Thorium

Bulk properties

Thorium is a soft, paramagnetic, bright silvery radioactive actinide metal. In the periodic table, it is located to the right of the actinide actinium, to the left of the actinide protactinium, and below the lanthanide cerium. Pure thorium is very ductile and, as normal for metals, can be cold-rolled, swaged, and drawn. At room temperature, thorium metal has a face-centred cubic crystal structure; additionally, it has two other forms at exotic conditions, one at high temperature (over 1360 °C; body-centred cubic) and one at high pressure (around 100 GPa; body-centred tetragonal).

The properties of thorium vary widely depending on the amount of impurities in the sample: the major impurity is usually thorium dioxide (ThO₂). The purest thorium specimens usually contain about a tenth of a percent of the dioxide. 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's lattice parameters, perhaps due to microscopic voids forming in the metal when it is cast. These values lie intermediate between those of its neighbours actinium (10.1 g/cm³) and protactinium (15.4 g/cm³), showing the continuity of trends across the early actinides.

Thorium's melting point of 1750 °C is above both that of actinium (1227 °C) and that of protactinium (approximately 1560 °C). In the beginning of period 7, from francium to thorium, the melting points of the elements increase (following the trend in the other periods): this is because the number of delocalised electrons that each atom contributes increases from one in francium to four in thorium, and there is a greater attraction between these electrons and the metal ions as their charge increases from one in francium to four in thorium. After thorium, there is a new smooth trend downward in the melting points of the early actinides from thorium to plutonium where the number of f electrons increases from about 0.4 to about 6, due to the itinerance of the f-orbitals, increasing hybridisation of the 5f and 6d orbitals and the formation of directional bonds in the metal resulting in increasingly complex crystal structures and weakened metallic bonding. (The f-electron count for thorium is listed as a non-integer due to a 5f–6d overlap.) Among the actinides, thorium has the highest melting and boiling points and second-lowest density (second only to actinium) Its boiling point of 4788 °C is the fifth-highest among all the elements with known boiling points, behind only osmium, tantalum, tungsten, and rhenium.

Thorium has a bulk modulus of 54 GPa, comparable to those of tin and scandium. The hardness of thorium is similar to that of soft steel, so heated pure thorium can be rolled in sheets and pulled into wire. Nevertheless, while thorium is nearly half as dense as uranium and plutonium, it is harder than either of them. Thorium becomes superconductive below 1.4 K.

Thorium can also form alloys with many other metals. Addition of small amounts of thorium improves the mechanical strength of magnesium, and thorium-aluminium alloys have been considered as a way to store thorium in proposed future thorium nuclear reactors. With chromium and uranium, it forms eutectic mixtures, and thorium is completely miscible in both solid and liquid states with its lighter congener cerium.

Isotopes

Although every element up to bismuth (element 83) has an isotope that is practically stable for all purposes ("classically stable"), with the exceptions of technetium and promethium (elements 43 and 61), all elements from polonium (element 84) onward are noticeably radioactive. Of these, thorium (element 90) is the most stable, closely followed by uranium (element 92): the isotope ²³²Th has a half-life of 14.05 billion years, about three times the age of the earth, and even slightly longer than the generally accepted age of the universe (about 13.8 billion years). As such, ²³²Th still occurs naturally today: four-fifths of the thorium present at Earth's formation has survived to the present. Thorium and uranium are thus the most well-studied of all the radioactive elements.

The heaviest three primordial nuclides, ²³²Th, ²³⁵U, and ²³⁸U, are the only isotopes beyond bismuth that have half-lives measured in billions of years. Thus, they are the only such heavy isotopes to have survived since their production around ten billion years ago. Hence, thorium and uranium are the only important primordial radioactive elements. Even then, the half-life of ²³²Th is only a billionth that of ²⁰⁹Bi, the last classically stable nuclide. ²³²Th is the only isotope of thorium occurring in significant quantities in nature today, and thus thorium is usually considered to be a mononuclidic element. The reason for the existence of this "island of relative stability" at thorium and uranium, where the longest-lived isotopes have half-lives of millions or billions of years, is because the most stable isotopes of these elements have closed nuclear shells; nevertheless, since thorium and uranium have more protons than bismuth, their nuclei are thus still susceptible to alpha decay because the strong nuclear force is not strong enough to overcome the electromagnetic repulsion between their protons.

