Thorium

Thorium
Atomic number	90
Symbol	Th
Boiling point	4788 °C
Electronegativity	1.3 (Pauling)
Electron configuration	[Rn] 6d2 7s2
Melting point	1750 °C
Period	7
Main isotopes	232Th, 230Th, 229Th
Phase STP	Solid
Block	f
Oxidation states	+4, +3
Wikidata	Q1115

Thorium (symbol Th, atomic number 90) is a lustrous, silvery metal in the actinide series of the periodic table. It is heavy and malleable but tarnishes in air to a dull gray oxide (ThO₂). Thorium is weakly radioactive; all its isotopes are unstable but the most abundant isotope, ^232Th, has a half-life of about 14.0 billion years – comparable to the age of the universe – so thorium persists in nature. At standard temperature and pressure (0 °C and 1 atm, or STP), thorium is a solid metal with a high melting point. It belongs to the f-block (actinide series, Period 7) and is a relatively electropositive element. Thorium typically forms compounds in the +4 oxidation state (losing four electrons), though +3 and +2 states are also known in some more reducing environments. In compounds it behaves much like its lighter homologues (zirconium and hafnium) but is more reactive and more easily oxidized. Chemically, thorium is the only naturally occurring actinide (aside from uranium) that is not dominated by a +3 state; its stable +4 ions give colorless salts in solution.

Thorium occurs in nature as a primordial element (present since Earth’s formation); it is about three times more abundant in the Earth’s crust than uranium. The main commercial source of thorium is the rare-earth phosphate mineral monazite, which can contain several percent thorium. Thorium was discovered in 1828 by Swedish chemist Jöns J. Berzelius, who named it after Thor, the Norse god of thunder and war. Historically it was used in gas lamp mantles (as thorium dioxide) and other high-temperature applications. More recently thorium has attracted interest as a nuclear fuel: ^232Th can absorb a neutron and eventually transmute to uranium-233, a fissile isotope usable in reactors.

Overview and Key Facts

Element and category: Thorium is an actinide metal (f-block) with atomic number 90. It lies in Period 7 of the periodic table. (Actinides are heavy, typically radioactive elements with atomic numbers 89–103.)
Symbol: Th (from Thorium, after Thor).
Atomic weight: ≈ 232.0377 (the standard atomic weight is taken essentially from ^232Th).
Phase at STP: Solid metallic. It is silvery-white when freshly prepared, but quickly tarnishes to gray in air as ThO₂ forms.
Melting/boiling points: Very high – melting at about 1750 °C and boiling around 4788 °C. These are among the highest of all elements (the boiling point is higher than osmium’s).
Density: ~11.7 g/cm³ at room temperature. This is comparatively low for an actinide metal (only actinium is lighter).
Crystal structure: At ambient conditions thorium metal adopts a face-centered cubic (fcc) lattice. Above about 1360 °C it transforms to a body-centered cubic (bcc) phase, and under extreme pressure it takes a body-centered tetragonal form. These transitions are allotropes (different solid phases) of the metal.
Common oxidation states: +4 (dominant), +3 and +2. In practice thorium forms mostly +4 compounds (Th^4+). The +3 state appears in some complexes and salts, and +2 is very rare but known.
Electron configuration: [Rn] 6d² 7s² in the neutral atom. In the ground state, the valence electrons occupy two 6d orbitals and two 7s orbitals beyond the radon core. These four valence electrons (6d²7s²) mean thorium has a formal valence of four at least.
Valence electrons: 4 (the 6d and 7s electrons outside the noble-gas core). These electrons are available for bonding or ionization.
Electronegativity: Thorium’s Pauling electronegativity is about 1.3 (on Pauling scale). This relatively low value (less than half of fluorine’s 4.0) reflects that thorium is only moderately willing to attract bonding electrons; it tends to form ionic or partially ionic bonds in its +4 state.
First ionization energy: Approximately 6.3 eV (≈ 610 kJ/mol). This is one of the lower ionization energies among the actinides (lower than most lanthanides and similar heavy elements), consistent with thorium’s large atomic size and relatively low nuclear charge felt by the outer electrons.
Atomic radius: Characteristic metallic radius about 180 pm, covalent radius ~165 pm. (For comparison, lanthanum is about 187 pm, reflecting that thorium is somewhat smaller than it might have been without the actinide contraction.)
Isotopes: All thorium isotopes are radioactive. Natural thorium is almost entirely ^232Th (half-life 1.40×10^10 y), with trace amounts of daughter isotopes like ^230Th (t½ ≈ 7.54×10^4 y). Thorium’s isotopic composition is essentially mononuclidic from a practical standpoint.
Primordial element: Thorium (along with uranium and potassium-40, etc.) is primordial, meaning it has existed on Earth since its formation due to its long half-life. (No significant amount is produced in ongoing processes.)
Abundance: Thorium is about 6–10 parts per million (ppm) by weight in the Earth’s crust. This makes it one of the more abundant heavy (high-atomic-number) elements – roughly on par with lead. It is about three times as abundant as uranium in the crust. In the universe it is quite rare (an r-process element), ranking around 77th in cosmic abundance, because it can only be made in extreme stellar processes and it decays over time.
Ores: The most important mineral is monazite ((Ce,La,Nd,Th)PO₄), a phosphate that often contains ~2–7% ThO₂. Thorium is also found in thorite (ThSiO₄) and thorianite (natural ThO₂). Many zircon and other heavy minerals contain a few percent Th.

