Thorium

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Thorium, 90Th
Small (3 cm) ampule with a tiny (5 mm) square of metal in it
Thorium
Pronunciation/ˈθɔːriəm/ ​(THOR-ee-əm)
Appearancesilvery
Standard atomic weight Ar°(Th)
Thorium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Ce
↑
Th
↓
—
actinium ← thorium → protactinium
kJ/mol
Heat of vaporisation514 kJ/mol
Molar heat capacity26.230 J/(mol·K)
Vapour pressure
P (Pa) 1 10 100 1 k 10 k 100 k
at T (K) 2633 2907 3248 3683 4259 5055
Atomic properties
Oxidation statescommon: +4
−1,
Jöns Jakob Berzelius (1829)
Isotopes of thorium
Main isotopes[8] Decay
abun­dance half-life (t1/2) mode pro­duct
227Th trace 18.68 d α 223Ra
228Th trace 1.9116 y α
224Ra
229Th trace 7917 y[9] α
225Ra
230Th 0.02% 75400 y α 226Ra
231Th trace 25.5 h
β−
231Pa
232Th 100.0% 1.405×1010 y α
228Ra
233Th trace 21.83 min β−
233Pa
234Th trace 24.1 d β−
234Pa
 Category: Thorium
| references

Thorium is a

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

thorium series that ends at stable 208Pb. On Earth, thorium and uranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities as primordial elements.[a] Thorium 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 elements
.

Thorium was discovered in 1828 by the Swedish chemist Jöns Jacob Berzelius, who named it after Thor, the Norse god 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

light bulbs. Ocean science has utilised 231Pa/230Th isotope ratios to understand the ancient ocean.[10]

Bulk properties

Thorium is a moderately soft,

cold-rolled, swaged, and drawn.[11] At room temperature, thorium metal has a face-centred cubic crystal structure; it has two other forms, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).[11]

Thorium metal has a

GPa, about the same as tin's (58.2 GPa). Aluminium's is 75.2 GPa; copper's 137.8 GPa; and mild steel's is 160–169 GPa.[12] Thorium is about as hard as soft steel, so when heated it can be rolled into sheets and pulled into wire.[13]

Thorium is nearly half as dense as

superconductive below 1.4 K.[11] Thorium's melting point of 1750 °C is above both those of actinium (1227 °C) and protactinium (1568 °C). At the start of period 7, from francium to thorium, the melting points of the elements increase (as in other periods), because the number of delocalised electrons each atom contributes increases from one in francium to four in thorium, leading to greater attraction between these electrons and the metal ions as their charge increases from one to four. After thorium, there is a new downward trend in melting points from thorium to plutonium, where the number of f-electrons increases from about 0.4 to about 6: this trend is due to the increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds resulting in more complex crystal structures and weakened metallic bonding.[13][14] (The f-electron count for thorium metal is a non-integer due to a 5f–6d overlap.)[14] Among the actinides up to californium, which can be studied in at least milligram quantities, thorium has the highest melting and boiling points and second-lowest density; only actinium is lighter. Thorium's boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points.[b]

The properties of thorium vary widely depending on the degree of impurities in the sample. The major impurity is usually

lattice parameters, perhaps due to microscopic voids forming in the metal when it is cast.[11] These values lie between those of its neighbours actinium (10.1 g/cm3) and protactinium (15.4 g/cm3), part of a trend across the early actinides.[11]

Thorium can form

eutectic mixtures with chromium and uranium, and it is completely miscible in both solid and liquid states with its lighter congener cerium.[11]

Isotopes

Main article: Isotopes of thorium

There are seven naturally occurring isotopes of Thorium but none are stable. 232Th is one of the two nuclides beyond bismuth (the other being

nuclear subshell with 142 neutrons.[19][20] Thorium has a characteristic terrestrial isotopic composition, with atomic weight 232.0377±0.0004.[2] It is one of only four radioactive elements (along with bismuth, protactinium and uranium) that occur in large enough quantities on Earth for a standard atomic weight to be determined.[2]

Thorium nuclei are susceptible to

partial half-life of this process is very large at over 1021 years and alpha decay predominates.[27][28]

Ball-and-arrow presentation of the thorium decay series
The 4n decay chain of 232Th, commonly called the "thorium series"

In total, 32

radioisotopes have been characterised, which range in mass number from 207[29] to 238.[27] After 232Th, the most stable of them (with respective half-lives) are 230Th (75,380 years), 229Th (7,917 years), 228Th (1.92 years), 234Th (24.10 days), and 227Th (18.68 days). All of these isotopes occur in nature as trace radioisotopes due to their presence in the decay chains of 232Th, 235U, 238U, and 237Np: the last of these is long extinct in nature due to its short half-life (2.14 million years), but is continually produced in minute traces from neutron capture in uranium ores. All of the remaining thorium isotopes have half-lives that are less than thirty days and the majority of these have half-lives that are less than ten minutes.[16] 233Th (half-life 22 minutes) occurs naturally as the result of neutron activation of natural 232Th.[30] 226Th (half-life 31 minutes) has not yet been observed in nature, but would be produced by the still-unobserved double beta decay of natural 226Ra.[31]

In deep seawaters the isotope 230Th makes up to 0.02% of natural thorium.[8] This is because its parent 238U is soluble in water, but 230Th is insoluble and precipitates into the sediment. Uranium ores with low thorium concentrations can be purified to produce gram-sized thorium samples of which over a quarter is the 230Th isotope, since 230Th is one of the daughters of 238U.[27] The International Union of Pure and Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element in 2013; it had formerly been considered a mononuclidic element.[32]

Thorium has three known

isomeric transition, the emitted gamma radiation is in the ultraviolet range.[34][35][c] The nuclear transition from 229Th to 229mTh is being investigated for a nuclear clock.[35]

Different isotopes of thorium are chemically identical, but have slightly differing physical properties: for example, the densities of pure 228Th, 229Th, 230Th, and 232Th are respectively expected to be 11.5, 11.6, 11.6, and 11.7 g/cm3.

fissionable with a bare critical mass of 2839 kg, although with steel reflectors this value could drop to 994 kg.[37][d] 232Th is not fissionable, but it is fertile as it can be converted to fissile 233U by neutron capture and subsequent beta decay.[37][38]

Radiometric dating

Two radiometric dating methods involve thorium isotopes: uranium–thorium dating, based on the decay of 234U to 230Th, and ionium–thorium dating, which measures the ratio of 232Th to 230Th.[e] These rely on the fact that 232Th is a primordial radioisotope, but 230Th only occurs as an intermediate decay product in the decay chain of 238U.[39] Uranium–thorium dating is a relatively short-range process because of the short half-lives of 234U and 230Th relative to the age of the Earth: it is also accompanied by a sister process involving the alpha decay of 235U into 231Th, which very quickly becomes the longer-lived 231Pa, and this process is often used to check the results of uranium–thorium dating. Uranium–thorium dating is commonly used to determine the age of calcium carbonate materials such as speleothem or coral, because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floor sediments, where their ratios are measured. The scheme has a range of several hundred thousand years.[39][40] Ionium–thorium dating is a related process, which exploits the insolubility of thorium (both 232Th and 230Th) and thus its presence in ocean sediments to date these sediments by measuring the ratio of 232Th to 230Th.[41][42] Both of these dating methods assume that the proportion of 230Th to 232Th is a constant during the period when the sediment layer was formed, that the sediment did not already contain thorium before contributions from the decay of uranium, and that the thorium cannot migrate within the sediment layer.[41][42]

Chemistry

Main article: Thorium compounds

A thorium atom has 90 electrons, of which four are

relativistic effects, which become stronger near the bottom of the periodic table, specifically the relativistic spin–orbit interaction. The closeness in energy levels of the 5f, 6d, and 7s energy levels of thorium results in thorium almost always losing all four valence electrons and occurring in its highest possible oxidation state of +4. This is different from its lanthanide congener cerium, in which +4 is also the highest possible state, but +3 plays an important role and is more stable. Thorium is much more similar to the transition metals zirconium and hafnium than to cerium in its ionization energies and redox potentials, and hence also in its chemistry: this transition-metal-like behaviour is the norm in the first half of the actinide series, from actinium to americium.[44][45][46]

