All known thoriumisotopes are unstable. The most stable isotope,232Th, has ahalf-life of 14.05 billion years, or about theage of the universe; it decays very slowly viaalpha decay, starting adecay chain named thethorium series that ends at stable208Pb. On Earth, thorium anduranium are the only elements with no stable or nearly-stable isotopes that still occur naturally in large quantities asprimordial elements.[a] Thorium is estimated to be over three times asabundant as uranium in the Earth's crust, and is chiefly refined frommonazite sands as a by-product of extractingrare-earth elements.
Thorium was discovered in 1828 by the Swedish chemistJöns Jacob Berzelius, who named it afterThor, theNorse 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 inTIG welding electrodes but is slowly being replaced in the field with different compositions. It was also material in high-end optics and scientific instrumentation, used in some broadcast vacuum tubes, and as the light source ingas mantles, but these uses have become marginal. It has been suggested as a replacement for uranium as nuclear fuel innuclear reactors, and severalthorium reactors have been built. Thorium is also used in strengtheningmagnesium, coatingtungsten wire in electrical and welding equipment, controlling the grain size of tungsten inelectric lamps, high-temperature crucibles, and glasses including camera and scientific instrument lenses. Other uses for thorium include heat-resistant ceramics,aircraft engines, and inlight bulbs. Ocean science has utilised231Pa/230Th isotope ratios to understand the ancient ocean.[10]
Thorium is a moderately soft,paramagnetic, bright silvery radioactive actinide metal that can be bent or shaped. In theperiodic table, it lies to the right ofactinium, to the left ofprotactinium, and belowcerium. Pure thorium is veryductile and, as normal for metals, can becold-rolled,swaged, anddrawn.[11] At room temperature, thorium metal has aface-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 abulk modulus (a measure of resistance to compression of a material) of 54 GPa, about the same astin'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 softsteel, so when heated it can be rolled into sheets and pulled into wire.[13]
Thorium is nearly half as dense asuranium andplutonium and is harder than both.[13] It becomessuperconductive below 1.4 K.[11] Thorium'smelting point of 1750 °C is above both those of actinium (1227 °C) and protactinium (1568 °C). At the start ofperiod 7, fromfrancium 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 toplutonium, 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 tocalifornium, 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 usuallythorium dioxideThO2); even the purest thorium specimens usually contain about a tenth of a per cent of the dioxide.[11] Experimental measurements of its density give values between 11.5 and 11.66 g/cm3: these are slightly lower than the theoretically expected value of 11.7 g/cm3 calculated from thorium'slattice 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 formalloys with many other metals. Addition of small proportions of thorium improves the mechanical strength ofmagnesium, and thorium-aluminium alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. Thorium formseutectic mixtures withchromium and uranium, and it is completelymiscible in both solid and liquidstates with its lightercongener cerium.[11]
There are seven naturally occurring isotopes of Thorium but none are stable.232Th is one of the two nuclides beyond bismuth (the other being238U) that have half-lives measured in billions of years; its half-life is 14.05 billion years, about three times theage of the Earth, and slightly longer than theage of the universe. Four-fifths of the thorium present at Earth's formation has survived to the present.[16][17][18]232Th is the only isotope of thorium occurring in quantity in nature.[16] Its stability is attributed to its closednuclear subshell with 142 neutrons.[19][20] Thorium has a characteristic terrestrial isotopic composition, withatomic weight232.0377±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 toalpha decay because the strong nuclear force cannot overcome the electromagnetic repulsion between their protons.[21] The alpha decay of232Th initiates the 4ndecay chain which includes isotopes with amass number divisible by 4 (hence the name; it is also called the thorium series after its progenitor). This chain of consecutive alpha andbeta decays begins with the decay of232Th to228Ra and terminates at208Pb.[16] Any sample of thorium or its compounds contains traces of these daughters, which are isotopes ofthallium,lead, bismuth, polonium,radon,radium, and actinium.[16] Natural thorium samples can be chemically purified to extract useful daughter nuclides, such as212Pb, which is used innuclear medicine forcancer therapy.[22][23]227Th (alpha emitter with an 18.68 days half-life) can also be used in cancer treatments such astargeted alpha therapies.[24][25][26]232Th also very occasionally undergoesspontaneous fission rather than alpha decay, and has left evidence of doing so in its minerals (as trappedxenon gas formed as a fission product), but thepartial half-life of this process is very large at over 1021 years and alpha decay predominates.[27][28]