²³²Th is the longest-lived isotope in the 4n decay chain which includes isotopes with a mass number divisible by 4 (hence the name: it is also called the thorium series after its progenitor). The chain begins with the alpha decay of ²³²Th to ²²⁸Ra, and terminates at stable ²⁰⁸Pb. (²³²Th very occasionally undergoes spontaneous fission rather than alpha decay, and has left evidence in doing so in its minerals, but the partial half-life of this process is very large at over 10²¹ years and hence alpha decay predominates.) Because ²³²Th is so long-lived, its daughters still exist in nature as radiogenic nuclides despite their much shorter half-lives, the longest among them being the 5.7-year half-life of ²²⁸Ra, the immediate alpha daughter of ²³²Th. Any sample of thorium or its compounds contain traces of these daughters, which are isotopes of thallium, lead, bismuth, polonium, radon, radium, and actinium. As such, natural thorium samples can be chemically purified to extract its useful daughter nuclides, such as ²¹²Pb, which is used in nuclear medicine for cancer therapy.

Thirty radioisotopes have been characterised, which range in mass number from 209 to 238. The most stable of them (after ²³²Th) are ²³⁰Th with a half-life of 75,380 years, ²²⁹Th with a half-life of 7,340 years, ²²⁸Th with a half-life of 1.92 years, ²³⁴Th with a half-life of 24.10 days, and ²²⁷Th with a half-life of 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 (produced in minute traces in nuclear reactions 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.

In deep seawaters the isotope ²³⁰Th becomes significant enough that the International Union of Pure and Applied Chemistry (IUPAC) reclassified thorium as a binuclidic element in 2013, as it can then make up to 0.04% of natural thorium, with ²³²Th being the remainder. The reason for this is that while its parent ²³⁸U is soluble in water, ²³⁰Th is insoluble and thus precipitates to form part of the sediment, and may be observed doing so. 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. Thorium thus has a characteristic terrestrial isotopic composition, and so an atomic mass can be given, which is 232.0377(4) u. Thorium is one of only three significantly radioactive elements (the others being protactinium and uranium) that occur in large enough quantities on Earth for this to be possible.

Thorium also has three known nuclear isomers (or metastable states), ^216m1Th, ^216m2Th, and ^229mTh. ^229mTh has the lowest known excitation energy of any isomer, measured to be (7.6 ± 0.5) eV. This is so low that when it undergoes isomeric transition, the emitted gamma radiation is in the ultraviolet range.

In the early history of the study of radioactivity, the different natural isotopes of thorium were given different names. In this scheme, ²²⁷Th was named radioactinium (RdAc), ²²⁸Th radiothorium (RdTh), ²³⁰Th ionium (Io), ²³¹Th uranium Y (UY), ²³²Th thorium (Th), and ²³⁴Th uranium X1 (UX₁). This reflects that ²²⁷Th and ²³¹Th occur in the decay chain of natural ²³⁵U (the actinium series), ²²⁸Th occurs in the decay chain of ²³²Th (the thorium series), and that ²³⁰Th occurs in the decay chain of ²³⁸U (the uranium or radium series). The remaining natural thorium isotope, ²²⁹Th, occurs in minute traces as part of the decay chain of ²³⁷Np, the neptunium series, which is much less abundant than the other decay chains in nature and was hence discovered much later: therefore it does not have a historical name. When it was realised that all of these are isotopes of thorium, many of these names fell out of use, and "thorium" came to refer to all isotopes, not just ²³²Th. The name ionium is still encountered for ²³⁰Th in the context of ionium-thorium dating.

Different isotopes of thorium behave identically chemically, but have slightly differing physical properties: for example, the densities of isotopically pure ²²⁸Th, ²²⁹Th, ²³⁰Th, and ²³²Th in g/cm³ are respectively expected to be 11.5, 11.6, 11.6, and 11.7. The isotope ²²⁹Th is expected to be fissionable with a bare critical mass of 2839 kg, although with steel reflectors this value could drop to 994 kg. While ²³²Th is not fissionable, it is fertile as it can be converted to fissile ²³³U using neutron capture.

Radiometric dating

Two radiometric dating methods involve thorium isotopes: uranium-thorium dating, involving the decay of ²³⁴U to ²³⁰Th (ionium), and ionium-thorium dating, which measures the ratio of ²³²Th to ²³⁰Th. 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. 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 while uranium is rather soluble in water, thorium and protactinium are not, and so they are selectively precipitated into ocean-floor sediments, from which their ratios are measured. The scheme has a range of several hundred thousand years. 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. Both of these dating methods assume that the proportion of ²³⁰Th to ²³²Th is a constant during the time 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 shift within the sediment layer.

Chemistry

A thorium atom has 90 electrons, of which four are valence electrons. Three atomic orbitals are theoretically available for the valence electrons to occupy: 5f, 6d, and 7s. Despite thorium's position in the f-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. Such unusual behaviour is due to relativistic effects, which are increasingly 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 result in thorium almost always losing all four of its valence electrons and hence occurring in its highest possible oxidation state of +4. This behaviour is quite distinct from that of its lanthanide congener cerium, for whom +4 is the highest possible state but +3 also plays an important role and is more stable. Therefore, thorium is much more similar to the transition metals zirconium and hafnium than to its true lanthanide congener cerium in its properties such as ionisation energies and redox potentials, and hence also in its chemistry.