Thorium’s combination of radioactivity and metallic properties made it historically useful (e.g. as ThO₂ in incandescent mantles) and now of interest for nuclear fuel. Despite its radioactivity, a pure piece of thorium emits weak radiation and has properties much like a typical heavy metal.

Atomic Structure and Electron Configuration

Thorium’s atomic structure can be understood by its electron configuration. The neutral atom has 90 electrons and a closed radon core Rn], which is krypton + fourteen electrons), followed by 6d²7s². This means the four outer electrons (6d and 7s) are the valence electrons. In chemical compounds, Thorium commonly uses all four to form Th^4+ ions. In special cases (for example, strong reducing agents), it can be reduced to Th^3+ (using one fewer electron) or even Th^2+ (though Th^2+ salts are much rarer).

In the periodic table, thorium is the first element of the actinide series. As the first f-block element, its 5f orbitals begin to fill (the ground state of Th has no 5f electrons, but relativistic effects mix 5f and 6d). Compared to its lanthanide analogs (like La and Ce), thorium’s atomic radius (about 180 pm) is somewhat contracted due to the actinide contraction (a trend similar to the lanthanide contraction). Because thorium is so heavy and its electrons are far from the nucleus, it has a low first ionization energy (~6.3 eV) – quite low for a heavy element. This low ionization energy and low electronegativity (1.3) reflect that thorium readily loses electrons and forms cations in its compounds.

As an actinide, thorium shows periodic trends characteristic of the f-block. Its atomic radius is larger than most of the following actinides, and decreases slightly across the series. Its electronegativity is low (only about 1.3 on Pauling’s scale) because it has relatively few protons attracting many electrons at long distances. The low ionization energy means noble-gas-like core with loosely bound outer electrons. In the actinide series up to uranium, thorium has the highest melting and boiling points; after thorium, the melting points generally fall off across protactinium, uranium, plutonium, due to 5f orbital effects.

Thorium’s valence shell orbitals (6d and 7s) are quite large, so in compounds thorium typically attains a coordination number of 8 or 9. This is why ThO₂ has a fluorite (cubic) structure with each Th surrounded by 8 oxygens. The ionic radius of Th^4+ (in coordination VIII) is about 111 pm, comparable to Ce^4+ or Zr^4+.

Isotopes and Nuclear Properties

Thorium has no stable isotopes in the sense of indefinite half-life. The overwhelmingly predominant isotope is ^232Th, which is essentially “stable” on human timescales because of its extremely long half-life (1.40×10^10 years, longer than Earth’s age). In practice, natural thorium is nearly mononuclidic in ^232Th. However, small traces of other isotopes occur from decay chains, especially ^230Th (half-life ~75,400 years) produced in the uranium-238 decay series, and ^228Th (1.91-year half-life) from the thorium-232 decay chain. Recently, IUPAC has noted that minor amounts of ^230Th are present in the environment, making thorium technically binuclidic, but these are trace.

^232Th decays almost entirely by alpha emission to radium-228, entering the “thorium series” of radioactive decay that ultimately leads to stable ^208Pb. Because ^232Th decays so slowly, the radiation from a bulk sample is relatively low (much lower activity than an equal mass of uranium or potassium). Thorium’s decay therefore contributes to natural background radiation but is not intensely radiological. (For example, a kilogram of thorium metal emits on the order of 30,000 Bq of alpha activity, much less than fuel-pellet materials.)

Important thorium isotopes for other reasons include ^229Th (half-life 7916 years) which has a remarkable low-energy nuclear isomeric state (^229mTh). This near-3.5 eV nuclear transition has been extensively studied for a potential “nuclear clock” because of its exceptionally low energy. Another isotope, ^230Th, is widely used in radiometric dating: the ratio ^230Th/^234U is used in U–Th dating of corals and carbonates, since ^234U decays to ^230Th. Thorium-228 (from ^232Th decay) is also important in geology and medicine (it decays to ^224Ra, which tends to accumulate in bone).

Thorium’s nuclear properties are central to its use as a “fertile” nuclear fuel. ^232Th is not fissile by itself (it cannot sustain a fission chain with thermal neutrons), but it can absorb a neutron to become ^233Th, which beta-decays to protactinium-233 and then to ^233U, a fissile isotope. In a reactor neutron flux, this breeding process yields ^233U (half-life 159,000 years for ^233Pa) which can undergo fission and release energy. This thorium fuel cycle has been tested (for example, the Shippingport reactor in the 1970s used a thorium/uranium-233 core) and is under renewed investigation (especially molten-salt reactor designs). The conversion efficiency and byproduct isotopes (like ^231Pa with a half-life of 32,760 years) are key aspects of thorium fuel technology.