The 4ndecay chain of232Th, commonly called the "thorium series"
In total, 32radioisotopes have been characterised, which range in mass number from 207[29] to 238.[27] After232Th, the most stable of them (with respective half-lives) are230Th (75,380 years),229Th (7,917 years),228Th (1.92 years),234Th (24.10 days), and227Th (18.68 days). All of these isotopes occur in nature astrace radioisotopes due to their presence in the decay chains of232Th,235U,238U, and237Np: the last of these is longextinct in nature due to its short half-life (2.14 million years), but is continually produced in minute traces fromneutron 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 ofneutron activation of natural232Th.[30]226Th (half-life 31 minutes) has not yet been observed in nature, but would be produced by the still-unobserveddouble beta decay of natural226Ra.[31]
In deepseawaters the isotope230Th makes up to0.02% of natural thorium.[8] This is because its parent238U is soluble in water, but230Th 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 the230Th isotope, since230Th is one of the daughters of238U.[27] TheInternational Union of Pure and Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element in 2013; it had formerly been considered amononuclidic element.[32]
Thorium has three knownnuclear isomers (or metastable states),216m1Th,216m2Th, and229mTh.229mTh has the lowest known excitation energy of any isomer,[33] measured to be7.6±0.5 eV. This is so low that when it undergoesisomeric transition, the emitted gamma radiation is in theultraviolet range.[34][35][c] The nuclear transition from229Th to229mTh is being investigated for anuclear clock.[35]
Different isotopes of thorium are chemically identical, but have slightly differing physical properties: for example, the densities of pure228Th,229Th,230Th, and232Th are respectively expected to be 11.5, 11.6, 11.6, and 11.7 g/cm3.[37] The isotope229Th is expected to befissionable with a barecritical mass of 2839 kg, although with steelreflectors this value could drop to 994 kg.[37][d]232Th is not fissionable, but it isfertile as it can be converted to fissile233U by neutron capture and subsequent beta decay.[37][38]
Two radiometric dating methods involve thorium isotopes:uranium–thorium dating, based on the decay of234U to230Th, andionium–thorium dating, which measures the ratio of232Th to230Th.[e] These rely on the fact that232Th is a primordial radioisotope, but230Th only occurs as an intermediate decay product in the decay chain of238U.[39] Uranium–thorium dating is a relatively short-range process because of the short half-lives of234U and230Th relative to the age of the Earth: it is also accompanied by a sister process involving the alpha decay of235U into231Th, which very quickly becomes the longer-lived231Pa, and this process is often used to check the results of uranium–thorium dating. Uranium–thorium dating is commonly used to determine the age ofcalcium carbonate materials such asspeleothem orcoral, because uranium is more soluble in water than thorium and protactinium, which are selectively precipitated into ocean-floorsediments, 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 (both232Th and230Th) and thus its presence in ocean sediments to date these sediments by measuring the ratio of232Th to230Th.[41][42] Both of these dating methods assume that the proportion of230Th to232Th 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]
A thorium atom has 90 electrons, of which four arevalence electrons. Fouratomic orbitals are theoretically available for the valence electrons to occupy: 5f, 6d, 7s, and 7p.[43] Despite thorium's position in thef-block of the periodic table, it has an anomalous [Rn]6d27s2 electron configuration in the ground state, as the 5f and 6d subshells in the early actinides are very close in energy, even more so than the 4f and 5d subshells of the lanthanides: thorium's 6d subshells are lower in energy than its 5f subshells, because its 5f subshells are not well-shielded by the filled 6s and 6p subshells and are destabilised. This is due torelativistic effects, which become stronger near the bottom of the periodic table, specifically the relativisticspin–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 thetransition 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]