Despite this anomalous electron configuration for gaseous thorium atoms, however, metallic thorium atoms show significant 5f involvement. This was first realised in 1995, when it was pointed out that a hypothetical metallic state of thorium that had the [Rn]6d²7s² configuration with the 5f orbitals above the Fermi level should be hexagonal close packed like the group 4 elements titanium, zirconium, and hafnium, and not face-centered cubic as it actually is. Indeed, the correct crystal structure can only be obtained when the 5f states are included, proving that thorium, and not protactinium, acts as the first actinide metallurgically with the clear influence of the 5f orbitals. The 5f character of thorium is also clear in the rare and highly unstable +3 oxidation state, in which thorium exhibits the electron configuration [Rn]5f¹.

Tetravalent thorium compounds are usually colourless or yellow, like those of silver or lead, as the Th⁴⁺ ion has no 5f or 6d electrons. Thorium and uranium are the most investigated of the radioactive elements because their radioactivity is slight enough to not pose major problems of handling and accessibility and they may be safely handled in a normal laboratory.

Reactivity

Thorium is a highly reactive and electropositive metal. Finely divided thorium metal presents a fire hazard due to its pyrophoricity and must therefore be handled carefully. When heated in air, thorium turnings ignite and burn brilliantly with a white light to produce the dioxide. In bulk, the reaction of pure thorium with air is slow, although corrosion may eventually occur after several months; most thorium samples are contaminated with varying degrees of the dioxide, which greatly accelerates corrosion. Such samples slowly tarnish in air, becoming grey and finally black at the surface.

At standard temperature and pressure, thorium is slowly attacked by water, but does not readily dissolve in most common acids, with the exception of hydrochloric acid, where it dissolves leaving behind a black insoluble residue, ThO(OH,Cl)H. It dissolves in concentrated nitric acid containing a small amount of catalytic fluoride or fluorosilicate ions; if these are not present, passivation can occur, similarly to uranium and plutonium.

Inorganic compounds

Most binary compounds of thorium with nonmetals may simply be prepared by heating the elements together. In air, thorium burns to form the simple dioxide, ThO₂, also called thoria or thorina: this has the fluorite structure. Thoria, a refractory material, has the highest melting point (3390 °C) of all known oxides. It is somewhat hygroscopic and reacts readily with water and many gases, but dissolves easily in concentrated nitric acid in the presence of fluoride. When heated, it emits intense blue light through incandescence, which becomes white when mixed with its lighter homologue cerium dioxide (CeO₂, ceria): this is the basis for its previously common application in gas mantles. Several binary thorium chalcogenides and oxychalcogenides are also known with sulfur, selenium, and tellurium.

All four thorium tetrahalides are known, as are some low-valent bromides and iodides: the tetrahalides are all 8-coordinated hygroscopic compounds that dissolve easily in polar solvents such as water. Additionally, many related polyhalide ions are also known. Thorium tetrafluoride has a monoclinic crystal structure and is isotypic with zirconium tetrafluoride and hafnium tetrafluoride, where the Th⁴⁺ ions are coordinated with F⁻ ions in somewhat distorted square antiprisms. The other tetrahalides instead have dodecahedral geometry. Lower iodides ThI₃ (black) and ThI₂ (gold) can also be prepared by reducing the tetraiodide with thorium metal: These do not contain Th(III) and Th(II), but instead contain Th⁴⁺ and could be more clearly formulated as electride compounds. Many polynary halides with the alkali metals, barium, thallium, and ammonium are known for thorium fluorides, chlorides, and bromides. For example, when treated with potassium fluoride and hydrofluoric acid, Th⁴⁺ forms the complex anion ThF2−
6, which precipitates as an insoluble salt, K₂ThF₆.

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

Coordination compounds

In acidic aqueous solution, thorium occurs as the tetrapositive aqua ion [Th(H₂O)₉]⁴⁺, which has tricapped trigonal prismatic molecular geometry: at pH < 3, the solutions of thorium salts are dominated by this cation. 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. 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 is formed and precipitates out (though equilibrium may take weeks to be reached, because the polymerisation usually slows down significantly just before the precipitation). 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.

Large 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. The distinctive ability of thorium salts is their high solubility, not only in water, but also in polar organic solvents.