Crystal structure of fluorite
Thorium dioxide has the fluorite crystal structure.
Th4+: __  /  O2−: __

Despite the anomalous electron configuration for gaseous thorium atoms, metallic thorium shows significant 5f involvement. A hypothetical metallic state of thorium that had the [Rn]6d27s2 configuration with the 5f orbitals above the

hexagonal close packed like the group 4 elements titanium, zirconium, and hafnium, and not face-centred cubic as it actually is. The actual crystal structure can only be explained when the 5f states are invoked, proving that thorium is metallurgically a true actinide.[14]

Tetravalent thorium compounds are usually colourless or yellow, like those of

main group elements of the s-block.[47][f] Thorium and uranium are the most investigated of the radioactive elements because their radioactivity is low enough not to require special handling in the laboratory.[48]

Reactivity

Thorium is a highly

standard reduction potential of −1.90 V for the Th4+/Th couple, it is somewhat more electropositive than zirconium or aluminium.[49] Finely divided thorium metal can exhibit pyrophoricity, spontaneously igniting in air.[11] When heated in air, thorium turnings ignite and burn with a brilliant white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion.[11] Such samples slowly tarnish, becoming grey and finally black at the surface.[11]

At

fluorosilicate ions;[11][51] if these are not present, passivation by the nitrate can occur, as with uranium and plutonium.[11][52][53]

Crystal structure of thorium tetrafluoride
Crystal structure of thorium tetrafluoride
Th4+: __  /  F−: __

Inorganic compounds

Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.

hygroscopic and reacts readily with water and many gases;[57] it dissolves easily in concentrated nitric acid in the presence of fluoride.[58]

When heated in air, thorium dioxide emits intense blue light; the light becomes white when ThO2 is mixed with its lighter homologue

free radicals that appear in high concentration in a flame, whose deexcitation releases large amounts of energy. The addition of 1% cerium dioxide, as in gas mantles, heightens the effect by increasing emissivity in the visible region of the spectrum; but because cerium, unlike thorium, can occur in multiple oxidation states, its charge and hence visible emissivity will depend on the region on the flame it is found in (as such regions vary in their chemical composition and hence how oxidising or reducing they are).[59]

Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium.[60]

All four thorium tetrahalides are known, as are some low-valent bromides and iodides:[61] the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water.[62] Many related polyhalide ions are also known.[61] Thorium tetrafluoride has a monoclinic crystal structure like those of zirconium tetrafluoride and hafnium tetrafluoride, where the Th4+ ions are coordinated with F− ions in somewhat distorted square antiprisms.[61] The other tetrahalides instead have dodecahedral geometry.[62] Lower iodides ThI3 (black) and ThI2 (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead contain Th4+ and could be more clearly formulated as electride compounds.[61] Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides.[61] For example, when treated with potassium fluoride and hydrofluoric acid, Th4+ forms the complex anion [ThF6]2− (hexafluorothorate(IV)), which precipitates as an insoluble salt, K2[ThF6] (potassium hexafluorothorate(IV)).[51]

Thorium borides, carbides, silicides, and nitrides are refractory materials, like those of uranium and plutonium, and have thus received attention as possible nuclear fuels.[54] All four heavier pnictogens (phosphorus, arsenic, antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known.[63] Thorium reacts with hydrogen to form the thorium hydrides ThH2 and Th4H15, the latter of which is superconducting below 7.5–8 K; at standard temperature and pressure, it conducts electricity like a metal.[64] The hydrides are thermally unstable and readily decompose upon exposure to air or moisture.[65]

Structure of thorocene
Sandwich molecule structure of thorocene

Coordination compounds

In an acidic aqueous solution, thorium occurs as the tetrapositive

aqua ion [Th(H2O)9]4+, which has tricapped trigonal prismatic molecular geometry:[66][67] at pH < 3, the solutions of thorium salts are dominated by this cation.[66] The Th4+ ion is the largest of the tetrapositive actinide ions, and depending on the coordination number can have a radius between 0.95 and 1.14 Å.[66] It is quite acidic due to its high charge, slightly stronger than sulfurous acid: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent than Fe3+), predominantly to [Th2(OH)2]6+ in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxide Th(OH)4 forms and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down before the precipitation).[68] As a hard Lewis acid, Th4+ favours hard ligands with oxygen atoms as donors: complexes with sulfur atoms as donors are less stable and are more prone to hydrolysis.[44]

High coordination numbers are the rule for thorium due to its large size. Thorium nitrate pentahydrate was the first known example of coordination number 11, the oxalate tetrahydrate has coordination number 10, and the borohydride (first prepared in the Manhattan Project) has coordination number 14.[68] These thorium salts are known for their high solubility in water and polar organic solvents.[13]

Many other inorganic thorium compounds with polyatomic anions are known, such as the

EDTA, are much more stable. In natural thorium-containing waters, organic thorium complexes usually occur in concentrations orders of magnitude higher than the inorganic complexes, even when the concentrations of inorganic ligands are much greater than those of organic ligands.[66]

Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2
Piano-stool molecule structure of (η8-C8H8)ThCl2(THF)2

In January 2021, the aromaticity has been observed in a large

metal cluster anion consisting of 12 bismuth atoms stabilised by a center thorium cation.[70] This compound was shown to be surprisingly stable, unlike many previous known aromatic metal clusters
.

Organothorium compounds

Most of the work on organothorium compounds has focused on the

K2C8H8 with thorium tetrachloride in tetrahydrofuran (THF) at the temperature of dry ice, or by reacting thorium tetrafluoride with MgC8H8.[71] It is unstable in air and decomposes in water or at 190 °C.[71] Half sandwich compounds are also known, such as (η8-C8H8)ThCl2(THF)2, which has a piano-stool structure and is made by reacting thorocene with thorium tetrachloride in tetrahydrofuran.[44]

The simplest of the cyclopentadienyls are Th(C5H5)3 and Th(C5H5)4: many derivatives are known. The former (which has two forms, one purple and one green)

aryl derivatives are prepared from the chloride derivative and have been used to study the nature of the Th–C sigma bond.[72]

Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH2C6H5)4, and tetraallylthorium, Th(CH2CH=CH2)4, are known, but their structures have not been determined. They decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion [Th(CH3)7]3−, heptamethylthorate(IV), which forms the salt [Li(tmeda)]3[Th(CH3)7] (tmeda = (CH3)2NCH2CH2N(CH3)2). Although one methyl group is only attached to the thorium atom (Th–C distance 257.1 pm) and the other six connect the lithium and thorium atoms (Th–C distances 265.5–276.5 pm), they behave equivalently in solution. Tetramethylthorium, Th(CH3)4, is not known, but its adducts are stabilised by phosphine ligands.[44]

Occurrence

Main article: Occurrence of thorium

Formation

232Th is a primordial nuclide, having existed in its current form for over ten billion years; it was formed during the

neutron drip line, as neutrons are captured much faster than the resulting nuclides can beta decay back toward stability. Neutron capture is the only way for stars to synthesise elements beyond iron because of the increased Coulomb barriers that make interactions between charged particles difficult at high atomic numbers and the fact that fusion beyond 56Fe is endothermic.[76] Because of the abrupt loss of stability past 209Bi, the r-process is the only process of stellar nucleosynthesis that can create thorium and uranium; all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.[74][77][78]

Abundance

In the universe, thorium is among the rarest of the primordial elements at rank 77th in cosmic abundance[74][79] because it is one of the two elements that can be produced only in the r-process (the other being uranium), and also because it has slowly been decaying away from the moment it formed. The only primordial elements rarer than thorium are thulium, lutetium, tantalum, and rhenium, the odd-numbered elements just before the third peak of r-process abundances around the heavy platinum group metals, as well as uranium.[74][76][g] In the distant past the abundances of thorium and uranium were enriched by the decay of plutonium and curium isotopes, and thorium was enriched relative to uranium by the decay of 236U to 232Th and the natural depletion of 235U, but these sources have long since decayed and no longer contribute.[80]