Thorium dioxide has thefluorite 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 theFermi level should behexagonal close packed like thegroup 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 ofsilver or lead, as theTh4+ ion has no 5f or 6d electrons.[13] Thorium chemistry is therefore largely that of an electropositive metal forming a singlediamagnetic ion with a stable noble-gas configuration, indicating a similarity between thorium and themain 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]
Thorium is a highlyreactive and electropositive metal. With astandard reduction potential of −1.90 V for theTh4+/Th couple, it is somewhat more electropositive than zirconium or aluminium.[49] Finely divided thorium metal can exhibitpyrophoricity, spontaneously igniting in air.[11] When heated in air, thoriumturnings 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]
Most binary compounds of thorium with nonmetals may be prepared by heating the elements together.[54] In air, thorium burns to formThO2, which has thefluorite structure.[55] Thorium dioxide is arefractory material, with the highest melting point (3390 °C) of any known oxide.[56] It is somewhathygroscopic 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 whenThO2 is mixed with its lighter homologuecerium dioxide (CeO2, ceria): this is the basis for its previously common application ingas mantles.[57] A flame is not necessary for this effect: in 1901, it was discovered that a hot Welsbach gas mantle (usingThO2 with 1%CeO2) remained at "full glow" when exposed to a cold unignited mixture of flammable gas[which?] and air.[59] The light emitted by thorium dioxide is higher in wavelength than theblackbody emission expected fromincandescence at the same temperature, an effect calledcandoluminescence. It occurs becauseThO2 : Ce acts as a catalyst for the recombination offree 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]
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 amonoclinic crystal structure like those ofzirconium tetrafluoride andhafnium tetrafluoride, where theTh4+ ions are coordinated withF− ions in somewhat distortedsquare antiprisms.[61] The other tetrahalides instead have dodecahedral geometry.[62] Lower iodidesThI3 (black) andThI2 (gold-coloured) can also be prepared by reducing the tetraiodide with thorium metal: they do not contain Th(III) and Th(II), but instead containTh4+ and could be more clearly formulated aselectride 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 withpotassium fluoride andhydrofluoric 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 possiblenuclear fuels.[54] All four heavierpnictogens (phosphorus,arsenic,antimony, and bismuth) also form binary thorium compounds. Thorium germanides are also known.[63] Thorium reacts with hydrogen to form the thorium hydridesThH2 andTh4H15, 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]
In an acidic aqueous solution, thorium occurs as the tetrapositiveaqua ion[Th(H2O)9]4+, which hastricapped trigonal prismatic molecular geometry:[66][67] at pH < 3, the solutions of thorium salts are dominated by this cation.[66] TheTh4+ 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 thansulfurous acid: thus it tends to undergo hydrolysis and polymerisation (though to a lesser extent thanFe3+), predominantly to[Th2(OH)2]6+ in solutions with pH 3 or below, but in more alkaline solution polymerisation continues until the gelatinous hydroxideTh(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 ahard 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 theManhattan 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 theperchlorates,sulfates,sulfites, nitrates, carbonates,phosphates,vanadates,molybdates, andchromates, and their hydrated forms.[69] They are important in thorium purification and the disposal of nuclear waste, but most of them have not yet been fully characterised, especially regarding their structural properties.[69] For example, thorium nitrate is produced by reacting thorium hydroxide with nitric acid: it is soluble in water and alcohols and is an important intermediate in the purification of thorium and its compounds.[69] Thorium complexes with organic ligands, such asoxalate,citrate, andEDTA, 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
In January 2021, the aromaticity has been observed in a largemetal cluster anion consisting of 12bismuth atoms stabilised by a center thorium cation.[70] This compound was shown to be surprisingly stable, unlike many previous knownaromatic metal clusters.