Many other inorganic thorium compounds with polyatomic anions are known, such as the perchlorates, sulfates, sulfites, nitrates, carbonates, phosphates, vanadates, molybdates, and chromates, and their hydrated forms. 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. 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. Thorium complexes with organic ligands, such as oxalate, citrate, and EDTA, are much stronger and tend to occur naturally in natural thorium-containing waters in concentrations orders of magnitude higher than the inorganic complexes.

Organothorium compounds

Most of the work on organothorium compounds has focused on the cyclopentadienyls and cyclooctatetraenyls. Like many of the early and middle actinides (up to americium, and also expected for curium), thorium forms the yellow cyclooctatetraenide complex Th(C₈H₈)₂, thorocene. It is isotypic with the better-known analogous uranium compound, uranocene. It can be prepared by reacting K₂C₈H₈ with thorium tetrachloride in tetrahydrofuran (THF) at the temperature of dry ice, or by reacting thorium tetrafluoride with MgC₈H₈. It is an unstable compound in air and outright decomposes in water or at 190 °C. 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.

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) is a rare example of thorium in the formal +3 oxidation state; a formal +2 oxidation state even occurs in a derivative. The chloride derivative [Th(C₅H₅)₃Cl] is prepared by heating thorium tetrachloride with limiting K(C₅H₅) used (other univalent metal cyclopentadienyls can also be used). The alkyl and aryl derivatives are prepared from the chloride derivative and have received attention due to the insight they give regarding the nature of the Th–C sigma bond.

Other organothorium compounds are not well-studied. Tetrabenzylthorium, Th(CH₂C₆H₅), and tetraallylthorium, Th(C₃H₅)₄, are known, but their structures have not yet been determined and they decompose slowly at room temperature. Thorium forms the monocapped trigonal prismatic anion [Th(CH₃)₇]³⁻, heptamethylthorate, which forms the salt [Li(tmeda)]₃[ThMe₇] (tmeda = Me₂NCH₂CH₂NMe₂). 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.

Formation

²³²Th is a primordial nuclide, having existed in its current form for over ten billion years; it was forged in the cores of dying stars through the r-process and scattered across the galaxy by supernovae. The letter "r" stands for "rapid neutron capture", and occurs in core-collapse supernovae, where heavy seed nuclei such as ⁵⁶Fe rapidly capture neutrons, running up against 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. Because of the abrupt loss of stability past ²⁰⁹Bi, the r-process is the only process of stellar nucleosynthesis that can create isotopes of thorium and uranium, because all other processes are too slow and the intermediate nuclei alpha decay before they capture enough neutrons to reach these elements.

In the universe, thorium is among the rarest of the primordial elements: it achieves this position not only because it is one of the two elements that can be produced only in the r-process (with the other one being uranium), but 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. Furthermore, neutron capture by nuclides beyond the end of the line of beta stability at A = 209 often results in nuclear fission instead of neutron absorption, reducing the fraction of nuclei that cross the gap of instability past bismuth to become thorium. On Earth, thorium is much more abundant: with an abundance of 8.1 ppm in the Earth's crust, it is one of the most abundant of the heavy elements, almost as abundant as lead (13 ppm) and significantly more abundant than tin (2.1 ppm). This is because thorium, being a lithophile, is likely to form oxide minerals that do not sink into the core. On the other hand, the heavy platinum group metals have no affinity for oxygen and their oxides may even be thermodynamically unstable with respect to the elements. Instead, they readily dissolve in iron and sink down with it into the core. Furthermore, thorium compounds are also poorly soluble in water.

On Earth

Natural thorium is essentially isotopically pure ²³²Th, which is the longest-lived and most stable isotope of thorium, having a half-life comparable to the age of the universe. Its radioactive decay is the largest single contributor to the Earth's internal heat; the other major contributors are the shorter-lived primordial nuclides, ²³⁸U, ⁴⁰K and ²³⁵U. 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.

On Earth, thorium is not a rare element as was previously thought, having a crustal abundance comparable to that of lead and molybdenum, twice that of arsenic, and thrice that of tin. Thorium only occurs as a minor constituent of most minerals. Soil normally contains about 6 parts per million (ppm) of thorium.

In nature, thorium occurs in the +4 oxidation state, together with uranium(IV), zirconium(IV), hafnium(IV), and cerium(IV), but also with scandium, yttrium, and the trivalent lanthanides which have similar ionic radii. Because of thorium's radioactivity, minerals containing significant quantities of thorium are often metamict, their crystal structure having been partially or totally destroyed by the alpha radiation produced in the radioactive decay of thorium. An extreme example is ekanite, (Ca,Fe,Pb)₂(Th,U)Si₈O₂₀, which almost never occurs in nonmetamict form due to thorium being an essential part of its chemical composition.

