thorium

Dark-grey, radioactive, metallic element of the actinide series, atomic number 90, relative atomic mass 232.038. It occurs throughout the world in small quantities in minerals such as thorite and is widely distributed in monazite beach sands. It is one of the fissile elements (others include uranium and plutonium) and its longest-lived isotope has a half-life of 1.39 × 10¹⁰ years. Thorium is used to strengthen alloys. It was discovered by Jöns Berzelius in 1828 and was named by him after the Norse god Thor.