In the Earth's crust, thorium is much more abundant: with an abundance of 8.1 g/tonne, it is one of the most abundant of the heavy elements, almost as abundant as lead (13 g/tonne) and more abundant than tin (2.1 g/tonne).[81] This is because thorium is likely to form oxide minerals that do not sink into the core; it is classified as a lithophile under the Goldschmidt classification, meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though the refractory elements have the same relative abundances in the Earth as in the Solar System as a whole, there is more accessible thorium than heavy platinum group metals in the crust.[82]

radiogenic heat from the decay of 232Th (violet) is a major contributor to the earth's internal heat budget. Of the four major nuclides providing this heat, 232Th has grown to provide the most heat as the other ones decayed faster than thorium.[83][84][85][86]

On Earth

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.[27] 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.)[87] 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.[56] 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.[27][h]

Thorium only occurs as a minor constituent of most minerals, and was for this reason previously thought to be rare.

metamict (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium.[92] 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.[93]

zirconium silicate, ZrSiO4) up to 0.4% thorium.[89]

Thorium dioxide occurs as the rare mineral thorianite. Due to its being isotypic with uranium dioxide, these two common actinide dioxides can form solid-state solutions and the name of the mineral changes according to the ThO2 content.[89][i] Thorite (chiefly thorium silicate, ThSiO4), also has a high thorium content and is the mineral in which thorium was first discovered.[89] In thorium silicate minerals, the Th4+ and SiO4−4 ions are often replaced with M3+ (where M = Sc, Y, or Ln) and phosphate (PO3−4) ions respectively.[89] Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released. The Th4+ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can be higher.[56]

History

Norse god of thunder, raising his hammer Mjölnir in a battle against the giants.[95]

Erroneous report

In 1815, the Swedish chemist

yttrium orthophosphate.[38][96][99][100]

Discovery

In 1828,

Royal Frederick University in Christiania (today called Oslo).[101] The elder Esmark determined that it was not a known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element.[38] He published his findings in 1829, having isolated an impure sample by reducing K[ThF5] (potassium pentafluorothorate(IV)) with potassium metal.[102][103][104] Berzelius reused the name of the previous supposed element discovery[102][105] and named the source mineral thorite.[38]

Jöns Jacob Berzelius
Jöns Jacob Berzelius, who first identified thorium as a new element

Berzelius made some initial characterisations of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio of thorium oxide was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and so calculated that the atomic mass was 7.5 times that of oxygen (120

amu); it is actually 15 times as large.[j] He determined that thorium was a very electropositive metal, ahead of cerium and behind zirconium in electropositivity.[106] Metallic thorium was isolated for the first time in 1914 by Dutch entrepreneurs Dirk Lely Jr. and Lodewijk Hamburger.[k]

Initial chemical classification

In the periodic table published by Dmitri Mendeleev in 1869, thorium and the rare-earth elements were placed outside the main body of the table, at the end of each vertical period after the alkaline earth metals. This reflected the belief at that time that thorium and the rare-earth metals were divalent. With the later recognition that the rare earths were mostly trivalent and thorium was tetravalent, Mendeleev moved cerium and thorium to group IV in 1871, which also contained the modern carbon group (group 14) and titanium group (group 4), because their maximum oxidation state was +4.[109][110] Cerium was soon removed from the main body of the table and placed in a separate lanthanide series; thorium was left with group 4 as it had similar properties to its supposed lighter congeners in that group, such as titanium and zirconium.[111][l]

First uses

Gas mantle
World War II thorium dioxide gas mantle

While thorium was discovered in 1828 its first application dates only from 1885, when Austrian chemist Carl Auer von Welsbach invented the gas mantle, a portable source of light which produces light from the incandescence of thorium oxide when heated by burning gaseous fuels.[38] Many applications were subsequently found for thorium and its compounds, including ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid.[112]

Radioactivity

Thorium was first observed to be radioactive in 1898, by the German chemist Gerhard Carl Schmidt and later that year, independently, by the Polish-French physicist Marie Curie. It was the second element that was found to be radioactive, after the 1896 discovery of radioactivity in uranium by French physicist Henri Becquerel.[113][114][115] Starting from 1899, the New Zealand physicist Ernest Rutherford and the American electrical engineer Robert Bowie Owens studied the radiation from thorium; initial observations showed that it varied significantly. It was determined that these variations came from a short-lived gaseous daughter of thorium, which they found to be a new element. This element is now named radon, the only one of the rare radioelements to be discovered in nature as a daughter of thorium rather than uranium.[116]

After accounting for the contribution of radon, Rutherford, now working with the British physicist

sexual impotence. In 1932, most of these uses were banned in the United States after a federal investigation into the health effects of radioactivity.[119] 10,000 individuals in the United States had been injected with thorium during X-ray diagnosis; they were later found to suffer health issues such as leukaemia and abnormal chromosomes.[56] Public interest in radioactivity had declined by the end of the 1930s.[119]

Glenn T. Seaborg
Glenn T. Seaborg, who settled thorium's location in the f-block

Further classification

Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 and group 6 respectively; the existence of the lanthanides in the sixth row was considered to be a one-off fluke. In 1892, British chemist Henry Bassett postulated a second extra-long periodic table row to accommodate known and undiscovered elements, considering thorium and uranium to be analogous to the lanthanides. In 1913, Danish physicist Niels Bohr published a theoretical model of the atom and its electron orbitals, which soon gathered wide acceptance. The model indicated that the seventh row of the periodic table should also have f-shells filling before the d-shells that were filled in the transition elements, like the sixth row with the lanthanides preceding the 5d transition metals.[109] The existence of a second inner transition series, in the form of the actinides, was not accepted until similarities with the electron structures of the lanthanides had been established;[120] Bohr suggested that the filling of the 5f orbitals may be delayed to after uranium.[109]

It was only with the discovery of the first

transuranic elements, which from plutonium onward have dominant +3 and +4 oxidation states like the lanthanides, that it was realised that the actinides were indeed filling f-orbitals rather than d-orbitals, with the transition-metal-like chemistry of the early actinides being the exception and not the rule.[121] In 1945, when American physicist Glenn T. Seaborg and his team had discovered the transuranic elements americium and curium, he proposed the actinide concept, realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener of hafnium in a fourth d-block row.[111][m]

Phasing out

In the 1990s, most applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements were found.[38][124] Despite its radioactivity, the element has remained in use for applications where no suitable alternatives could be found. A 1981 study by the Oak Ridge National Laboratory in the United States estimated that using a thorium gas mantle every weekend would be safe for a person,[124] but this was not the case for the dose received by people manufacturing the mantles or for the soils around some factory sites.[125] Some manufacturers have changed to other materials, such as yttrium.[126] As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even falsely claiming them to be non-radioactive.[124][127]

Nuclear power

Main articles: Thorium-based nuclear power and Thorium fuel cycle
Indian Point Energy Center
The Indian Point Energy Center (Buchanan, New York, United States), home of the world's first thorium reactor

Thorium has been used as a power source on a prototype scale. The earliest thorium-based reactor was built at the Indian Point Energy Center located in Buchanan, New York, United States in 1962.[128] China may be the first to have a shot at commercialising the technology.[129] The country with the largest estimated reserves of thorium in the world is India, which has sparse reserves of uranium. In the 1950s, India targeted achieving energy independence with their three-stage nuclear power programme.[130][131] In most countries, uranium was relatively abundant and the progress of thorium-based reactors was slow; in the 20th century, three reactors were built in India and twelve elsewhere.[132] Large-scale research was begun in 1996 by the International Atomic Energy Agency to study the use of thorium reactors; a year later, the United States Department of Energy started their research. Alvin Radkowsky of Tel Aviv University in Israel was the head designer of Shippingport Atomic Power Station in Pennsylvania, the first American civilian reactor to breed thorium.[133] He founded a consortium to develop thorium reactors, which included other laboratories: Raytheon Nuclear Inc. and Brookhaven National Laboratory in the United States, and the Kurchatov Institute in Russia.[134]

In the 21st century, thorium's potential for reducing nuclear proliferation and its

Advanced Heavy Water Reactor
(AHWR). In 2009, the chairman of the Indian Atomic Energy Commission said that India has a "long-term objective goal of becoming energy-independent based on its vast thorium resources."