Monazite is the most important commercial source of thorium because it occurs in large deposits worldwide, principally in India, South Africa, Brazil, Australia, and Malaysia. It contains around 2.5% thorium on average, although some deposits may contain up to 20% thorium. Monazite is a chemically unreactive phosphate mineral that is found as yellow or brown sand; its low reactivity makes it difficult to extract thorium from it. Allanite can have 0.1–2% thorium and zircon up to 0.4% thorium.

Thorium dioxide occurs as the rare mineral thorianite, which usually contains up to 12% ThO₂. 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. Thorite, or tetragonal thorium silicate (ThSiO₄), also has a high thorium content and is the mineral in which thorium was first discovered. In thorium silicate minerals, the Th⁴⁺ and SiO4−
4 ions are often replaced with M³⁺ (M = Sc, Y, Ln) and phosphate (PO3−
4) ions respectively. Because of the great insolubility of thorium dioxide, thorium does not usually spread quickly through the environment when released in significant quantities: the Th⁴⁺ ion is soluble, especially in acidic soils, and in such conditions the thorium concentration can reach 40 ppm.

History

In 1815, the Swedish chemist Jöns Jakob Berzelius analysed an unusual sample of gadolinite from a copper mine in Falun, 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. (By that time, Berzelius had already discovered two elements, cerium and selenium, but he had made a public mistake once, announcing a new element, gahnium, that turned out to be simply zinc oxide.) Berzelius privately named the supposed tentative element "thorium" in 1817 and its supposed oxide "thorina" after Thor, the Norse god of thunder. In 1824, after more deposits of the same mineral in Vest-Agder, Norway, were discovered, he retracted his findings, as the mineral in question proved to actually be an yttrium mineral, primarily composed of yttrium orthophosphate. As the yttrium in this mineral was initially mistaken as being a new element, the mineral was named kenotime by the French mineralogist François Sulpice Beudant as a rebuke of Berzelius, from the Greek words κενός (vain) and τιμή (honour). This became "xenotime" as a misprint from the beginning, blunting the criticism. This misspelt form was later explained as being from ξενός (stranger to) and τιμή (honour), supposedly referencing the small, rare and easily overlooked crystals that xenotime occurs as.

In 1828, Morten Thrane Esmark found a black mineral on Løvøya island, Telemark county, Norway. He was a Norwegian priest and amateur mineralogist who studied the minerals in Telemark, where he served as vicar. 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 the University of Oslo. The elder Esmark determined that it was not any known mineral and sent a sample to Berzelius for examination. Berzelius determined that it contained a new element. He published his findings in 1829, having isolated an impure sample for the first time by reducing KThF₅ with potassium metal. Berzelius reused the name of the previous supposed element discovery. Thus, he named the source mineral thorite, which has the chemical composition (Th,U)SiO₄. Berzelius also made some initial characterisation of the new metal and its chemical compounds: he correctly determined that the thorium–oxygen mass ratio was 7.5 (its actual value is close to that, ~7.3), but he assumed the new element was divalent rather than tetravalent, and as such assumed that the atomic mass was 7.5 times that of oxygen (120 amu), while it is actually 15 times as large. He determined that thorium was a very electropositive metal, that he placed ahead of cerium and behind zirconium in electropositivity. Berzelius did not isolate the element in its metallic state; for the first time, thorium was isolated in 1914 by D. Lely Jr. and L. Hamburger. They obtained 99% pure thorium metal by reducing thorium chloride with sodium metal. A simpler method leading to even higher purity was discovered in 1927 by Marden and Rentschler, involving the reduction of thorium oxide with calcium when calcium chloride was present.

In Dmitri Mendeleev's 1869 periodic table, 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 contained the modern carbon group (group 14), titanium group (group 4), cerium, and thorium, because their maximum oxidation state was +4. Cerium was soon removed from the main body of the table and placed in a separate lanthanide series, while thorium remained with group 4 as it had similar properties to its supposed lighter congeners in that group, such as titanium and zirconium.

Although thorium was discovered in 1828, it had no applications until 1885, when Carl Auer von Welsbach invented the gas mantle, a portable source of light which produces light from the incandescence of very hot thorium oxide, heated to extremely high temperatures by burning gaseous fuels. After 1885, many applications were found for thorium and its compounds, such as in ceramics, carbon arc lamps, heat-resistant crucibles, and as catalysts for industrial chemical reactions such as the oxidation of ammonia to nitric acid.

In the late 19th century onward, the atomic theory underwent significant improvements, which shaped the further history of thorium. Thorium was first observed to be radioactive in 1898, independently, by the German chemist Gerhard Carl Schmidt and later that year, 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 Henri Becquerel. Between 1900 and 1903, Ernest Rutherford and Frederick Soddy showed how thorium decayed at a fixed rate over time into a series of other elements. This observation led to the identification of half-life as one of the outcomes of the alpha particle experiments that led to their disintegration theory of radioactivity. The biological effect of radiation was discovered in 1903; the danger presented by radioactivity to health and environment was the reason thorium was phased out of use in applications that did not explicitly use the radioactivity. Since the 1930s, it has been widely acknowledged that thorium possesses a minor threat to human organisms in large quantities.

