Thorium

Thorium is a <a href="/facts/Chemical_element/0jducgWD">chemical element</a>; it has <a href="/facts/Chemical_symbol/2Fe1VAL0">symbol</a> Th and <a href="/facts/Atomic_number/bgAr581S">atomic number</a> 90. Thorium is a weakly <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> light silver metal which <a href="/facts/Tarnish/GCskvFrN">tarnishes</a> olive grey when it is exposed to air, forming <a href="/facts/Thorium_dioxide/wboPx2It">thorium dioxide</a>; it is moderately soft, <a href="/facts/Malleable/SBJsahEk">malleable</a>, and has a high <a href="/facts/Melting_point/MY0w0h0k">melting point</a>. Thorium is an electropositive <a href="/facts/Actinide/kHC4lxJV">actinide</a> whose chemistry is dominated by the +4 <a href="/facts/Oxidation_state/W53W6s6V">oxidation state</a>; it is quite reactive and can ignite in air when finely divided.
All known thorium <a href="/facts/Isotope/KD9B1y6o">isotopes</a> are unstable. The most stable isotope, <a href="/facts/Thorium-232/lRb3opzg">232Th</a>, has a <a href="/facts/Half-life/kGVH8BAB">half-life</a> of 14.05 billion years, or about the <a href="/facts/Age_of_the_universe/BaByputq">age of the universe</a>; it decays very slowly via <a href="/facts/Alpha_decay/rCsHNIUE">alpha decay</a>, starting a <a href="/facts/Decay_chain/JvTwPNBn">decay chain</a> named the <a href="/facts/Thorium_series/JvTwPNBn">thorium series</a> that ends at stable 208<a href="/facts/Lead/EYljasJN">Pb</a>. On Earth, thorium and <a href="/facts/Uranium/n2sGWBzU">uranium</a> are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as <a href="/facts/Primordial_nuclide/lFAUqHDv">primordial elements</a>. Thorium is estimated to be over three times as <a href="/facts/Natural_abundance/dGjaAwc9">abundant</a> as uranium in the Earth's crust, and is chiefly refined from <a href="/facts/Monazite/an71GkIH">monazite</a> sands as a by-product of extracting <a href="/facts/Rare-earth_element/PQSUQumk">rare-earth elements</a>.
Thorium was discovered in 1828 by the Swedish chemist <a href="/facts/J%C3%B6ns_Jacob_Berzelius/HakSv3sP">Jöns Jacob Berzelius</a>, who named it after <a href="/facts/Thor/Ww3MSrq6">Thor</a>, the <a href="/facts/List_of_Germanic_deities/qtye9fxo">Norse god</a> of thunder and war. Its first applications were developed in the late 19th century. Thorium's radioactivity was widely acknowledged during the first decades of the 20th century. In the second half of the 20th century, thorium was replaced in many uses due to concerns about its radioactivity properties.
Thorium is still used as an alloying element in <a href="/facts/TIG_welding/EY7f43rh">TIG welding</a> electrodes but is slowly being replaced in the field with different compositions. It was also material in high-end optics and scientific instrumentation, used in some broadcast vacuum tubes, and as the light source in <a href="/facts/Gas_mantle/d56ATf6b">gas mantles</a>, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel in <a href="/facts/Nuclear_reactor/8xqjIbge">nuclear reactors</a>, and several <a href="/facts/Thorium_fuel_cycle/mohxxkx3">thorium reactors</a> have been built. Thorium is also used in strengthening <a href="/facts/Magnesium/XyIhd2FG">magnesium</a>, coating <a href="/facts/Tungsten/WYwbNDYX">tungsten</a> wire in electrical and welding equipment, controlling the grain size of tungsten in <a href="/facts/Electric_lamps/v3fb5WBp">electric lamps</a>, high-temperature crucibles, and glasses including camera and scientific instrument lenses. Other uses for thorium include heat-resistant ceramics, <a href="/facts/Aircraft_engines/XJo2Bd2e">aircraft engines</a>, and in <a href="/facts/Light_bulbs/v3fb5WBp">light bulbs</a>. Ocean science has utilised 231<a href="/facts/Protactinium/v87VqTPf">Pa</a>/230Th isotope ratios to understand the ancient ocean.

Thorium open-in-new

Thorium