On 16 June 2023 China's National Nuclear Safety Administration issued a licence to the Shanghai Institute of Applied Physics (SINAP) of the Chinese Academy of Sciences to begin operating the TMSR-LF1, 2 MWt liquid fuel thorium-based molten salt experimental reactor which was completed in August 2021.[138] China is believed to have one of the largest thorium reserves in the world. The exact size of those reserves has not been publicly disclosed, but it is estimated to be enough to meet the country's total energy needs for more than 20,000 years.[139]

Nuclear weapons

When gram quantities of plutonium were first produced in the Manhattan Project, it was discovered that a minor isotope (240Pu) underwent significant spontaneous fission, which brought into question the viability of a plutonium-fuelled gun-type nuclear weapon. While the Los Alamos team began work on the implosion-type weapon to circumvent this issue, the Chicago team discussed reactor design solutions. Eugene Wigner proposed to use the 240Pu-contaminated plutonium to drive the conversion of thorium into 233U in a special converter reactor. It was hypothesized that the 233U would then be usable in a gun-type weapon, though concerns about contamination from 232U were voiced. Progress on the implosion weapon was sufficient, and this converter was not developed further, but the design had enormous influence on the development of nuclear energy. It was the first detailed description of a highly enriched water-cooled, water-moderated reactor similar to future naval and commercial power reactors.[140]

During the

nuclear bomb; they fired a test bomb in 1955.[141] They concluded that a 233U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary uranium–plutonium bombs,[142] especially since 233U is difficult to produce in isotopically pure form.[141]

Thorium metal was used in the radiation case of at least one nuclear weapon design deployed by the United States (the W71).[143]

Production

See also: List of countries by thorium resources
Lower-bound estimates of thorium reserves in thousand tonnes, 2014[141]
Country Reserves
India 846
Brazil 632
Australia 595
United States 595
Egypt 380
Turkey 374
Venezuela 300
Canada 172
Russia 155
South Africa 148
China 100
Norway 87
Greenland 86
Finland 60
Sweden 50
Kazakhstan 50
Other countries 1725
World total 6355

The low demand makes working mines for extraction of thorium alone not profitable, and it is almost always extracted with the rare earths, which themselves may be by-products of production of other minerals.[144] The current reliance on monazite for production is due to thorium being largely produced as a by-product; other sources such as thorite contain more thorium and could easily be used for production if demand rose.[145] Present knowledge of the distribution of thorium resources is poor, as low demand has led to exploration efforts being relatively minor.[146] In 2014, world production of the monazite concentrate, from which thorium would be extracted, was 2,700 tonnes.[147]

The common production route of thorium constitutes concentration of thorium minerals; extraction of thorium from the concentrate; purification of thorium; and (optionally) conversion to compounds, such as thorium dioxide.[148]

Concentration

There are two categories of thorium minerals for thorium extraction: primary and secondary. Primary deposits occur in acidic granitic magmas and pegmatites. They are concentrated, but of small size. Secondary deposits occur at the mouths of rivers in granitic mountain regions. In these deposits, thorium is enriched along with other heavy minerals.[49] Initial concentration varies with the type of deposit.[148]

For the primary deposits, the source pegmatites, which are usually obtained by mining, are divided into small parts and then undergo

thickening, filtration, and calcination. The result is a concentrate with rare-earth content of up to 90%.[148] Secondary materials (such as coastal sands) undergo gravity separation. Magnetic separation follows, with a series of magnets of increasing strength. Monazite obtained by this method can be as pure as 98%.[148]

Industrial production in the 20th century relied on treatment with hot, concentrated sulfuric acid in cast iron vessels, followed by selective precipitation by dilution with water, as on the subsequent steps. This method relied on the specifics of the technique and the concentrate grain size; many alternatives have been proposed, but only one has proven effective economically: alkaline digestion with hot sodium hydroxide solution. This is more expensive than the original method but yields a higher purity of thorium; in particular, it removes phosphates from the concentrate.[148]

Acid digestion

Acid digestion is a two-stage process, involving the use of up to 93% sulfuric acid at 210–230 °C. First, sulfuric acid in excess of 60% of the sand mass is added, thickening the reaction mixture as products are formed. Then, fuming sulfuric acid is added and the mixture is kept at the same temperature for another five hours to reduce the volume of solution remaining after dilution. The concentration of the sulfuric acid is selected based on reaction rate and viscosity, which both increase with concentration, albeit with viscosity retarding the reaction. Increasing the temperature also speeds up the reaction, but temperatures of 300 °C and above must be avoided, because they cause insoluble thorium pyrophosphate to form. Since dissolution is very exothermic, the monazite sand cannot be added to the acid too quickly. Conversely, at temperatures below 200 °C the reaction does not go fast enough for the process to be practical. To ensure that no precipitates form to block the reactive monazite surface, the mass of acid used must be twice that of the sand, instead of the 60% that would be expected from stoichiometry. The mixture is then cooled to 70 °C and diluted with ten times its volume of cold water, so that any remaining monazite sinks to the bottom while the rare earths and thorium remain in solution. Thorium may then be separated by precipitating it as the phosphate at pH 1.3, since the rare earths do not precipitate until pH 2.[148]

Alkaline digestion

Alkaline digestion is carried out in 30–45%

titanium hydroxide is deleterious as it binds thorium and prevents it from dissolving fully.[148]

Purification

High thorium concentrations are needed in nuclear applications. In particular, concentrations of atoms with high neutron capture

complexation of Th4+ are used. For example, following alkaline digestion and the removal of phosphate, the resulting nitrato complexes of thorium, uranium, and the rare earths can be separated by extraction with tributyl phosphate in kerosene.[148]

Modern applications

Non-radioactivity-related uses of thorium have been in decline since the 1950s[149] due to environmental concerns largely stemming from the radioactivity of thorium and its decay products.[38][124]

Most thorium applications use its dioxide (sometimes called "thoria" in the industry), rather than the metal. This compound has a melting point of 3300 °C (6000 °F), the highest of all known oxides; only a few substances have higher melting points.[56] This helps the compound remain solid in a flame, and it considerably increases the brightness of the flame; this is the main reason thorium is used in gas lamp mantles.[150] All substances emit energy (glow) at high temperatures, but the light emitted by thorium is nearly all in the visible spectrum, hence the brightness of thorium mantles.[59]

Energy, some of it in the form of visible light, is emitted when thorium is exposed to a source of energy itself, such as a cathode ray, heat, or

pseudoscientific health bracelets.[153]

During the production of

incandescent filaments, recrystallisation of tungsten is significantly lowered by adding small amounts of thorium dioxide to the tungsten sintering powder before drawing the filaments.[149] A small addition of thorium to tungsten thermocathodes considerably reduces the work function of electrons; as a result, electrons are emitted at considerably lower temperatures.[38] Thorium forms a one-atom-thick layer on the surface of tungsten. The work function from a thorium surface is lowered possibly because of the electric field on the interface between thorium and tungsten formed due to thorium's greater electropositivity.[154] Since the 1920s, thoriated tungsten wires have been used in electronic tubes and in the cathodes and anticathodes of X-ray tubes and rectifiers.The reactivity of thorium with atmospheric oxygen required the introduction of an evaporated magnesium layer as a getter for impurities in the evacuated tubes, giving them their characteristic metallic inner coating.[155]: 16  The introduction of transistors in the 1950s significantly diminished this use, but not entirely.[149] Thorium dioxide is used in gas tungsten arc welding (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability.[38] Thorium oxide is being replaced in this use with other oxides, such as those of zirconium, cerium, and lanthanum.[156][157]

Thorium dioxide is found in refractory ceramics, such as high-temperature laboratory crucibles,[38] either as the primary ingredient or as an addition to zirconium dioxide. An alloy of 90% platinum and 10% thorium is an effective catalyst for oxidising ammonia to nitrogen oxides, but this has been replaced by an alloy of 95% platinum and 5% rhodium because of its better mechanical properties and greater durability.[149]

Three lenses from yellowed to transparent left-to-right
Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)

When added to

lenses for cameras and scientific instruments.[50] The radiation from these lenses can darken them and turn them yellow over a period of years and it degrades film, but the health risks are minimal.[158] Yellowed lenses may be restored to their original colourless state by lengthy exposure to intense ultraviolet radiation. Thorium dioxide has since been replaced in this application by rare-earth oxides, such as lanthanum, as they provide similar effects and are not radioactive.[149]

Thorium tetrafluoride is used as an anti-reflection material in multilayered optical coatings. It is transparent to electromagnetic waves having wavelengths in the range of 0.350–12 μm, a range that includes near ultraviolet, visible and mid infrared light. Its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material.[159] Replacements for thorium tetrafluoride are being developed as of the 2010s,[160] which include Lanthanum trifluoride.