In 1922, 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. Such a second extra-long periodic table row, to accommodate known and undiscovered elements with an atomic weight greater than bismuth (thorium, protactinium, and uranium, for example), had been postulated as far back as 1892 by Henry Bassett, who considered thorium and uranium to be members of a second series of rare earths, and predicted that more elements would be found within this series. Most investigators considered on the basis of their chemical properties that these elements were analogous to the 5d transition elements hafnium, tantalum, and tungsten, and considered the existence of the lanthanides in the sixth row to be a one-off fluke. 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. It was only with the discovery of the first transuranic elements, which from plutonium onward have dominant +3 and +4 oxidation states (neptunium having +4 and +5 as dominant states and +7 as an unstable state), that it was realised that the actinides were indeed filling f-orbitals rather than d-orbitals. Thus it was not until 1945, after Glenn T. Seaborg and his team had discovered the transuranic elements americium and curium, that Seaborg realised that thorium was the second member of the actinide series and was filling an f-block row, instead of being the heavier congener of hafnium and filling a fourth d-block row. Today, thorium's similarities to hafnium are still sometimes acknowledged by calling it a "pseudo group-4 element".

Despite thorium's radioactivity, the element has remained in use for a long time for applications not exploiting the effect as no suitable alternatives could be found. While a 1981 study estimated that a dose from using a thorium mantle every weekend would be safe for a person, this was not the case for people manufacturing the mantles (and thus contacting many) as well as soils around some factory sites. A major shift occurred as late as in the 1990s, when most of these applications that do not depend on thorium's radioactivity declined quickly due to safety and environmental concerns as suitable safer replacements have been found. Due to concerns, some manufacturers have switched to other materials, such as yttrium, although these are usually either more expensive or less efficient. Other manufacturers continue to make thorium mantles, but moved their factories to developing countries. As recently as 2007, some companies continued to manufacture and sell thorium mantles without giving adequate information about their radioactivity, with some even fraudulently claiming them to be non-radioactive while in reality using large quantities of thorium, up to 259 milligrams per mantle.

Efforts have been applied to initiate usage of thorium and its radioactivity as a power source; the earliest thorium-based reactor was made in U.S: the first core at the Indian Point Energy Center in 1962. India has one of the largest supplies of thorium in the world but does not have much uranium used elsewhere, and targeted in the 1950s at achieving energy independence for the country with their three-stage nuclear power programme. On the other hand, in most countries, the progress staggered because uranium was relatively abundant and the progress of thorium-based reactors was therefore slow (in the 20th century, 3 reactors were opened in India and 12 elsewhere). Large-scale research was begun in 1996 by the International Atomic Energy Agency (IAEA) to study the use of thorium reactors; a year later, the U.S. Energy Department began their research on the matter. Nuclear scientist Alvin Radkowsky of Tel Aviv University in Israel, the head designer of the American first civilian nuclear power plant at Shippingport, Pennsylvania whose third core bred thorium, founded a consortium to develop thorium reactors, which included other companies: Raytheon Nuclear Inc. and Brookhaven National Laboratory in the U.S. and the Kurchatov Institute in Russia. In the 21st century, thorium's potential for improving proliferation resistance and waste characteristics led to renewed interest in the thorium fuel cycle.

Production

Thorium is extracted mostly from monazite: thorium pyrophosphate (ThP₂O₇) is reacted with nitric acid, and the produced thorium nitrate treated with tributyl phosphate. Rare-earth impurities are separated by increasing the pH in sulfate solution.

In another extraction method, monazite is decomposed with a 45% aqueous solution of sodium hydroxide at 140 °C. Mixed metal hydroxides are extracted first, filtered at 80 °C, washed with water and dissolved with concentrated hydrochloric acid. Next, the acidic solution is neutralised with hydroxides to pH = 5.8 that results in precipitation of thorium hydroxide (Th(OH)₄) contaminated with ~3% of rare-earth hydroxides; the remaining rare-earth hydroxides remain in solution. Thorium hydroxide is dissolved in an inorganic acid and then purified from the rare earth elements. An efficient method is the dissolution of thorium hydroxide in nitric acid, because the resulting solution can be purified by extraction with organic solvents:

Th(OH)₄ + 4 HNO₃ → Th(NO₃)₄ + 4 H₂O

Metallic thorium is separated from the anhydrous oxide or chloride by reacting it with calcium in an inert atmosphere:

ThO₂ + 2 Ca → 2 CaO + Th

Sometimes thorium is extracted by electrolysis of a fluoride in a mixture of sodium and potassium chloride at 700–800 °C in a graphite crucible. Highly pure thorium can be extracted from its iodide with the crystal bar process.

