Thorium – facts and properties

Position in the periodic table

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Occurrence and extraction

Thorium is clearly more abundant than uranium and occurs in small amounts in many rocks and soils. The main ore is monazite, a phosphate of the rare earth metals that collects in coastal sands, for example in India, Brazil and Australia. Thorium is a by-product of extracting the rare earths and is used only on a small scale. Almost all natural thorium is the isotope thorium-232.

The discovery began with a mineral. The Norwegian clergyman and mineral collector Morten Thrane Esmark found it on an island in 1828 and sent a sample to Berzelius in Stockholm. Berzelius recognised a new element in it, described it the following year and named it after Thor. The mineral is now called thorite. The radiation of thorium was demonstrated in 1898 by Gerhard Schmidt and Marie Curie, working independently.

Properties and reactions

Thorium is in period 7 and is the second actinoid, in the actinoid row of the table between actinium on its left and protactinium on its right. Its electron configuration is [Rn] 6d² 7s², and its electronegativity is 1.3. The atomic radius is 180 pm and the density 11.72 g/cm³. Thorium melts at 1750 °C and boils only at 4788 °C, one of the highest boiling points of any metal. Its atomic mass is given as 232.038 u, the mass of the isotope thorium-232, which makes up almost all of the natural element.

Thorium is a soft, silvery metal that tarnishes grey to black in air. Finely divided, it can ignite by itself. In compounds it appears almost only as a quadruply charged positive ion. Thorium dioxide melts at a higher temperature than any other oxide, far above 3000 °C, and is therefore used as a heat-resistant material.

Thorium-232 has a half-life of about 14 billion years, roughly the age of the universe. Over such a span pure thorium radiates only weakly, and its alpha radiation is stopped by skin. In time, however, decay products such as radium-228 and radon-220 build up, which strengthen the radiation and have to be taken into account when the material is processed. The decay series ends in stable lead-208.

The main danger is dust that is inhaled or swallowed and settles in lungs, liver and bone. An example is Thorotrast, a thorium-containing liquid injected as an X-ray contrast agent from the late 1920s onwards. Decades later many patients developed liver tumours and blood diseases. The product was banned afterwards.

Mantles, lenses and reactor concepts

The biggest everyday use was the gas mantle. Around 1890 Carl Auer von Welsbach soaked a fabric in thorium nitrate. On burning, a framework of thorium dioxide with a little cerium dioxide remained, which glows brilliant white in a gas flame. Gas street lamps and later camping lamps became much brighter. Today most makers use thorium-free mantles.

Glass for camera lenses also contained thorium dioxide, because it raises the refraction of light. Such lenses from the mid twentieth century radiate weakly and yellow with age. Thoriated tungsten electrodes made welding easier but are increasingly replaced by other types.

As a nuclear fuel, thorium is a concept. Thorium-232 cannot be split by slow neutrons, but it absorbs them and turns via thorium-233 and protactinium-233 into uranium-233, which can be split. Supporters point to large reserves, for example in India. Critics name high technical hurdles. There are experimental reactors, but no broad commercial use.

In research, thorium helps to determine ages. The isotope thorium-230 forms in the decay of uranium-234. Corals and stalagmites take up uranium as they grow, but hardly any thorium. The ratio between the two shows their age, up to several hundred thousand years. Uranium-thorium dating thus delivers climate data from the past.

Quick question

What is the atomic number of thorium (Th)?

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