Mag-Thor alloys (also called thoriated magnesium) found use in some aerospace applications, though such uses have been phased out due to concerns over radioactivity.

Potential use for nuclear energy

Main articles: Thorium-based nuclear power and Thorium fuel cycle

The main nuclear power source in a reactor is the neutron-induced fission of a nuclide; the synthetic fissile

233Pa and then the fissile 233U:[38]

Transmutations in the thorium fuel cycle
237Np
↑
231U ← 232U ↔ 233U ↔ 234U ↔ 235U ↔ 236U → 237U
↓ ↑ ↑ ↑
231Pa → 232Pa ← 233Pa → 234Pa
↑ ↑
230Th → 231Th ← 232Th → 233Th
  • Nuclides with a yellow background in italic have half-lives under 30 days
  • Nuclides in bold have half-lives over 1,000,000 years
  • Nuclides in red frames are
    fissile

233U is fissile and can be used as a nuclear fuel in the same way as 235U or

fast breeder reactors where 238U undergoes neutron capture to become 239U, beta decaying to first 239Np and then fissile 239Pu.[163]

The fission of 233
92U
 produces 2.48 neutrons on average.[164] One neutron is needed to keep the fission reaction going. For a self-contained continuous breeding cycle, one more neutron is needed to breed a new 233
92U
 atom from the fertile 232
90Th
. This leaves a margin of 0.45 neutrons (or 18% of the neutron flux) for losses.

Advantages

Thorium is more abundant than uranium, and can satisfy world energy demands for longer.

molten salt reactors
.

232Th absorbs neutrons more readily than 238U, and 233U has a higher probability of fission upon neutron capture (92.0%) than 235U (85.5%) or 239Pu (73.5%).[166] It also releases more neutrons upon fission on average.[165] A single neutron capture by 238U produces transuranic waste along with the fissile 239Pu, but 232Th only produces this waste after five captures, forming 237Np. This number of captures does not happen for 98–99% of the 232Th nuclei because the intermediate products 233U or 235U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide fuels to minimise the generation of transuranics and maximise the destruction of plutonium.[167]

Thorium fuels result in a safer and better-performing

coefficient of thermal expansion. It is more stable chemically than the now-common fuel uranium dioxide, because the latter oxidises to triuranium octoxide (U3O8), becoming substantially less dense.[168]

Disadvantages

The used fuel is difficult and dangerous to reprocess because many of the daughters of 232Th and 233U are strong gamma emitters.[165] All 233U production methods result in impurities of 232U, either from parasitic knock-out (n,2n) reactions on 232Th, 233Pa, or 233U that result in the loss of a neutron, or from double neutron capture of 230Th, an impurity in natural 232Th:[169]

230
90Th
 + n → 231
90Th
 + γ β−→25.5 h 231
91Pa
 ( α→3.28 × 104
 y 227
89Ac
 )
231
91Pa
 + n → 232
91Pa
 + γ β−→1.3 d 232
92U
 α→69 y

232U by itself is not particularly harmful, but quickly decays to produce the strong gamma emitter

molten salt reactors during their operation, so that it does not have a chance to capture a neutron and will only decay to 233U.[171]

The irradiation of 232Th with neutrons, followed by its processing, need to be mastered before these advantages can be realised, and this requires more advanced technology than the uranium and plutonium fuel cycle;

radiotoxicity of spent nuclear fuel.[171]

Hazards and health effects

timothy-grass
seed

Radiological

Natural thorium decays very slowly compared to many other radioactive materials, and the emitted

alpha radiation cannot penetrate human skin. As a result, handling small amounts of thorium, such as those in gas mantles, is considered safe, although the use of such items may pose some risks.[176] Exposure to an aerosol of thorium, such as contaminated dust, can lead to increased risk of cancers of the lung, pancreas, and blood, as lungs and other internal organs can be penetrated by alpha radiation.[176] Internal exposure to thorium leads to increased risk of liver diseases.[177]

The decay products of 232Th include more dangerous radionuclides such as radium and radon. Although relatively little of those products are created as the result of the slow decay of thorium, a proper assessment of the radiological toxicity of 232Th must include the contribution of its daughters, some of which are dangerous

mammogram.[181]

Some

nuclear safety agencies make recommendations about the use of thorium mantles and have raised safety concerns regarding their manufacture and disposal; the radiation dose from one mantle is not a serious problem, but that from many mantles gathered together in factories or landfills is.[177]

Biological

Thorium is odourless and tasteless.

toxic, especially in the presence of strong complex-forming ions such as citrate that carry the thorium into the body in soluble form.[179] If a thorium-containing object has been chewed or sucked, it loses 0.4% of thorium and 90% of its dangerous daughters to the body.[127] Three-quarters of the thorium that has penetrated the body accumulates in the skeleton. Absorption through the skin is possible, but is not a likely means of exposure.[176] Thorium's low solubility in water also means that excretion of thorium by the kidneys and faeces is rather slow.[179]

Tests on the thorium uptake of workers involved in monazite processing showed thorium levels above recommended limits in their bodies, but no adverse effects on health were found at those moderately low concentrations. No chemical toxicity has yet been observed in the

tracheobronchial tract and the lungs from exposure to thorium.[184] People who work with thorium compounds are at a risk of dermatitis. It can take as much as thirty years after the ingestion of thorium for symptoms to manifest themselves.[56] Thorium has no known biological role.[56]

Chemical

Powdered thorium metal is pyrophoric: it ignites spontaneously in air.[11] In 1964, the United States Department of the Interior listed thorium as "severe" on a table entitled "Ignition and explosibility of metal powders". Its ignition temperature was given as 270 °C (520 °F) for dust clouds and 280 °C (535 °F) for layers. Its minimum explosive concentration was listed as 0.075 oz/cu ft (0.075 kg/m3); the minimum igniting energy for (non-submicron) dust was listed as 5 mJ.[185]

In 1956, the

third-degree burns.[186][187][188]

Exposure routes

Thorium exists in very small quantities everywhere on Earth although larger amounts exist in certain parts: the average human contains about 40 micrograms of thorium and typically consumes three micrograms per day.[56] Most thorium exposure occurs through dust inhalation; some thorium comes with food and water, but because of its low solubility, this exposure is negligible.[179]

Exposure is raised for people who live near thorium deposits or radioactive waste disposal sites, those who live near or work in uranium, phosphate, or tin processing factories, and for those who work in gas mantle production.[189] Thorium is especially common in the Tamil Nadu coastal areas of India, where residents may be exposed to a naturally occurring radiation dose ten times higher than the worldwide average.[190] It is also common in northern Brazilian coastal areas, from south Bahia to Guarapari, a city with radioactive monazite sand beaches, with radiation levels up to 50 times higher than world average background radiation.[191]

Another possible source of exposure is thorium dust produced at weapons testing ranges, as thorium is used in the guidance systems of some missiles. This has been blamed for a high incidence of birth defects and cancer at Salto di Quirra on the Italian island of Sardinia.[192]

