Thorium is estimated to be over three times as abundant as uranium in the Earth's crust, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals. On Earth, thorium and uranium are the only significantly radioactive elements that still occur naturally in large quantities as primordial elements. The most stable isotope, 232Th, has a half-life of 14.05 billion years, or about the age of the universe it decays very slowly via alpha decay, starting a decay chain named the thorium series that ends at stable 208 Pb. Thorium is an electropositive actinide whose chemistry is dominated by the +4 oxidation state it is quite reactive and can ignite in air when finely divided.Īll known thorium isotopes are unstable. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide it is moderately soft, malleable, and has a high melting point. Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90.