Most of the work on organothorium compounds has focused on thecyclopentadienyl complexes andcyclooctatetraenyls. Like many of the early and middle actinides (up toamericium, and also expected forcurium), thorium forms a cyclooctatetraenide complex: the yellowTh(C8H8)2,thorocene. It isisotypic with the better-known analogous uranium compounduranocene.[71] It can be prepared by reactingK2C8H8 with thorium tetrachloride intetrahydrofuran (THF) at the temperature ofdry ice, or by reacting thorium tetrafluoride withMgC8H8.[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 areTh(C5H5)3 andTh(C5H5)4: many derivatives are known. The former (which has two forms, one purple and one green)[71] is a rare example of thorium in the formal +3 oxidation state;[72] a formal +2 oxidation state occurs in a derivative.[73] The chloride derivative[Th(C5H5)3Cl] is prepared by heating thorium tetrachloride withlimitingKC5H5 used (other univalent metal cyclopentadienyls can also be used). Thealkyl andaryl derivatives are prepared from the chloride derivative and have been used to study the nature of the Th–Csigma 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 itsadducts are stabilised byphosphine ligands.[44]
232Th is a primordial nuclide, having existed in its current form for over ten billion years; it was formed during ther-process, which probably occurs insupernovae andneutron star mergers. These violent events scattered it across the galaxy.[74][75] The letter "r" stands for "rapid neutron capture", and occurs in core-collapse supernovae, where heavy seed nuclei such as56Fe rapidly capture neutrons, running up against theneutron 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 increasedCoulomb barriers that make interactions between charged particles difficult at high atomic numbers and the fact that fusion beyond56Fe isendothermic.[76] Because of the abrupt loss of stability past209Bi, 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]
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 arethulium,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 of236U to232Th and the natural depletion of235U, but these sources have long since decayed and no longer contribute.[80]
In the Earth's crust, thorium is much more abundant: with anabundance 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 alithophile under theGoldschmidt classification, meaning that it is generally found combined with oxygen. Common thorium compounds are also poorly soluble in water. Thus, even though therefractory 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]
Theradiogenic heat from the decay of232Th (violet) is a major contributor to theearth'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]
Natural thorium is usually almost pure232Th, 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 theEarth's internal heat; the other major contributors are the shorter-lived primordial radionuclides, which are238U,40K, and235U in descending order of their contribution. (At the time of the Earth's formation,40K and235U contributed much more by virtue of their short half-lives, but they have decayed more quickly, leaving the contribution from232Th and238U 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, only230Th is usually detectable, occurring insecular equilibrium with its parent238U, 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.[89][90] In fact, it is the 37th most abundant element in the Earth's crust with an abundance of 12 parts per million.[91] In nature, thorium occurs in the +4 oxidation state, together with uranium(IV),zirconium(IV), hafnium(IV), and cerium(IV), and also withscandium,yttrium, and the trivalent lanthanides which have similarionic radii.[89] Because of thorium's radioactivity, minerals containing it are oftenmetamict (amorphous), their crystal structure having been damaged by the alpha radiation produced by thorium.[92] An extreme example isekanite,(Ca,Fe,Pb)2(Th,U)Si8O20, which almost never occurs in nonmetamict form due to the thorium it contains.[93]
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 India, South Africa, Brazil, Australia, andMalaysia. It contains around 2.5% thorium on average, although some deposits may contain up to 20%.[89][94] Monazite is a chemically unreactive mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it.[89]Allanite (chiefly silicates-hydroxides of various metals) can have 0.1–2% thorium andzircon (chieflyzirconium silicate,ZrSiO4) up to 0.4% thorium.[89]
Thorium dioxide occurs as the rare mineralthorianite. Due to its being isotypic withuranium dioxide, these two common actinide dioxides can form solid-state solutions and the name of the mineral changes according to theThO2 content.[89][i]Thorite (chieflythorium silicate,ThSiO4), also has a high thorium content and is the mineral in which thorium was first discovered.[89] In thorium silicate minerals, theTh4+ andSiO4−4 ions are often replaced withM3+ (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. TheTh4+ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can be higher.[56]
