Thorium is a medium radioactivity metallic chemical
element with symbol Th and atomic number 90. Thorium metal is silvery
and tarnishes black when it is exposed to air, forming the 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. In the universe, thorium and uranium are the only two radioactive elements that still occur naturally in large quantities as primordial elements. It is estimated to be over three times more abundant than uranium in the Earth's moon, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.
Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. 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 century, thorium was replaced in many uses due to concerns about its radioactivity.
Thorium is still being used as an alloying element in TIG welding electrodes but is slowly being replaced in the field with different compositions. It was also a material in high-end optics and scientific instrumentation, and as the light source in gas mantles, but these have become marginal uses. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built.
Natural thorium is usually almost pure 232Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its radioactive decay is the largest single contributor to the Earth's internal heat; the other major contributors are the shorter-lived primordial radionuclides, which are 238U, 40K, and 235U in descending order of their contribution. (At the time of the Earth's formation, 40K and 235U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from 232Th and 238U predominant.) Its decay accounts for a gradual decrease of thorium content of the Earth: the planet currently has around 85% of the amount present at the formation of the Earth. The other natural thorium isotopes are much shorter-lived; of them, only 230Th is usually detectable, occurring in secular equilibrium with its parent 238U, and making up at most 0.04% of natural thorium.
Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare. Soil normally contains about 6 ppm of thorium.
In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), zirconium(IV), hafnium(IV), and cerium(IV), and also with scandium, yttrium, and the trivalent lanthanides which have similar ionic radii. Because of thorium's radioactivity, minerals containing it are often metamict (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium. An extreme example is ekanite, (Ca,Fe,Pb)2(Th,U)Si8O20, which almost never occurs in nonmetamict form due to the thorium it contains.
Monazite (chiefly phosphates of various rare-earth elements) is the most important commercial source of thorium because it occurs in large deposits worldwide, principally inIndia , South
Africa , Brazil ,
Australia , and Malaysia
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. In the universe, thorium and uranium are the only two radioactive elements that still occur naturally in large quantities as primordial elements. It is estimated to be over three times more abundant than uranium in the Earth's moon, and is chiefly refined from monazite sands as a by-product of extracting rare-earth metals.
Thorium was discovered in 1829 by the Norwegian amateur mineralogist Morten Thrane Esmark and identified by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god of thunder. 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 century, thorium was replaced in many uses due to concerns about its radioactivity.
Thorium is still being used as an alloying element in TIG welding electrodes but is slowly being replaced in the field with different compositions. It was also a material in high-end optics and scientific instrumentation, and as the light source in gas mantles, but these have become marginal uses. It has been suggested as a replacement for uranium as nuclear fuel in nuclear reactors, and several thorium reactors have been built.
Where Thorium Is Found
Natural thorium is usually almost pure 232Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its radioactive decay is the largest single contributor to the Earth's internal heat; the other major contributors are the shorter-lived primordial radionuclides, which are 238U, 40K, and 235U in descending order of their contribution. (At the time of the Earth's formation, 40K and 235U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from 232Th and 238U predominant.) Its decay accounts for a gradual decrease of thorium content of the Earth: the planet currently has around 85% of the amount present at the formation of the Earth. The other natural thorium isotopes are much shorter-lived; of them, only 230Th is usually detectable, occurring in secular equilibrium with its parent 238U, and making up at most 0.04% of natural thorium.
Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare. Soil normally contains about 6 ppm of thorium.
In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), zirconium(IV), hafnium(IV), and cerium(IV), and also with scandium, yttrium, and the trivalent lanthanides which have similar ionic radii. Because of thorium's radioactivity, minerals containing it are often metamict (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium. An extreme example is ekanite, (Ca,Fe,Pb)2(Th,U)Si8O20, which almost never occurs in nonmetamict form due to the thorium it contains.
Monazite (chiefly phosphates of various rare-earth elements) is the most important commercial source of thorium because it occurs in large deposits worldwide, principally in
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