium is soft, very ductile, and can be swaged, drawn and cold-rolled. When heated in air, its turnings ignite and burn with a brilliantly white light. Powdered thorium is pyrophoric, requiring careful handling.
Thorium is dimorphic, changing from a cubic to a body-centered cubic structure at 1,400 C (2,552 F). Thorium does not dissolve easily in most common acids (with the exception of hydrochloric) but water slowly attacks it.
Much of the earth’s internally produced heat is attributed to thorium and uranium.
Sources of thorium
As a primordial nuclide, 232Th has existed in its current form for more than 4.5 billion years. Its existence predates the formation of Earth. Thorium was formed in the cores of dying stars through the r-process, and supernovas eventually scattered it across the galaxy. Its half-life is comparable to the age of the universe.
Small amounts of thorium are found in most rocks and soils. Soil usually has an about 6 parts per million of thorium. Thorium is found in several minerals, including thorianite, monazite, and thorite. They occur on all continents, and thorium is now considered three times more abundant than uranium, or about as common as lead and molybdenum.
Thorium is recovered commercially from rare-earth minerals and monazite, which is anything from 3-to-9 percent thorium. High-purity thorium has been made, and there are several methods for producing thorium metal.
Uses of thorium
Historically, thorium’s primary use was for the Welsbach mantle used in portable gaslights. Along with other ingredients, the thorium in these mantles produced a dazzling light when heated with a gas flame.
Today, thorium metal is used as a source for nuclear power. Thorium-cycle converter-reactor systems are in development. Thorium’s abundance means that there is probably more energy available from thorium than from both uranium and fossil fuels, but any significant demand for thorium as a nuclear fuel is still several years in the future.
Thorium is also used to coat the tungsten wire found in electronic equipment. Its presence as an alloying element in magnesium, imparting high strength and slowing resistance at elevated temperatures, plus its low-work function and high electron emission make it an excellent source for coating tungsten wire. Thorium oxide is also used to control grain size in tungsten when used in electric lamps and in high-temperature laboratory crucibles. Additionally, thorium oxide is useful as a catalyst in ammonia-to-nitric acid conversion, in petroleum cracking, and in sulfuric acid production.
Gases containing thorium oxide are useful in producing high-quality camera lenses and scientific instruments. These gases have a high refractive index and low dispersion. 232Th is radioactive enough to expose a photographic plate in a few hours.
Isotopes of thorium
Thorium has 27 known radioisotopes. They range in atomic weight from 210 to 236 and all are unstable. 232Th is by far the most stable with a half-life as long as the universe — 14.05 billion years. Other isotopes are short lived, and are actually intermediates in the decay chain of higher elements. Only trace amounts of them are found. The longer-lived of these trace isotopes include: 230Th with a half-life of 75,380 years which is a daughter product of 238U decay; 229Th with a half-life of 7,340 years and 228Th with a half-life of 1.92 years. All of the remaining radioactive isotopes have half-lives that are less than 30 days and the majority of these have half-lives less than 10 minutes.
232Th contains almost all naturally occurring thorium. It is an alpha emitter and goes through six alpha and four beta decay steps before becoming the stable isotope 208Pb.