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Thorium

physical science Maturity 11-13 Next article →

Thorium is a chemical element with the symbol Th and the atomic number 90. It is classified as an actinide, which is a group of heavy, radioactive metallic elements.

Thorium half sandwich
Thorium half sandwich

Thorium is a bright, silvery metal that is moderately soft and malleable. This means it can be bent, shaped, or pressed into thin sheets. When exposed to air, it reacts to form thorium dioxide, which gives the metal an olive-grey tarnish.

CaF2 polyhedra
CaF2 polyhedra

Thorium is an electropositive element, meaning it tends to lose electrons easily. Its chemistry is largely defined by the +4 oxidation state. This metal is also quite reactive; if it is divided into fine particles, it can actually ignite in the air.

Decay Chain Thorium
Decay Chain Thorium

The physical structure of thorium changes depending on its environment. At room temperature, it has a face-centered cubic crystal structure. However, if you heat it above 1360 °C, it shifts to a body-centered cubic structure. Under extreme pressure of around 100 GPa, it adopts a body-centered tetragonal structure. Thorium has a very high melting point of 1750 °C. This is significantly higher than its neighbors, actinium and protactinium. This high melting point occurs because the number of delocalized electrons increases as you move toward thorium. These electrons create a stronger attraction between the metal ions, which holds the structure together more tightly.

J J Berzelius
J J Berzelius

Thorium was discovered in 1828 by the Swedish chemist Jöns Jacob Berzelius. He identified the element while analyzing a new mineral found in Norway. This mineral was discovered by Morten Thrane Esmark on the island of Løvøya. Berzelius chose to name the element after Thor, the Norse god of thunder and war.

In the early 20th century, scientists began to widely acknowledge its radioactivity. During the second half of the 20th century, many applications of thorium changed. People began to replace thorium in various products due to concerns regarding its radioactive properties.

Decay Chain Thorium
Decay Chain Thorium

All known isotopes of thorium are unstable, meaning they undergo radioactive decay. The most stable isotope is 232Th, which has a half-life of 14.0 billion years. This duration is roughly the age of the universe. This isotope decays through a process called alpha decay, where the nucleus releases an alpha particle. This begins the thorium series, a chain of consecutive alpha and beta decays. This long process eventually results in the stable element lead-208.

Evolution of Earth's radiogenic heat-no total
Evolution of Earth's radiogenic heat-no total

Thorium and uranium are the only elements that occur naturally in large quantities as primordial elements. This means they have existed since the Earth was formed without being completely decayed away.

Old thorium dioxide gas mantle - oblong shape
Old thorium dioxide gas mantle - oblong shape

Thorium is surprisingly common in the Earth's crust. It is estimated to be over three times as abundant as uranium. Most thorium is refined from monazite sands. It is usually collected as a by-product during the extraction of rare-earth elements. Because of its abundance and potential, researchers have suggested using thorium as a replacement for uranium in nuclear reactors. Several thorium-based reactors have already been built to explore this possibility.

Yellowing of thorium lenses
Yellowing of thorium lenses

Thorium has been used in many specialized industrial and scientific roles. It is used as an alloying element in TIG welding electrodes. It is also used to strengthen magnesium and to coat tungsten wire in electrical equipment. In the past, thorium was a common material in high-end optics and scientific lenses.

Indian Point Nuclear Power Plant
Indian Point Nuclear Power Plant

However, thorium can cause lenses to yellow over time. It was also used in gas mantles to provide light and in certain broadcast vacuum tubes. While some of these uses are now marginal, thorium remains important in heat-resistant ceramics and aircraft engines.

Decay Chain Thorium
Decay Chain Thorium

Scientists also use thorium to study the history of our planet. Two specific methods, uranium–thorium dating and ionium–thorium dating, help determine the age of materials. Uranium–thorium dating is used on calcium carbonate, such as coral or cave formations called speleothems. This works because uranium is more soluble in water than thorium. Ionium–thorium dating measures the ratio of 232Th to 230Th in ocean sediments. These methods allow oceanographers to understand the ancient ocean by looking at how these isotopes settle over time.

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🖼️ Images & Media (13)
File:Decay Chain Thorium.svg
Decay Chain Thorium.svg
File:CaF2 polyhedra.png
CaF2 polyhedra.png
File:Kristallstruktur Uran(IV)-fluorid.png
Kristallstruktur Uran(IV)-fluorid.png
File:Uranocene-3D-balls.png
Uranocene-3D-balls.png
File:Thorium half sandwich.svg
Thorium half sandwich.svg
File:Evolution of Earth's radiogenic heat-no total.svg
Evolution of Earth's radiogenic heat-no total.svg
File:Mårten Eskil Winge - Tor's Fight with the Giants - Google Art Project.jpg
Mårten Eskil Winge - Tor's Fight with the...
File:J J Berzelius.jpg
J J Berzelius.jpg
File:Old thorium dioxide gas mantle - oblong shape.JPG
Old thorium dioxide gas mantle - oblong shape.JPG
File:Seaborg in lab - restoration.jpg
Seaborg in lab - restoration.jpg
File:Indian_Point_Nuclear_Power_Plant.jpg
Indian_Point_Nuclear_Power_Plant.jpg
File:Yellowing of thorium lenses.jpg
Yellowing of thorium lenses.jpg

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