In 1815, the Swedish chemistJöns Jacob Berzelius analysed an unusual sample ofgadolinite from a copper mine inFalun, central Sweden. He noted impregnated traces of a white mineral, which he cautiously assumed to be an earth (oxide in modern chemical nomenclature) of an unknown element. Berzelius had already discovered two elements,cerium andselenium, but he had made a public mistake once, announcing a new element,gahnium, that turned out to bezinc oxide.[96] Berzelius privately named the putative element "thorium" in 1817[97] and its supposed oxide "thorina" afterThor, theNorse god of thunder.[98] In 1824, after more deposits of the same mineral inVest-Agder, Norway, were discovered, he retracted his findings, as the mineral (later namedxenotime) proved to be mostlyyttrium orthophosphate.[38][96][99][100]
In 1828,Morten Thrane Esmark found a black mineral onLøvøya island,Telemark county, Norway. He was a Norwegianpriest and amateurmineralogist who studied the minerals in Telemark, where he served asvicar. He commonly sent the most interesting specimens, such as this one, to his father,Jens Esmark, a noted mineralogist and professor of mineralogy and geology at theRoyal Frederick University in Christiania (today calledOslo).[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 reducingK[ThF5] (potassium pentafluorothorate(IV)) withpotassium metal.[102][103][104] Berzelius reused the name of the previous supposed element discovery[102][105] and named the source mineral thorite.[38]
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 (120amu); it is actually 15 times as large.[j] He determined that thorium was a veryelectropositive 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]
In the periodic table published byDmitri 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 thealkaline 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 moderncarbon 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 astitanium and zirconium.[111][l]
While thorium was discovered in 1828 its first application dates only from 1885, when Austrian chemistCarl Auer von Welsbach invented thegas 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]
Thorium was first observed to be radioactive in 1898, by the German chemistGerhard Carl Schmidt and later that year, independently, by the Polish-French physicistMarie Curie. It was the second element that was found to be radioactive, after the 1896 discovery of radioactivity in uranium by French physicistHenri Becquerel.[113][114][115] Starting from 1899, the New Zealand physicistErnest Rutherford and the American electrical engineerRobert 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 namedradon, 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 physicistFrederick Soddy, showed how thorium decayed at a fixed rate over time into a series of other elements in work dating from 1900 to 1903. This observation led to the identification of thehalf-life as one of the outcomes of thealpha particle experiments that led to the disintegration theory ofradioactivity.[117] The biological effect of radiation was discovered in 1903.[118] The newly discovered phenomenon of radioactivity excited scientists and the general public alike. In the 1920s, thorium's radioactivity was promoted as a cure forrheumatism,diabetes, andsexual 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]
Up to the late 19th century, chemists unanimously agreed that thorium and uranium were the heaviest members of group 4 andgroup 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 physicistNiels Bohr published atheoretical 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 firsttransuranic 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 physicistGlenn T. Seaborg and his team had discovered the transuranic elements americium and curium, he proposed theactinide concept, realising that thorium was the second member of an f-block actinide series analogous to the lanthanides, instead of being the heavier congener ofhafnium in a fourth d-block row.[111][m]
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 theOak 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]
In the 21st century, thorium's potential for reducing nuclear proliferation and itswaste characteristics led to renewed interest in the thorium fuel cycle.[135][136][137] India has projected meeting as much as 30% of its electrical demands through thorium-basednuclear power by 2050. In February 2014,Bhabha Atomic Research Centre (BARC), inMumbai, India, presented their latest design for a "next-generation nuclear reactor" that burns thorium as its fuel core, calling it theAdvanced 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 theTMSR-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]
When gram quantities ofplutonium were first produced in theManhattan Project, it was discovered that a minor isotope (240Pu) underwent significantspontaneous fission, which brought into question the viability of a plutonium-fuelledgun-type nuclear weapon. While theLos Alamos team began work on theimplosion-type weapon to circumvent this issue, theChicago team discussed reactor design solutions.Eugene Wigner proposed to use the240Pu-contaminated plutonium to drive the conversion of thorium into233U in a special converter reactor. It was hypothesized that the233U would then be usable in a gun-type weapon, though concerns about contamination from232U 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 theCold War the United States explored the possibility of using232Th as a source of233U to be used in anuclear bomb; they fireda test bomb in 1955.[141] They concluded that a233U-fired bomb would be a very potent weapon, but it bore few sustainable "technical advantages" over the contemporary uranium–plutonium bombs,[142] especially since233U is difficult to produce in isotopically pure form.[141]
