- Thorium (Th) is a silvery-white, moderately soft, and slightly radioactive metal with atomic number 90, belonging to the actinide series of the periodic table.
- Its electron configuration is [Rn] 6d² 7s², and it typically forms Th⁴⁺ ions in chemical compounds. The atomic structure contains ninety protons, most commonly one hundred and forty-two neutrons in its most abundant isotope, and ninety electrons distributed in seven shells.
- Naturally occurring thorium consists almost entirely of the isotope thorium-232 (²³²Th), which has an extraordinarily long half-life of about 14.05 billion years—roughly the age of the universe—making it only weakly radioactive. Other isotopes such as thorium-230 (²³⁰Th) and thorium-228 (²²⁸Th) occur in small amounts as part of natural decay chains.
- Thorium is relatively abundant in the Earth’s crust, occurring at about 6–10 parts per million, which is more common than tin, lead, or silver. It is widely distributed in minerals such as monazite ((Ce,La,Nd,Th)PO₄) and thorite (ThSiO₄), with major deposits found in India, Brazil, the United States, Australia, and Norway. Monazite sands can contain up to 12% thorium oxide (ThO₂), making them the primary commercial source of the element.
- The element was discovered in 1828 by Swedish chemist Jöns Jakob Berzelius, who named it after Thor, the Norse god of thunder. Initially thought to be non-radioactive (radioactivity was not yet discovered), thorium was later recognized for its nuclear properties in the early 20th century.
- Thorium has multiple important applications. Historically, thorium oxide was used in gas mantles for portable lighting, producing an intense white light when heated. In the modern era, thorium has gained attention as a potential nuclear fuel. In a thorium fuel cycle, ²³²Th absorbs a neutron to become fissile uranium-233 (²³³U), which can sustain a chain reaction. Thorium-based nuclear reactors offer potential advantages such as higher fuel efficiency, reduced long-lived nuclear waste, and lower proliferation risks compared to uranium reactors. Thorium is also used in high-temperature-resistant ceramics, specialized optics, and certain metal alloys for aerospace applications.
- Chemically, thorium is reactive, especially when finely divided. It tarnishes in air, forming a layer of thorium dioxide, and reacts slowly with water but readily with acids. Thorium dioxide has one of the highest melting points of any known oxide, around 3390 °C.
- Biologically, thorium has no known role in living organisms and is considered toxic mainly due to its radioactivity. Inhalation of thorium-containing dust poses significant health risks, including lung cancer.
- Environmentally, thorium is stable in natural minerals, but mining and processing can generate radioactive waste, requiring careful handling and long-term storage.
