   #copyright

Thorium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


              90              actinium ← thorium → protactinium
              Ce
             ↑
             Th
             ↓
             (Uqn)

                                  Periodic Table - Extended Periodic Table

                                                                   General
                                      Name, Symbol, Number thorium, Th, 90
                                                 Chemical series Actinides
                                            Group, Period, Block n/a, 7, f
                                                  Appearance silvery white
                                           Atomic mass 232.03806 (2) g/mol
                                     Electron configuration [Rn] 6d^2 7s^2
                               Electrons per shell 2, 8, 18, 32, 18, 10, 2
                                                       Physical properties
                                                               Phase solid
                                       Density (near r.t.) 11.7 g·cm^−3
                                                     Melting point 2115  K
                                                    (1842 ° C, 3348 ° F)
                                                      Boiling point 5061 K
                                                    (4788 ° C, 8650 ° F)
                                         Heat of fusion 13.81 kJ·mol^−1
                                     Heat of vaporization 514 kJ·mol^−1
                         Heat capacity (25 °C) 26.230 J·mol^−1·K^−1

   CAPTION: Vapor pressure

                                      P/Pa   1    10  100  1 k  10 k 100 k
                                     at T/K 2633 2907 3248 3683 4259 5055

                                                         Atomic properties
                                     Crystal structure cubic face centered
                                                        Oxidation states 4
                                                      (weakly basic oxide)
                                     Electronegativity 1.3 (Pauling scale)
                                                       Ionization energies
                                             ( more) 1st: 587 kJ·mol^−1
                                                    2nd: 1110 kJ·mol^−1
                                                    3rd: 1930 kJ·mol^−1
                                                      Atomic radius 180 pm
                                                             Miscellaneous
                                                 Magnetic ordering no data
                                 Electrical resistivity (0 °C) 147 nΩ·m
                       Thermal conductivity (300 K) 54.0 W·m^−1·K^−1
                       Thermal expansion (25 °C) 11.0 µm·m^−1·K^−1
                               Speed of sound (thin rod) (20 °C) 2490 m/s
                                                    Young's modulus 79 GPa
                                                      Shear modulus 31 GPa
                                                       Bulk modulus 54 GPa
                                                        Poisson ratio 0.27
                                                         Mohs hardness 3.0
                                                  Vickers hardness 350 MPa
                                                  Brinell hardness 400 MPa
                                             CAS registry number 7440-29-1
                                                         Selected isotopes

                 CAPTION: Main article: Isotopes of thorium

                         iso    NA       half-life     DM DE ( MeV)   DP
                        ^228Th syn   1.9116 years      α  5.520     ^224Ra
                        ^229Th syn   7340 years        α  5.168     ^225Ra
                        ^230Th syn   75380 years       α  4.770     ^226Ra
                        ^231Th trace 25.5 hours        β  0.39      ^231Pa
                        ^232Th 100%  1.405×10^10 years α  4.083     ^228Ra
                        ^234Th trace 24.1 days         β  0.27      ^234Pa

                                                                References

   Thorium ( IPA: /ˈθɔːriəm/) is a chemical element in the periodic table
   that has the symbol Th and atomic number 90. As a naturally occurring,
   slightly radioactive metal, it has been considered as an alternative
   nuclear fuel to uranium.

Notable characteristics

   When pure, thorium is a silvery white metal that retains its lustre for
   several months. However, when it is contaminated with the oxide,
   thorium slowly tarnishes in air, becoming grey and eventually black.
   Thorium dioxide (ThO[2]), also called thoria, has one of the highest
   melting points of all oxides (3300°C). When heated in air, thorium
   metal turnings ignite and burn brilliantly with a white light.

   See Actinides in the environment for details of the environmental
   aspects of thorium.

Applications

   Applications of thorium:
     * As an alloying element in magnesium, imparting high strength and
       creep resistance at elevated temperatures.
     * Thorium is used to coat tungsten wire used in electronic equipment,
       improving the electron emission of heated cathodes.
     * Thorium has been used in gas tungsten arc welding electrodes and
       heat-resistant ceramics.
     * Uranium-thorium age dating has been used to date hominid fossils.
     * As a fertile material for producing nuclear fuel. In particular,
       the proposed energy amplifier reactor design would employ thorium.
       Since thorium is more abundant than uranium, some designs of
       nuclear reactor incorporate thorium in their nuclear fuel cycle.
     * Thorium is a very effective radiation shield, although it has not
       been used for this purpose as much as have lead or depleted
       uranium.
     * Thorium may be used in subcritical reactors instead of uranium as
       fuel. This produces less waste and cannot melt down.

   Applications of thorium dioxide (ThO[2]):
     * Mantles in portable gas lights. These mantles glow with a dazzling
       light (unrelated to radioactivity) when heated in a gas flame.
     * Used to control the grain size of tungsten used for electric lamps.
     * Used for high-temperature laboratory crucibles.
     * Added to glass, it helps create glasses of a high refractive index
       and with low dispersion. Consequently, they find application in
       high-quality lenses for cameras and scientific instruments.
     * Has been used as a catalyst:
          + In the conversion of ammonia to nitric acid.
          + In petroleum cracking.
          + In producing sulfuric acid.
     * Thorium dioxide is the active ingredient of Thorotrast, which was
       used as part of X-ray diagnostics. This use has been abandoned due
       to the carcinogenic nature of Thorotrast.