Thorium metal was used in theradiation case of at least one nuclear weapon design deployed by the United States (theW71).[143]
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]
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 undergoflotation. Alkaline earth metal carbonates may be removed after reaction withhydrogen chloride; then followthickening, 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 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 is carried out in 30–45%sodium hydroxide solution at about 140 °C for about three hours. Too high a temperature leads to the formation of poorly soluble thorium oxide and an excess of uranium in the filtrate, and too low a concentration of alkali leads to a very slow reaction. These reaction conditions are rather mild and require monazite sand with a particle size under 45 μm. Following filtration, the filter cake includes thorium and the rare earths as their hydroxides, uranium assodium diuranate, and phosphate astrisodium phosphate. This crystallises trisodium phosphate decahydrate when cooled below 60 °C; uranium impurities in this product increase with the amount ofsilicon dioxide in the reaction mixture, necessitating recrystallisation before commercial use. The hydroxides are dissolved at 80 °C in 37% hydrochloric acid. Filtration of the remaining precipitates followed by addition of 47% sodium hydroxide results in the precipitation of thorium and uranium at about pH 5.8. Complete drying of the precipitate must be avoided, as air may oxidise cerium from the +3 to the +4 oxidation state, and the cerium(IV) formed can liberate freechlorine from the hydrochloric acid. The rare earths again precipitate out at higher pH. The precipitates are neutralised by the original sodium hydroxide solution, although most of the phosphate must first be removed to avoid precipitating rare-earth phosphates.Solvent extraction may also be used to separate out the thorium and uranium, by dissolving the resultant filter cake in nitric acid. The presence oftitanium hydroxide is deleterious as it binds thorium and prevents it from dissolving fully.[148]
High thorium concentrations are needed in nuclear applications. In particular, concentrations of atoms with high neutron capturecross-sections must be very low (for example,gadolinium concentrations must be lower than one part per million by weight). Previously, repeated dissolution and recrystallisation was used to achieve high purity. Today, liquid solvent extraction procedures involving selectivecomplexation ofTh4+ 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 withtributyl phosphate inkerosene.[148]
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 ingas lamp mantles.[150] All substances emit energy (glow) at high temperatures, but the light emitted by thorium is nearly all in thevisible 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, orultraviolet light. This effect is shared by cerium dioxide, which converts ultraviolet light into visible light more efficiently, but thorium dioxide gives a higher flame temperature, emitting lessinfrared light.[150] Thorium in mantles, though still common, has been progressively replaced with yttrium since the late 1990s.[151] According to the 2005 review by the United Kingdom'sNational Radiological Protection Board, "although [thoriated gas mantles] were widely available a few years ago, they are not any more."[152] Thorium is also used to make cheap permanentnegative ion generators, such as inpseudoscientific health bracelets.[153]
During the production ofincandescent filaments,recrystallisation of tungsten is significantly lowered by adding small amounts of thorium dioxide to the tungstensintering powder before drawing the filaments.[149] A small addition of thorium to tungstenthermocathodes considerably reduces thework 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 evaporatedmagnesium layer as agetter 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 ingas 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, andlanthanum.[156][157]
Thorium dioxide is found inrefractory ceramics, such as high-temperature laboratorycrucibles,[38] either as the primary ingredient or as an addition tozirconium dioxide. An alloy of 90%platinum and 10% thorium is an effective catalyst for oxidisingammonia 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]
Yellowed thorium dioxide lens (left), a similar lens partially de-yellowed with ultraviolet radiation (centre), and lens without yellowing (right)
When added toglass, thorium dioxide helps increase itsrefractive index and decreasedispersion. Such glass finds application in high-qualitylenses 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 aslanthanum, 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 andmid 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 includeLanthanum 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.