Since thorium has few applications today, only a few hundred tonnes of it are produced each year, mostly as a by-product of the production of uranium and the lanthanides, and even these meagre quantities exceed the current demand for thorium. Half of this thorium produced is used in gas mantles. Nevertheless, production could easily be increased, and probably would be if thorium became used in large quantities as nuclear fuel. Present knowledge of the distribution of thorium resources is poor because of the relatively low-key exploration efforts arising out of insignificant demand.

Applications

Many applications of thorium are becoming obsolete due to environmental concerns largely stemming from the radioactivity of thorium and its decay products. Thorium is thus being phased out of many of its uses.

Thorium is used in gas tungsten arc welding (GTAW) to increase the high-temperature strength of tungsten electrodes and improve arc stability. In electronic equipment, thorium coating of tungsten wire improves the electron emission of heated cathodes.

Thorium is added to tungsten because it lowers the effective work function with the result that the thoriated tungsten thermocathode emits electrons at considerably lower temperatures. This is used in thoriated tungsten elements found in the filaments of vacuum tubes, e.g. magnetron found in microwave oven.

Many applications of thorium use, rather than the metal, its dioxide, commonly called "thoria" in the industry. The melting point of thoria is 3300 °C – the highest of all known oxides; only a few substances have higher melting points. This means that when heated to high temperatures, it does not melt, but merely glows with an intense blue light; addition of cerium dioxide gives a bright white light. Sometimes thorium nitrate is used instead, because it will thermally decompose to thoria at high temperatures. This property of thoria is used in thorium mantles of portable gas lights, including natural gas lamps, oil lamps and camping lights. Additionally, thoria is a material for heat-resistant ceramics, as used in high-temperature laboratory crucibles.

When added to glass, thoria helps increase refractive index and decrease dispersion. Such glass finds application in high-quality lenses for cameras and scientific instruments. The radiation from these lenses can darken them and turn them yellow over a period of years and degrade film, but the health risks are minimal. Yellowed lenses may be restored to their original colourless state with lengthy exposure to intense ultraviolet radiation.

Thoria has been used as a chemical catalyst in the conversion of ammonia to nitric acid, in petroleum cracking and in producing sulfuric acid.

Thorium tetrafluoride is used as an antireflection material in multilayered optical coatings. It has an optical transparency in the range of 0.35–12 µm, and its radiation is primarily due to alpha particles, which can be easily stopped by a thin cover layer of another material. Thorium tetrafluoride was also used in manufacturing carbon arc lamps, which provided high-intensity illumination for movie projectors and search lights.

Potential use for nuclear energy

In thermal breeder reactors, the fertile isotope ²³²Th is bombarded by slow neutrons, undergoing neutron capture to become ²³³Th, which undergoes two consecutive beta decays to become first ²³³Pa and then the fissile ²³³U:

232
90Th
+ n → 233
90Th
+ γ β⁻→21.8 min 233
91Pa
β⁻→27.0 days 233
92U
α→1.59 × 105
y

²³³U is fissile and hence can be used as a nuclear fuel in much the same way as the more-commonly used ²³⁵U or ²³⁹Pu. When ²³³U undergoes nuclear fission, the neutrons emitted can strike further ²³²Th nuclei, restarting the cycle. This closely parallels the uranium fuel cycle in fast breeder reactors where ²³⁸U undergoes neutron capture to become ²³⁹U, beta decaying to first ²³⁹Np and then fissile ²³⁹Pu. The main advantage of the thorium fuel cycle is that thorium is more abundant than uranium and hence can satisfy world energy demands for longer. Furthermore, ²³²Th absorbs neutrons more readily than ²³⁸U, and not only does ²³³U have a higher probability of fission upon neutron capture (92.0%) than ²³⁵U (85.5%) or ²³⁹Pu (73.5%), it also releases more neutrons upon fission on average. An added advantage ²³³U and ²³⁹Pu enjoy over all other fissile nuclei (except the naturally occurring ²³⁵U) is that they can be bred from neutron capture by the naturally occurring quantity isotopes ²³²Th and ²³⁸U. Thorium fuels also result in a safer and better-performing reactor core because thoria has a higher melting point, higher thermal conductivity, and lower coefficient of thermal expansion than the now-common fuel uranium dioxide (UO₂): thoria is also more chemically stable as, unlike uranium dioxide, it does not further oxidise to triuranium octoxide (U₃O₈). Furthermore, while a single neutron capture by ²³⁸U would produce transuranic waste, along with it fissile ²³⁹Pu, five captures are generally necessary to do so from ²³²Th. 98–99% of thorium-cycle fuel nuclei would fission at either ²³³U or ²³⁵U, so fewer long-lived transuranics are produced. Because of this, thorium is a potentially attractive alternative to uranium in mixed oxide (MOX) fuels to minimise the generation of transuranics and maximise the destruction of plutonium.

