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Thorium is a heavy element in the periodic table, belonging to the actinide series (elements 89 through 102). Thorium has the atomic number 90, with an atomic mass of 232.038. The stable isotope of thorium is thorium-232, or Th-232, which consists of 90 electrons, 90 protons and 142 neutrons. In its normal phase, it occurs as a silvery white metal, which is soft and highly ductile. With prolonged exposure to oxide, it transmutes into gray, and eventually black. Thorium oxide (ThO2) does not oxidize, unlike uranium oxide (UO2), which is beneficial for various practical purposes. Statistically, thorium is three to four times as abundant in nature as uranium, and approximately as abundant as lead.
Thorium emits low-level alpha-wave radiation in its natural state, and is somewhat less radiotoxic than uranium. It is not dangerous to the touch, however ingestion, inhalation or prolonged contact would be toxic for biological life forms. The half-life of natural thorium is approximately 14 billion years. Th-232 decays initially into radium-228, and ultimately decays into the stable lead-208. Thorium has the largest range between melting point and boiling point of any element: 2,946 degrees Celsius. Thorium does not dissolve in most acids, with the exception of hydrochloric acid.
Thorium has some special features that have caused it to be used in some industrial applications. It has been used as an additive to tungsten arc welding electrodes, for example. Being a radioactive element, thorium is also used for radioactive age dating, for example, of fossils. Thorium oxide has the highest melting point of any oxide material: 3,300 degrees Celsius. This is likely the reason why it has been used for various heat-intensive applications, such as in the mantles of portable gas lanterns; high temperature ceramics including crucibles; catalysts in the petroleum industry; and various specialized aerospace applications. Most industrial applications for thorium have more recently substituted other materials, owing to the availability of less radiotoxic alternatives. Because it is very seldom used, there is no supply or demand of thorium in the world at present, outside of sundry research applications, which would come from recycled supply.
Natural thorium has many names, which all refer to the same substance: thorium oxide, thorium dioxide, ThO2, and thoria. In nature, thorium does not occur with any other resident isotopes besides Th-232. Thorium is said to be fertile material. This means that thorium can accept a neutron and transmute into fissile material, which, if highly concentrated enough, can sustain a nuclear chain reaction. Thorium is the only naturally occurring fertile material on earth, besides uranium. Unlike thorium, however, uranium occurs naturally with 0.711 wt% of Uranium-235 isotope, or U-235, which is itself a fissile material. If highly concentrated enough, U-235 can ignite its own nuclear chain reaction.
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