The main nuclear power source in a reactor is the neutron-induced fission of a nuclide; the synthetic fissile[d] nuclei233U and239Pu can bebred from neutron capture by the naturally occurring quantity nuclides232Th and238U.235U occurs naturally in significant amounts and is also fissile.[161][162][n] In the thorium fuel cycle, the fertile isotope232Th is bombarded byslow neutrons, undergoing neutron capture to become233Th, which undergoes two consecutive beta decays to become first233Pa and then the fissile233U:[38]
233U is fissile and can be used as a nuclear fuel in the same way as235U or239Pu. When233U undergoes nuclear fission, the neutrons emitted can strike further232Th nuclei, continuing the cycle.[38] This parallels the uranium fuel cycle infast breeder reactors where238U undergoes neutron capture to become239U, beta decaying to first239Np and then fissile239Pu.[163]
The fission of233 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 new233 92U atom from the fertile232 90Th . This leaves a margin of 0.45 neutrons (or 18% of the neutron flux) for losses.
Thorium is more abundant than uranium, and can satisfy world energy demands for longer.[165] It is particularly suitable for being used as fertile material inmolten salt reactors.
232Th absorbs neutrons more readily than238U, and233U has a higher probability of fission upon neutron capture (92.0%) than235U (85.5%) or239Pu (73.5%).[166] It also releases more neutrons upon fission on average.[165] A single neutron capture by238U produces transuranic waste along with the fissile239Pu, but232Th only produces this waste after five captures, forming237Np. This number of captures does not happen for 98–99% of the232Th nuclei because the intermediate products233U or235U undergo fission, and fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium inmixed oxide fuels to minimise the generation of transuranics and maximise the destruction ofplutonium.[167]
The used fuel is difficult and dangerous to reprocess because many of the daughters of232Th and233U are strong gamma emitters.[165] All233U production methods result in impurities of232U, either from parasitic knock-out (n,2n) reactions on232Th,233Pa, or233U that result in the loss of a neutron, or from double neutron capture of230Th, an impurity in natural232Th:[169]
232U by itself is not particularly harmful, but quickly decays to produce the strong gamma emitter208Tl. (232Th follows the same decay chain, but its much longer half-life means that the quantities of208Tl produced are negligible.)[170] These impurities of232U make233U easy to detect and dangerous to work on, and the impracticality of their separation limits the possibilities ofnuclear proliferation using233U as the fissile material.[169]233Pa has a relatively long half-life of 27 days and a highcross section for neutron capture. Thus it is aneutron poison: instead of rapidly decaying to the useful233U, a significant amount of233Pa converts to234U and consumes neutrons, degradingthe reactor efficiency. To avoid this,233Pa is extracted from the active zone of thoriummolten salt reactors during their operation, so that it does not have a chance to capture a neutron and will only decay to233U.[171]
The irradiation of232Th 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;[38] research continues in this area. Others cite the low commercial viability of the thorium fuel cycle:[172][173][174] the internationalNuclear Energy Agency predicts that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which may persist "in the coming decades".[175] The isotopes produced in the thorium fuel cycle are mostly not transuranic, but some of them are still very dangerous, such as231Pa, which has a half-life of 32,760 years and is a major contributor to the long-termradiotoxicity of spent nuclear fuel.[171]
Natural thorium decays very slowly compared to many other radioactive materials, and the emittedalpha 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 ofcancers of thelung,pancreas, andblood, as lungs and other internal organs can be penetrated by alpha radiation.[176] Internal exposure to thorium leads to increased risk ofliver diseases.[177]
The decay products of232Th 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 of232Th must include the contribution of its daughters, some of which are dangerousgamma emitters,[178] and which are built up quickly following the initial decay of232Th due to the absence of long-lived nuclides along the decay chain.[179] As the dangerous daughters of thorium have much lower melting points than thorium dioxide, they are volatilised every time the mantle is heated for use. In the first hour of use large fractions of the thorium daughters224Ra,228Ra,212Pb, and212Bi are released.[180] Most of the radiation dose by a normal user arises from inhaling the radium, resulting in a radiation dose of up to 0.2 millisieverts per use, about a third of the dose sustained during amammogram.[181]
Somenuclear safety agencies make recommendations about the use of thorium mantles and have raised safety concerns regarding theirmanufacture 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]
Thorium is odourless and tasteless.[182] The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water,[183] precipitating out before entering the body as the hydroxide.[184] Some thorium compounds are chemically moderatelytoxic, 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 theskeleton. 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 thetracheobronchial tract and the lungs from exposure to thorium.[184] People who work with thorium compounds are at a risk ofdermatitis. 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]
Powdered thorium metal is pyrophoric: it ignites spontaneously in air.[11] In 1964, theUnited 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]
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 theTamil 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 northernBrazilian coastal areas, from southBahia toGuarapari, 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 atSalto di Quirra on the Italian island ofSardinia.[192]
^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.
