Thorium (Th) is a heavy metal with the atomic number 90. This silvery-white element is found in small quantities in most rocks and soils, making it about three times more abundant in the Earth’s crust than uranium. Thorium metal is moderately soft and malleable, but it readily oxidizes when exposed to air, tarnishing to a gray or black surface. Its primary isotope, Thorium-232, has an extremely long half-life of about 14 billion years.
Thorium’s Place on the Periodic Table
Thorium is located in the seventh period of the periodic table, corresponding to its seven electron shells. It is the first element in the Actinide series, the inner transition metal group typically displayed as a separate row beneath the main body of the table. The elements in this series formally belong to the larger f-block of the periodic table. Some older designs place Thorium directly below Group 4 (Titanium and Zirconium), reflecting its electronic structure and properties similar to transition metals, even though it is fundamentally an Actinide.
Defining Characteristics of the Actinides
The Actinide series, ranging from element 89 (Actinium) to element 103 (Lawrencium), shares distinct properties. All Actinides are heavy metals characterized by high atomic masses, though Thorium is an exception to the high-density rule. Every Actinide element is radioactive, meaning their atomic nuclei are unstable and spontaneously break down over time. Although Thorium is radioactive, its long half-life classifies it as only weakly radioactive. These elements also exhibit variable valence states, forming ions with different positive charges; however, Thorium’s chemistry is predominantly governed by a stable positive four (+4) oxidation state.
Primary Applications and Uses
The most significant contemporary application of Thorium is its potential role in the Thorium fuel cycle for nuclear energy generation. Unlike fissile Uranium-235, Thorium-232 is a fertile material that must first absorb a neutron inside a reactor to transmute into the fissile isotope Uranium-233. This breeding capability allows the Thorium cycle to use a more abundant resource and produce less long-lived radioactive waste compared to the conventional uranium cycle. Thorium also has non-nuclear uses stemming from its robust chemical properties. For example, its oxide has one of the highest known melting points, leading to its historical use in Welsbach gas mantles, and it is used as an alloying agent to strengthen metals like magnesium.