History

   Thorium was discovered in 1828 by the Swedish chemist Jöns Jakob
   Berzelius, who named it after Thor, the Norse god of thunder. The metal
   had virtually no uses until the invention of the lantern mantle in
   1885.

   The crystal bar process (or Iodide process) was discovered by Anton
   Eduard van Arkel and Jan Hendrik de Boer in 1925 to produce high-purity
   metallic thorium.

   The name ionium was given early in the study of radioactive elements to
   the ^230Th isotope produced in the decay chain of ^238U before it was
   realized that ionium and thorium were chemically identical. The symbol
   Io was used for this supposed element.

Occurrence

   Monazite, a rare-earth-and-thorium-phosphate mineral is the primary
   source of the world's thorium
   Enlarge
   Monazite, a rare-earth-and-thorium-phosphate mineral is the primary
   source of the world's thorium

   Thorium is found in small amounts in most rocks and soils, where it is
   about three times more abundant than uranium, and is about as common as
   lead. Soil commonly contains an average of around 12 parts per million
   (ppm) of thorium. Thorium occurs in several minerals, the most common
   being the rare earth-thorium-phosphate mineral, monazite, which
   contains up to about 12% thorium oxide. There are substantial deposits
   in several countries. ^232Th decays very slowly (its half-life is about
   three times the age of the earth) but other thorium isotopes occur in
   the thorium and uranium decay chains. Most of these are short-lived and
   hence much more radioactive than ^232Th, though on a mass basis they
   are negligible.

Thorium as a nuclear fuel

   Thorium, as well as uranium and plutonium, can be used as fuel in a
   nuclear reactor. Although not fissile itself, ^232Th will absorb slow
   neutrons to produce uranium-233 (^233U), which is fissile. Hence, like
   ^238U, it is fertile. In one significant respect ^233U is better than
   the other two fissile isotopes used for nuclear fuel, ^235U and
   plutonium-239 (^239Pu), because of its higher neutron yield per neutron
   absorbed. Given a start with some other fissile material (^235U or
   ^239Pu), a breeding cycle similar to, but more efficient than that
   currently possible with the ^238U-to-^239Pu cycle (in slow-neutron
   reactors), can be set up. The ^232Th absorbs a neutron to become ^233Th
   which normally decays to protactinium-233 (^233Pa) and then ^233U. The
   irradiated fuel can then be unloaded from the reactor, the ^233U
   separated from the thorium (a relatively simple process since it
   involves chemical instead of isotopic separation), and fed back into
   another reactor as part of a closed nuclear fuel cycle.

   Problems include the high cost of fuel fabrication due partly to the
   high radioactivity of ^233U which is a result of its contamination with
   traces of the short-lived ^232U; the similar problems in recycling
   thorium due to highly radioactive ^228Th; some weapons proliferation
   risk of ^233U; and the technical problems (not yet satisfactorily
   solved) in reprocessing. Much development work is still required before
   the thorium fuel cycle can be commercialised, and the effort required
   seems unlikely while (or where) abundant uranium is available.

   Nevertheless, the thorium fuel cycle, with its potential for breeding
   fuel without the need for fast neutron reactors, holds considerable
   potential long-term. Thorium is significantly more abundant than
   uranium, so it is a key factor in the sustainability of nuclear energy.

   Australia and India have particularly large reserves of thorium. India
   has planned its nuclear power program to eventually use thorium
   exclusively, phasing out uranium as an input material. This ambitious
   plan uses both fast and thermal breeder reactors. The Advanced Heavy
   Water Reactor and KAMINI reactor are efforts in this direction.

   The current thorium mineral reserve estimates (in tons)
     * 300,000 Australia
     * 290,000 India
     * 170,000 Norway
     * 160,000 United States
     * 100,000 Canada
     * 35,000 South Africa
     * 16,000 Brazil
     * 95,000 Others

Isotopes

   Naturally occurring thorium is composed of one isotope: ^232Th. Twenty
   seven radioisotopes have been characterized, with the most {abundant
   and/or stable} being ^232Th with a half-life of 14.05 billion years,
   ^230Th with a half-life of 75,380 years, ^229Th with a half-life of
   7340 years, and ^228Th with a half-life of 1.92 years. All of the
   remaining radioactive isotopes have half-lifes that are less than
   thirty days and the majority of these have half lifes that are less
   than ten minutes. This element also has one meta state.

   The known isotopes of thorium range in atomic weight from 210 amu
   (^210Th) to 236 amu (^236Th).

Precautions

   Powdered thorium metal is often pyrophoric and should be handled
   carefully.

   Exposure to aerosolized thorium can lead to increased risk of cancers
   of the lung, pancreas and blood. Exposure to thorium internally leads
   to increased risk of liver diseases. This element has no known
   biological role. See also Thorotrast.

In popular culture

   David Hahn, the so-called "radioactive boy scout," bombarded thorium
   from lantern mantles with neutrons to produce small quantities of
   fissionable material in his backyard. He had to abandon his project
   when he began to detect elevated radiation levels several houses away
   from his own.
   Retrieved from " http://en.wikipedia.org/wiki/Thorium"
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