The main disadvantage is that it is difficult and dangerous to reprocess the used fuel because many of the daughters of ²³²Th and ²³³U are strong gamma emitters. Additionally, all ²³³U production methods other than mercury fluourescence always result in significant impurities of the very dangerous gamma emitter ²³²U, either from parasitic (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:

230
90Th
+ n → 231
90Th
+ γ β⁻→25.5 h 231
91Pa
α→3.28 × 104
y231
91Pa
+ n → 232
91Pa
+ γ β⁻→1.3 d 232
92U
α→69 y

These impurities of ²³²U make ²³³U very easy to detect and very dangerous to work on, and the impracticality of their separation limits the possibilities of nuclear proliferation using ²³³U as the fissile material. Additionally, ²³³Pa has a relatively long half-life of 27 days and a high cross section for neutron capture. Thus it is a neutron poison: instead of rapidly decaying to the useful ²³³U, a significant amount of ²³³Pa converts to ²³⁴U and consumes neutrons, degrading the reactor efficiency. To avoid this, ²³³Pa is extracted from the active zone of thorium molten salt reactors during their operation, so that it only decays to ²³³U.

Another disadvantage of the thorium fuel cycle is the need to neutron-irradiate and process natural ²³²Th before these advantages become real, and this requires more advanced technology than the presently used fuels based on uranium and plutonium; nevertheless, advances are being made in this technology. Another common criticism centres around the low commercial viability of the thorium fuel cycle: some entities like the Nuclear Energy Agency go further and predict that the thorium cycle will never be commercially viable while uranium is available in abundance—a situation which Trevor Findlay predicts will persist "in the coming decades". Furthermore, though the isotopes produced in the thorium fuel cycle are not transuranic, some of them are still very dangerous, such as ²³¹Pa, which has a long half-life of 32760 years and is a major contributor to the long term radiotoxicity of spent nuclear fuel.

Radiological

Thorium is odourless and tasteless. As thorium occurs naturally, it exists in very small quantities almost everywhere on Earth: the average human contains about 100 micrograms of thorium and typically consumes three micrograms per day of thorium. This exposure is raised for people who live near uranium, phosphate, or tin processing factories, thorium deposits, radioactive waste disposal sites, and for those who work in uranium, thorium, tin, or phosphate mining or gas mantle production industries. 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. When thorium is ingested, 99.98% does not remain in the body. Out of the thorium that does remain in the body, three quarters of it accumulates in the skeleton. While absorption through the skin is possible, it is not a likely means of thorium exposure.

Natural thorium decays very slowly compared to many other radioactive materials, and the alpha radiation emitted cannot penetrate human skin. As a result, owning and handling small amounts of thorium, such as those in a gas mantle, is considered safe, although usage of such items may pose some risks. 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. Exposure to thorium internally leads to increased risk of liver diseases.

Although ²³²Th is only mildly radioactive, it decays into more dangerous radionuclides such as radium and radon. A proper assessment of the radiological toxicity of ²³²Th must include the contribution of its daughters, such as ²²⁸Ra, which are dangerous gamma emitters. There are concerns about the safety of thorium mantles, as the dangerous daughters of thorium have much lower melting points than thoria (except of course ²²⁸Th, which is chemically identical to its parent ²³²Th), and they would be volatilised every time the mantle is heated for use. For example, during burning, significant fractions of the thorium daughters ²²⁴Ra, ²²⁸Ra, ²¹²Pb, and ²¹²Bi are released in the first hour of use alone. 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 a mammogram. If a mantle is ingested, 0.4% of the thorium and 90% of its dangerous daughters are leached into the body. Some nuclear safety agencies make recommendations about their use, and have raised some safety concerns regarding their manufacture and disposal, because while the radiation dose from one mantle is not a serious problem, that from many mantles gathered together in factories or landfills is.

Chemical

The chemical toxicity of thorium is low because thorium and its most common compounds (mostly the dioxide) are poorly soluble in water. Nevertheless, some thorium compounds are chemically moderately toxic. 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.

Powdered thorium metal is pyrophoric and often ignites spontaneously in air. In the 1956 Sylvania Electric Products explosion, the burning of thorium metal powder sludge led to nine injuries, some severe, as well as one death from thorium poisoning.