See also

Explanatory notes

  1. ^ Bismuth is very slightly radioactive, but its half-life (1.9×1019 years) is so long that its decay is negligible even over geological timespans.
  2. ^ Behind osmium, tantalum, tungsten, and rhenium;[11] higher boiling points are speculated to be found in the 6d transition metals, but they have not been produced in large enough quantities to test this prediction.[15]
  3. ^ Gamma rays are distinguished by their origin in the nucleus, not their wavelength; hence there is no lower limit to gamma energy derived from radioactive decay.[36]
  4. ^ a b A fissionable nuclide is capable of undergoing fission (even with a low probability) after capturing a high-energy neutron. Some of these nuclides can be induced to fission with low-energy thermal neutrons with a high probability; they are referred to as fissile. A fertile nuclide is one that could be bombarded with neutrons to produce a fissile nuclide. Critical mass is the mass of a ball of a material which could undergo a sustained nuclear chain reaction.
  5. ^ The name ionium for 230Th is a remnant from a period when different isotopes were not recognised to be the same element and were given different names.
  6. p-block.[47]
  7. ^ An even number of either protons or neutrons generally increases nuclear stability of isotopes, compared to isotopes with odd numbers. Elements with odd atomic numbers have no more than two stable isotopes; even-numbered elements have multiple stable isotopes, with tin (element 50) having ten.[16]
  8. neutron irradiation of 232Th, and is extremely rare in nature.[27]
  9. ^ Thorianite refers to minerals with 75–100 mol% ThO2; uranothorianite, 25–75 mol% ThO2; thorian uraninite, 15–25 mol% ThO2; uraninite, 0–15 mol% ThO2.[89]
  10. atomic weight
    values two-thirds of their actual ones, and thorium and uranium are given values half of the actual ones.
  11. ^ The main difficulty in isolating thorium lies not in its chemical electropositivity, but in the close association of thorium in nature with the rare-earth elements and uranium, which collectively are difficult to separate from each other. Swedish chemist Lars Fredrik Nilson, the discoverer of scandium, had previously made an attempt to isolate thorium metal in 1882, but was unsuccessful at achieving a high degree of purity.[107] Lely and Hamburger obtained 99% pure thorium metal by reducing thorium chloride with sodium metal.[108] A simpler method leading to even higher purity was discovered in 1927 by American engineers John Marden and Harvey Rentschler, involving the reduction of thorium oxide with calcium in presence of calcium chloride.[108]
  12. Julius Lothar Meyer in 1870, all of which exclude the rare earths and thorium.[109]
  13. ^ The filling of the 5f subshell from the beginning of the actinide series was confirmed when the 6d elements were reached in the 1960s, proving that the 4f and 5f series are of equal length. Lawrencium has only +3 as an oxidation state, breaking from the trend of the late actinides towards the +2 state; it thus fits as a heavier congener of lutetium. Even more importantly, the next element, rutherfordium, was found to behave like hafnium and show only a +4 state.[46][122] Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group 4 element".[123]
  14. 252Es. Of these, only 235U have significant amounts in nature, and only 233U and 239Pu can be bred from naturally occurring nuclei with a single neutron capture.[162]

