thorium

Thorium (Th), atomic number 90, is a weakly radioactive metallic element in the actinide series. It is silvery and relatively soft, tarnishing in air. Its density is about 11.7 g/cm3; melting point around 1,755°C and boiling point near 4,700°C. Nearly all natural thorium is the isotope Th-232, with a half-life of about 14 billion years. Th-232 decays by alpha emission through a long decay chain to stable lead-208 and is considered fertile rather than fissile.

Thorium is relatively abundant in the Earth's crust (about 6 parts per million) and occurs mainly in minerals such as monazite and thorite. It is mined in various regions, including India, Australia, and parts of Africa. In nuclear contexts, Th-232 can absorb neutrons to become U-233, a fissile isotope, via neutron capture and beta decays, making thorium a candidate for the thorium fuel cycle. Proponents cite potential advantages such as reduced long-term waste and limited plutonium production, though technical and regulatory challenges remain.

Historically, thorium oxide was used in gas mantles (Welsbach mantles) to produce bright white light; usage declined due to radioactivity concerns. Today thorium has limited commercial use and is mainly studied for nuclear fuel applications, specialty ceramics, and certain high-temperature alloys. Handling requires radiological controls to minimize exposure, especially if materials are ingested or inhaled.