^Behindosmium,tantalum,tungsten, andrhenium;[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]
^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]
^abAfissionable 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 asfissile. Afertile 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 sustainednuclear chain reaction.
^The nameionium for230Th is a remnant from a period when different isotopes were not recognised to be the same element and were given different names.
^Unlike the previous similarity between the actinides and the transition metals, the main-group similarity largely ends at thorium before being resumed in the second half of the actinide series, because of the growing contribution of the 5f orbitals to covalent bonding. The only other commonly-encountered actinide, uranium, retains some echoes of main-group behaviour. The chemistry of uranium is more complicated than that of thorium, but the two most common oxidation states of uranium are uranium(VI) and uranium(IV); these are two oxidation units apart, with the higher oxidation state corresponding to formal loss of all valence electrons, which is similar to the behaviour of the heavy main-group elements in thep-block.[47]
^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]
^Other isotopes may occur alongside232Th, but only in trace quantities. If the source contains no uranium, the only other thorium isotope present would be228Th, which occurs in thedecay chain of232Th (thethorium series): the ratio of228Th to232Th would be under 10−10.[27] If uranium is present, tiny traces of several other isotopes will also be present:231Th and227Th from the decay chain of235U (theactinium series), and slightly larger but still tiny traces of234Th and230Th from the decay chain of238U (theuranium series).[27]229Th is also been produced in the decay chain of237Np (theneptunium series): all primordial237Np isextinct, but it is still produced as a result of nuclear reactions in uranium ores.[88]229Th is mostly produced as adaughter of artificial233U made byneutron irradiation of232Th, and is extremely rare in nature.[27]
^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]
^At the time, therare-earth elements, among which thorium was found and with which it is closely associated in nature, were thought to be divalent; the rare earths were givenatomic weight values two-thirds of their actual ones, and thorium and uranium are given values half of the actual ones.
^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 chemistLars 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]
^Thorium also appears in the 1864 table by British chemistJohn Newlands as the last and heaviest element, as it was initially thought that uranium was a trivalent element with an atomic weight of around 120: this is half of its actual value, since uranium is predominantly hexavalent. It also appears as the heaviest element in the 1864 table by British chemistWilliam Odling under titanium, zirconium, andtantalum. It does not appear in the periodic systems published by French geologistAlexandre-Émile Béguyer de Chancourtois in 1862, German-American musicianGustav Hinrichs in 1867, or German chemistJulius Lothar Meyer in 1870, all of which exclude the rare earths and thorium.[109]
^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 oflutetium. 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]
^The thirteen fissile actinide isotopes with half-lives over a year are229Th,233U,235U,236Np,239Pu,241Pu,242mAm,243Cm,245Cm,247Cm,249Cf,251Cf, and252Es. Of these, only235U have significant amounts in nature, and only233U and239Pu can be bred from naturally occurring nuclei with a single neutron capture.[162]
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