Citations

  1. ^ "Standard Atomic Weights: Thorium". CIAAW. 2013.
  2. ^
    ISSN 1365-3075
    .
  3. ^
    ISBN 978-1-62708-155-9
    .
  4. PMID 31276242
    .
  5. ^
    ISBN 978-0-08-037941-8
    .
  6. ISBN 978-0-8493-0486-6
    .
  7. ISBN 978-0-8493-0464-4
    .
  8. ^
    doi:10.1088/1674-1137/abddae
    .
  9. doi:10.1103/PhysRevC.89.064310
    .
  10. PMID 21048764
    .
  11. ^ a b c d e f g h i j k l m n o Wickleder, Fourest & Dorhout 2006, pp. 61–63.
  12. ISBN 978-0-08-048096-1
    .
  13. ^
    ISBN 978-5-7695-2533-9
    .
  14. ^
    PMID 10059654. Archived
    from the original on 8 March 2023. Retrieved 24 August 2019.
  15. ISBN 978-3-540-07109-9
    . Retrieved 4 October 2013.
  16. ^
    doi:10.1016/j.nuclphysa.2003.11.001. Archived
    (PDF) from the original on 16 April 2023. Retrieved 10 October 2021.
  17. doi:10.1351/pac200375060683
    .
  18. S2CID 94552853
    .
  19. ISBN 978-1-84826-127-3
    .
  20. ISBN 978-1-4419-0719-6
    .
  21. ISBN 978-0-07-244848-1. Archived from the original
    (PDF) on 4 October 2016. Retrieved 4 July 2016.
  22. ^ "AREVA Med launches production of lead-212 at new facility" (Press release). Areva. 2013. Archived from the original on 19 December 2020. Retrieved 1 January 2017.
  23. ^ "Mineral Yearbook 2012" (PDF). United States Geological Survey. Archived (PDF) from the original on 11 April 2013. Retrieved 30 September 2017.
  24. PMID 27476138
    .
  25. S2CID 53115264. Archived
    from the original on 11 May 2021. Retrieved 3 February 2019.
  26. PMID 27802058. Archived
    from the original on 29 April 2021. Retrieved 3 February 2019.
  27. ^ a b c d e f g h Wickleder, Fourest & Dorhout 2006, pp. 53–55.
  28. PMID 9970335
    .
  29. S2CID 248935764
    .
  30. doi:10.1021/ja01143a074. Archived
    (PDF) from the original on 29 April 2019.
  31. doi:10.1006/adnd.2001.0873
    .
  32. CIAAW
    . 2013.
  33. hdl:10261/12130
    .
  34. PMID 17501268. Archived
    from the original on 13 April 2023. Retrieved 24 August 2019.
  35. ^
    S2CID 205248786
    .
  36. ISBN 978-0-201-02116-5. Archived from the original on 17 February 2021. Retrieved 13 January 2018. {{cite book}}: ISBN / Date incompatibility (help
    )
  37. ^ a b c "Evaluation of nuclear criticality safety data and limits for actinides in transport" (PDF). Institut de radioprotection et de sûreté nucléaire. p. 15. Archived from the original (PDF) on 10 July 2007. Retrieved 20 December 2010.
  38. ^ a b c d e f g h i j k l m n Wickleder, Fourest & Dorhout 2006, pp. 52–53.
  39. ^ a b "3–6: Uranium Thorium Dating" (PDF). Institute for Structure and Nuclear Astrophysics, University of Notre Dame. Archived from the original (PDF) on 21 April 2021. Retrieved 7 October 2017.
  40. ^ Davis, O. "Uranium-Thorium Dating". Department of Geosciences, University of Arizona. Archived from the original on 28 March 2017. Retrieved 7 October 2017.
  41. ^
    ISBN 978-1-61530-125-6
  42. ^
    ISBN 978-1-4419-0719-6
  43. ^ Wickleder, Fourest & Dorhout 2006, pp. 59–60.
  44. ^
    John Wiley & Sons
    .
  45. doi:10.1063/1.3253147
    .
  46. ^
    ISBN 0-85186-428-7. Archived
    (PDF) from the original on 31 March 2022. Retrieved 11 January 2024.
  47. ^
    ISBN 978-0-471-18602-1
    .
  48. ^ Greenwood & Earnshaw 1997, p. 1262.
  49. ^ a b Stoll 2005, p. 6.
  50. ^
    ISBN 978-0-8493-0485-9
    .
  51. ^ a b Hyde, E. K. (1960). The radiochemistry of thorium (PDF). National Academy of Sciences. Archived from the original (PDF) on 5 March 2021. Retrieved 29 September 2017.
  52. ^ Greenwood & Earnshaw 1997, p. 1264.
  53. doi:10.1021/ie50569a035
    .
  54. ^ a b Greenwood & Earnshaw 1997, p. 1267.
  55. doi:10.1016/S0022-3115(96)00750-7
    .
  56. ^
    ISBN 978-0-19-960563-7
    .
  57. ^ a b Wickleder, Fourest & Dorhout 2006, pp. 70–77.
  58. ^ Greenwood & Earnshaw 1997, p. 1269.
  59. ^
    doi:10.1016/0022-2313(74)90001-5
    .
  60. ^ Wickleder, Fourest & Dorhout 2006, pp. 95–97.
  61. ^ a b c d e Wickleder, Fourest & Dorhout 2006, pp. 78–94.
  62. ^ a b Greenwood & Earnshaw 1997, p. 1271.
  63. ^ Wickleder, Fourest & Dorhout 2006, pp. 97–101.
  64. ^ Wickleder, Fourest & Dorhout 2006, pp. 64–66.
  65. ^ Greenwood & Earnshaw 1997, p. 127.
  66. ^ a b c d Wickleder, Fourest & Dorhout 2006, pp. 117–134.
  67. doi:10.1351/PAC-CON-09-10-22
    .
  68. ^ a b Greenwood & Earnshaw 1997, pp. 1275–1277.
  69. ^ a b c Wickleder, Fourest & Dorhout 2006, pp. 101–115.
  70. ^ Krämer2021-01-04T14:40:00+00:00, Katrina. "Heavy-metal cluster sets size record for metal aromaticity". Chemistry World. Archived from the original on 4 January 2021. Retrieved 2 July 2022.{{cite web}}: CS1 maint: numeric names: authors list (link)
  71. ^ a b c d Wickleder, Fourest & Dorhout 2006, pp. 116–117.
  72. ^ a b Greenwood & Earnshaw 1997, pp. 1278–1280.
  73. PMID 29560172
    .
  74. ^
    S2CID 120201972
    .
  75. S2CID 4295865
    .
  76. ^
    S2CID 14814446
    .
  77. doi:10.1103/RevModPhys.29.547
    .
  78. ISBN 978-0-226-10953-4
    .
  79. ^ Helmenstine, Anne (28 June 2022). "Composition of the Universe – Element Abundance". Science Notes and Projects. Archived from the original on 24 May 2024. Retrieved 13 June 2024.
  80. ^ Stoll 2005, p. 2.
  81. ^ Greenwood & Earnshaw 1997, p. 1294.
  82. ISBN 978-0-521-89148-6
    .
  83. doi:10.1080/00033797800200441
    .
  84. doi:10.1103/PhysRev.105.679
    .
  85. S2CID 4226393
    .
  86. doi:10.1103/PhysRev.91.1474
    .
  87. doi:10.1038/ngeo1205. Archived from the original
    on 17 April 2023. Retrieved 3 February 2019.
  88. doi:10.1021/ja01143a074. Archived
    from the original on 28 July 2019. Retrieved 3 February 2019.
  89. ^ a b c d e f g h i Wickleder, Fourest & Dorhout 2006, pp. 55–56.
  90. ^ Agency for Toxic Substances and Disease Registry (2016). Thorium (PDF) (Report). Archived from the original (PDF) on 12 April 2021. Retrieved 30 September 2017.
  91. ISBN 978-0-19-257046-8
    .
  92. ^ Woodhead, James A.; Rossman, George R.; Silver, Leon T. (1 February 1991). "The metamictization of zircon: Radiation dose-dependent structural characteristics". American Mineralogist. 76 (1–2): 74–82. Archived from the original on 13 April 2023. Retrieved 10 October 2021.
  93. ^ Szymanski, J. T.; Owens, D. R.; Roberts, A. C.; Ansell, H. G.; Chao, George Y. (1 February 1982). "A mineralogical study and crystal-structure determination of nonmetamict ekanite, ThCa2Si8O20". The Canadian Mineralogist. 20 (1): 65–75. Archived from the original on 24 October 2021. Retrieved 10 October 2021.
  94. ^ Greenwood & Earnshaw 1997, p. 1255.
  95. ^ "Tor's Fight with the Giants". Google Arts & Culture. Retrieved 26 June 2016.
  96. ^
    ISBN 978-0-19-938334-4
    .
  97. ISBN 978-1-4831-5659-0
    .
  98. ^ Thomson, T. (1831). A System of Chemistry of Inorganic Bodies. Vol. 1. Baldwin & Cradock and William Blackwood. p. 475.
  99. Berzelius, J. J.
    (1824). "Undersökning af några Mineralier. 1. Phosphorsyrad Ytterjord" [Examining some minerals. 1st phosphoric yttria.]. Kungliga Svenska Vetenskapsakademiens Handlingar (in Swedish). 2: 334–338.
  100. ^ "Xenotime-(Y)". Mindat database. Archived from the original on 16 March 2017. Retrieved 7 October 2017.
  101. Store norske leksikon (in Norwegian). Kunnskapsforlaget. Archived
    from the original on 28 April 2021. Retrieved 16 May 2009.
  102. ^
    doi:10.1021/ed009p1231
    .
  103. doi:10.1002/andp.18290920702. Archived
    from the original on 27 April 2021. Retrieved 20 July 2009. (modern citation: Annalen der Physik, vol. 92, no. 7, pp. 385–415).
  104. ^ Berzelius, J. J. (1829). "Undersökning af ett nytt mineral (Thorit), som innehåller en förut obekant jord" [Investigation of a new mineral (thorite), as contained in a previously unknown earth]. Kungliga Svenska Vetenskaps Akademiens Handlingar (in Swedish): 1–30.
  105. doi:10.1002/ange.19020153703. Archived
    from the original on 13 April 2023. Retrieved 24 August 2019.
  106. ^ Leach, M. R. "The Internet Database of Periodic Tables: Berzelius' Electronegativity Table". Archived from the original on 28 April 2021. Retrieved 16 July 2016.
  107. doi:10.1002/cber.188201502213. Archived
    from the original on 13 April 2023. Retrieved 24 August 2019.
  108. ^ a b Meister, G. (1948). Production of Rarer Metals (PDF) (Report). United States Atomic Energy Commission. Archived from the original (PDF) on 24 February 2017. Retrieved 22 September 2017.
  109. ^ a b c d Leach, M. R. "The Internet Database of Periodic Tables". Archived from the original on 24 March 2016. Retrieved 14 May 2012.
  110. doi:10.1021/ed080p952
    .
  111. ^
    ISBN 978-1-111-42710-8
    .
  112. ^ Wickleder, Fourest & Dorhout 2006, p. 52.
  113. OL 24166254M
    .
  114. ^ Schmidt, G. C. (1898). "Über die vom Thorium und den Thoriumverbindungen ausgehende Strahlung" [On the radiation emitted by thorium and thorium compounds]. Verhandlungen der Physikalischen Gesellschaft zu Berlin (in German). 17: 14–16.
  115. doi:10.1002/andp.18983010512. Archived
    from the original on 28 April 2021. Retrieved 19 July 2009. (modern citation: Annalen der Physik, vol. 301, pp. 141–151 (1898)).
