page_title: Thorium
section_title: Hazards
hierarchical_section_title: Thorium / Hazards
caption_attribution_description: English: Thorium radioactive incandescent gas mantle placed above plant seeds
context_page_description: Thorium is a weakly radioactive metallic chemical element with the symbol Th and atomic number 90. Thorium is silvery and tarnishes black when it is exposed to air, forming thorium dioxide; it is moderately hard, malleable, and has a high melting point. 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.
  All known thorium isotopes are unstable. 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 ²⁰⁸Pb. On Earth, thorium, bismuth, and uranium are the only three radioactive elements that still occur naturally in large quantities as primordial elements. It 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.
caption: Experiment on the effect of radiation (from an unburned thorium gas mantle) on the germination and growth of timothy-grass seed
exif: {}