Thorium

With atomic number 90 and an electron configuration of [Rn] 7s2 6d2, thorium sits near the opening of the actinide series, where the 5f and 6d subshells compete closely in energy. At room temperature its two 6d electrons make it behave more like a transition metal than the later actinides, giving it a single dominant oxidation state of +4. Thorium is mildly radioactive: the most abundant isotope, Th-232, undergoes alpha decay with a half-life of 14.05 billion years — longer than the age of the universe — making fresh thorium metal feel almost inert compared to its heavier actinide neighbors. Dense at 11.72 g/cm³ and silvery-white when freshly cut, it slowly tarnishes to a dark oxide layer in air. Its high melting point of 1,750°C and good corrosion resistance made it valuable in high-temperature applications long before its nuclear potential was recognized.

Historically, thorium dioxide was the key ingredient in Welsbach gas mantles, the bright incandescent meshes that made gas lighting practical in the late 19th and early 20th centuries; thoriated mantles are still sold for camping lanterns. High-purity thorium oxide is used as a stabilizer in tungsten electrodes for TIG welding, where it raises the melting point and improves arc stability. In the nuclear energy field, thorium is the feedstock for the thorium fuel cycle: Th-232 absorbs a neutron and decays through protactinium-233 to U-233, a fissile isotope capable of sustaining a reactor chain reaction. Molten salt reactor designs, including India's advanced heavy-water reactor program, are actively developing thorium-based fuel cycles that produce far less long-lived transuranium waste than conventional uranium cycles. Minor uses include optical glass for high-refractive-index camera and telescope lenses, though thorium-free alternatives are replacing it in this application.

Jöns Jacob Berzelius discovered thorium in 1828 while analyzing a dark mineral sample sent to him from a parish in Telemark, Norway. He named it after Thor, the Norse god of thunder. The mineral, later named thorite, turned out to be a thorium silicate. Berzelius isolated thorium metal impurely by reducing thorium tetrachloride with potassium. For most of the 19th century thorium remained a laboratory curiosity, its radioactivity unrecognized. Ernest Rutherford and Frederick Soddy made a pivotal observation in 1902: thorium continuously generates a radioactive gas (thoron, now known as radon-220), the first direct evidence of radioactive transformation of one element into another. This launched the concept of radioactive decay chains. Interest in thorium as an energy source intensified during the Cold War, and experimental molten salt reactors at Oak Ridge National Laboratory in the 1960s demonstrated that thorium-fueled liquid-fluoride reactors were technically feasible, though uranium-based designs ultimately dominated commercial nuclear development.

Thorium is roughly three to four times more abundant in Earth's crust than uranium, averaging about 9.6 parts per million. Because it forms resistant, insoluble minerals, it concentrates in heavy mineral sands alongside zircon, ilmenite, and monazite. Monazite, a rare-earth phosphate mineral, is the chief commercial source and often contains 3–10 percent thorium dioxide by weight. Major deposits occur in India, Brazil, Australia, and the United States; India's Kerala coast holds particularly large monazite placer deposits. Thorium also occurs as a trace constituent in granites, which contain enough of it — along with uranium and potassium-40 — to generate measurable heat through radioactive decay. This radiogenic heat contributes a significant fraction of Earth's internal heat budget. Like uranium, all terrestrial thorium was produced by neutron-capture nucleosynthesis in stellar environments and incorporated into the solar system during its formation.

Thorium dioxide (ThO2), also called thoria, is the most important compound industrially. With one of the highest melting points of any oxide at roughly 3,300°C, it is used in crucibles for melting high-temperature metals and as a refractory lining. Thorium nitrate, Th(NO3)4, is the soluble salt most commonly used in chemical processing and was the active ingredient in the liquid solutions applied to early gas-mantle fabrics. Thorium tetrachloride (ThCl4) and thorium tetrafluoride (ThF4) are the halide precursors used in metallurgical reduction to produce thorium metal and in the preparation of liquid-fluoride thorium reactor fuels. Thorium sulfate and thorium oxalate serve as analytical reagents and as intermediates in ore processing. In the thorium fuel cycle, UO2-ThO2 mixed oxide ceramics are being developed as drop-in reactor fuel pellets that blend uranium-233 bred from thorium with existing reactor infrastructure.

• The thorium fuel cycle is so efficient that the amount of thorium in a standard automobile's worth of steel — from trace impurities — contains enough fissile potential, if fully exploited, to power that car for its entire lifetime.
• Thorium-232 has a half-life of 14.05 billion years, making it essentially as old as the universe itself; atoms of thorium present in Earth's mantle when the planet formed are still largely intact today.
• India, sitting atop some of the world's largest monazite deposits, has staked its long-term energy strategy partly on thorium reactors and operates an ambitious three-stage nuclear program explicitly designed to reach large-scale thorium utilization.
• Old thoriated tungsten welding electrodes are mildly radioactive enough that workers are advised not to grind them in confined spaces without ventilation — the fine tungsten-thorium dust, if inhaled, deposits alpha emitters directly in lung tissue.
• A complete thorium decay chain involves ten successive radioactive decay steps — alpha and beta emissions — before finally arriving at stable lead-208, releasing energy at each step along the way.