  116. ^ Rutherford, E.; Owens, R. B. (1899). "Thorium and uranium radiation". Trans. R. Soc. Can. 2: 9–12.: "The radiation from thorium oxide was not constant, but varied in a most capricious manner", whereas "All the compounds of Uranium give out a radiation which is remarkably constant."
  117. ISBN 978-0-8065-2139-8
    .
  118. Nobel Media AB. Archived
    from the original on 7 August 2018. Retrieved 11 May 2017.
  119. ^
    ISBN 978-0-87371-026-8
    .
  120. ISBN 978-0-444-40776-4
    ..
  121. ISBN 978-1-4516-7761-4
    .
  122. doi:10.1016/S0925-8388(98)00072-3
    .
  123. S2CID 122675117. Archived
    from the original on 17 April 2021. Retrieved 21 June 2023.
  124. ^
    S2CID 7368077
    .
  125. ^ New Jersey Department of Health (1996). "Health and hazardous waste" (PDF). A Practitioner's Guide to Patients' Environmental Exposures. 1 (3): 1–8. Archived from the original (PDF) on 15 April 2016.
  126. doi:10.1119/1.18463
    .
  127. ^
    PMID 17270253
    .
  128. ^ Kazimi, M. (2003). "Thorium Fuel for Nuclear Energy". American Scientist. Archived from the original on 1 January 2017. Retrieved 29 September 2017.
  129. S2CID 237471852
    .
  130. ^ Majumdar, S.; Purushotham, D. S. C. (1999). "Experience of thorium fuel development in India". Thorium fuel utilization: Options and trends (PDF) (Report). International Atomic Energy Agency. Archived (PDF) from the original on 12 April 2021. Retrieved 7 October 2017.
  131. ^ "Nuclear Power in India". World Nuclear Association. 2017. Archived from the original on 6 September 2016. Retrieved 29 September 2017.
  132. ^ "IAEA-TECDOC-1450 Thorium Fuel Cycle – Potential Benefits and Challenges" (PDF). International Atomic Energy Agency. 2005. Archived (PDF) from the original on 4 August 2016. Retrieved 23 March 2009.
  133. ^ Shippingport Atomic Power Station. "Historic Achievement Recognized: Shippingport Atomic Power Station, A National Engineering Historical Landmark" (PDF). p. 4. Archived from the original (PDF) on 17 July 2015. Retrieved 24 June 2006.
  134. doi:10.1080/00963402.1997.11456765
    .
  135. ^ "IAEA-TECDOC-1349 Potential of thorium-based fuel cycles to constrain plutonium and to reduce the long-lived waste toxicity" (PDF). International Atomic Energy Agency. 2002. Archived (PDF) from the original on 28 April 2021. Retrieved 24 March 2009.
  136. ^ Evans, B. (2006). "Scientist urges switch to thorium". ABC News. Archived from the original on 28 March 2010. Retrieved 17 September 2011.
  137. ^ Martin, R. (2009). "Uranium is So Last Century – Enter Thorium, the New Green Nuke". Wired. Archived from the original on 26 June 2010. Retrieved 19 June 2010.
  138. ^ Dr. Alfredo Carpineti (16 June 2023) Experimental Molten Salt Nuclear Reactor Gets Go-Ahead In China Archived 2 July 2023 at the Wayback Machine
  139. ^ "China gives green light to its first thorium-powered nuclear reactor". South China Morning Post. 15 June 2023.
  140. ISBN 978-1-56396-358-2
    .
  141. ^ a b c "Thorium". World Nuclear Association. 2017. Archived from the original on 8 May 2017. Retrieved 21 June 2017.
  142. OSTI 79078. Archived
    (PDF) from the original on 10 September 2023. Retrieved 24 August 2019.
  143. ^ "Classification Bulletin WNP-118" (PDF). U.S. Department of Energy. 12 March 2008. Archived (PDF) from the original on 3 February 2017. Retrieved 13 September 2019.
  144. ^ Stoll 2005, p. 7.
  145. ^ United States Geological Survey (2012). "Thorium" (PDF). Archived (PDF) from the original on 29 April 2017. Retrieved 12 May 2017.
  146. ^ Jayaram, K. M. V. (1987). An Overview of World Thorium Resources, Incentives for Further Exploration and Forecast for Thorium Requirements in the Near Future (PDF) (Report). Department of Atomic Energy. Archived from the original (PDF) on 28 June 2011.
  147. ^ Thorium. Statistics and Information (Report). United States Geological Survey. 2017. Archived from the original on 10 January 2019. Retrieved 6 January 2018.
  148. ^ a b c d e f g h Stoll 2005, p. 8.
  149. ^ a b c d e Stoll 2005, p. 32.
  150. ^ a b Stoll 2005, p. 31.
  151. ISBN 978-1-58157-829-4
    .
  152. ^ Shaw, J.; Dunderdale, J.; Paynter, R.A. (9 June 2005). "A review of consumer products containing radioactive substances in the European Union" (PDF). NRPB Occupational Services Department. Archived (PDF) from the original on 10 March 2021. Retrieved 21 July 2017.
  153. ^ ""Negative Ion" Technology—What You Should Know". U.S. Nuclear Regulatory Commission. 28 July 2014. Archived from the original on 8 December 2014. Retrieved 16 June 2021.
  154. ISBN 978-1-4831-3979-1
    .
  155. ISBN 978-0-911572-27-8
    .
  156. ISBN 978-1-61325-221-5
    .
  157. ISBN 978-1-305-49469-5
    .
  158. ^ "Thoriated Camera Lens (ca. 1970s)". Oak Ridge Associated Universities. 2021. Archived from the original on 24 June 2021. Retrieved 11 October 2021.
  159. ISBN 978-0-8194-2285-9
    .
  160. ISBN 978-3-540-36386-6
    .
  161. S2CID 116039197
    .
  162. ^
    doi:10.1016/j.anucene.2010.07.006
    .
  163. ^ "Plutonium". World Nuclear Association. 2017. Archived from the original on 5 October 2017. Retrieved 29 September 2017.
  164. ^ "Uranium 233 Fission". Nuclear Power. 2023. Retrieved 28 April 2023.
  165. ^ a b c Greenwood & Earnshaw 1997, p. 1259.
  166. ^ "Interactive Chart of Nuclides". Brookhaven National Laboratory. Archived from the original on 24 January 2017. Retrieved 12 August 2013.
  167. ^ "Thorium test begins". World Nuclear News. 2013. Archived from the original on 19 July 2013. Retrieved 21 July 2013.
  168. ^ "IAEA-TECDOC-1450 Thorium Fuel Cycle – Potential Benefits and Challenges" (PDF). International Atomic Energy Agency. 2005. Archived (PDF) from the original on 4 August 2016. Retrieved 23 March 2009.
  169. ^
    ISBN 978-0-471-46560-7
    .
  170. ^ Stoll 2005, p. 30.
  171. ^
    ISBN 978-0-08-044472-7
    .
  172. ^ Rees, E. (2011). "Don't believe the spin on thorium being a greener nuclear option". The Guardian. Archived from the original on 27 September 2017. Retrieved 29 September 2017.
  173. ISBN 978-1-136-29437-2
    .
  174. ^ "Nuclear Energy FAQs" (PDF). Argonne National Laboratory. 2014. Archived (PDF) from the original on 7 October 2014. Retrieved 13 January 2018.
  175. ISBN 978-1-136-84993-0
    .
  176. ^ a b c "Thorium: Radiation Protection". United States Environmental Protection Agency. Archived from the original on 1 October 2006. Retrieved 27 February 2016.
  177. ^ a b "Radioactivity in Lantern Mantles". Australian Radiation Protection and Nuclear Safety Agency. Archived from the original on 14 October 2007. Retrieved 29 September 2017.
  178. ^ "Natural Decay Series: Uranium, Radium, and Thorium" (PDF). Argonne National Laboratory. 2005. Archived from the original (PDF) on 17 August 2016. Retrieved 30 September 2017.
  179. ^ a b c d Stoll 2005, p. 35.
  180. PMID 6706595
    .
  181. S2CID 39901914
    .
  182. ^ "Toxicological Profile for Thorium" (PDF). Agency for Toxic Substances and Disease Registry U.S. Public Health Service. 1990. p. 4. Archived (PDF) from the original on 21 October 2004. Retrieved 5 September 2016.
  183. ^ Merkel, B.; Dudel, G.; et al. (1988). Untersuchungen zur radiologischen Emission des Uran-Tailings Schneckenstein (PDF) (Report) (in German). Sächsisches Staatsministerium für Umwelt und Landesentwicklung. Archived from the original (PDF) on 8 January 2013.
  184. ^ a b Stoll 2005, p. 34.
  185. ^ Jacobson, M.; Cooper, A. R.; Nagy, J. (1964). Explosibility of metal powders (PDF) (Report). United States Department of the Interior. Archived (PDF) from the original on 3 August 2016. Retrieved 29 September 2017.
  186. ^ "Nine Injured in Atomic Lab Blasts". Pittsburgh Post-Gazette. Associated Press. 1956. p. 2. Archived from the original on 16 March 2020. Retrieved 29 September 2017.
  187. ^ "No Radiation Threat Seen in A-laboratory Blast". The St. Petersburg Times. Associated Press. 1956. p. 2. Archived from the original on 16 March 2020. Retrieved 29 September 2017.
  188. ^ Harrington, M. (2003). "Sad Memories of '56 Sylvania Explosion". New York Newsday. Archived from the original on 4 February 2012. Retrieved 29 September 2017.
  189. ^ "Thorium ToxFAQs" (PDF). Agency for Toxic Substances and Disease Registry. Archived (PDF) from the original on 5 June 2012. Retrieved 29 September 2017.
  190. ^ "Compendium of Policy And Statutory Provisions Relating To Exploitation of Beach Sand Minerals". Department of Atomic Energy. Archived from the original on 4 December 2008. Retrieved 19 December 2008.
  191. PMID 7345962
    .
  192. ^ Alberici, Emma (29 January 2019). "Italian military officials' trial ignites suspicions of links between weapon testing and birth defects in Sardinia". ABC News. Australian Broadcasting Corporation. Archived from the original on 29 January 2019. Retrieved 29 January 2019.

General and cited references

Further reading
  • The dictionary definition of thorium at Wiktionary
  • Media related to Thorium at Wikimedia Commons
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