In summary, thorium’s nuclear properties include: no stable isotopes, a single long-lived isotope (^232Th), and the ability to breed fissile uranium-233. Its decay series produces some radioactive daughters (Ra, Ac, Hf, Pb). The isotope spins and nuclear structure are of academic interest (for example, ^232Th has spin 0+, ^229Th has spin 5/2+). The important point is that thorium is radiogenic but relatively gentle due to its low decay rate, and its nuclear transmutation under neutron irradiation is the basis for thorium-based nuclear fuels.

Allotropes and Typical Compounds

Thorium does not have dramatic allotropes like carbon’s graphite/diamond. Its allotropy is limited to metallic crystal phases. At room temperature thorium metal is the “alpha” phase with an fcc lattice. Upon heating above about 1360 °C it transforms to the beta phase (body-centered cubic, bcc), and under extremely high pressure it goes to a gamma phase (body-centered tetragonal). These are simply different atomic arrangements of the metal and are not usually called “allotropes” in the sense of totally different forms, but they are distinct solid phases. There are no known non-metallic allotropes (thorium is not molecular in the solid state).

Thorium’s chemistry is dominated by compounds in the +4 oxidation state (Th^4+). These include:

Oxides: Thorium dioxide (ThO₂), known as thoria, is the most important oxide. It is a very high-melting, inert ceramic (fluorite structure) used in high-temperature ceramics and previously in gas mantles. Lower oxides like Th₂O₃ or ThO are not stable under normal conditions. ThO₂ is very refractory (melting point ~3300 °C, among the highest of all oxides) and is chemically resistant to air, acids, and water.

Halides: Thorium forms tetravalent halides. For example, thorium tetrafluoride (ThF₄) is a white ionic solid (coordination VII or VIII around Th); ThF₄ is used in molten salt reactor concepts because it dissolves in fluoride salts. Thorium tetrachloride (ThCl₄) is a volatile white solid; it sublimates and was used historically in thorium metal production by reduction. Similarly ThBr₄ and ThI₄ can be made. Divalent or trivalent halides (ThCl₃ etc.) are known under very reducing conditions, but are less common.

Hydrides and other binaries: Thorium metal reacts with hydrogen to form hydrides such as ThH₂ and Th₄H₁₅. ThH₂ is a plutonium-like hydride (non-stoichiometric, metallic). Interestingly, Th₄H₁₅ is a superconductor below ~7.5 K. Thorium is unusual in that it forms hydrides with more hydrogen than MH₃ (M = metal). Similarly, thorium forms borides (ThB₆, ThB₁₂, etc.), carbides (ThC, ThC₂, Th₂C₃), and silicides (ThSi, ThSi₂, Th₃Si₅). These refractories are of interest in high-temperature materials and nuclear fuel. For example, ThC and ThC₂ have melting points above 2600 °C.

Sulfides, Nitrides, Phosphides: Thorium forms a sulfide ThS, a nitride ThN, and phosphide ThP (all formally Th^4+S^2–, Th^4+N^3– etc. but require reduction to isolate). These are mostly prepared in inert atmospheres and are radioactive solids.

Nitrates, Carbonates, etc.: In solution chemistry, Th^4+ behaves like a highly-charged lanthanide. It forms insoluble hydroxide Th(OH)₄ in neutral water, and soluble complexes in strongly acidic solutions. Thorium nitrate [Th(NO₃)₄] is soluble in nitric acid; thorium sulfate (Th(SO₄)₂) and thorium phosphate (Th₃(PO₄)₄) are very insoluble. Thorium carbonates also precipitate. Essentially, Th^4+ is a hard Lewis acid that strongly hydrolyzes.

Complexes: Thorium can form coordination complexes with various ligands (e.g. EDTA, citrate, oxalate), always as Th^4+. There are organic compounds of thorium (organothorium compounds with Th–C bonds), but these are mainly of research interest.

In all these compounds, thorium generally shows +4, which can give colorless or pale-colored salts (since Th^4+ has no f-electrons to give f–f transitions). Indeed, many thorium salts and complexes are white or colorless. Some Th(IV) compounds (e.g. ThSiO₄) are yellow due to trace impurities.

Thorium’s chemistry is broadly ionic with some covalent character similar to the chemistry of zirconium/hafnium. The bonding can accommodate eight or nine nearest neighbors around Th (as in ThO₂). Unlike uranium, thorium does not easily reach higher oxidation (no stable Th(V) or Th(VI) oxo cations), nor does it easily become +3 (except under strong reduction). The +4 state dominates because the 5f and 6d electrons are sufficiently high in energy that losing four electrons is relatively easy for thorium.

Physical Properties

Thorium metal is a silvery, slightly lustrous metal with physical properties of a typical refractory metal. It is relatively soft and forged in air; it is malleable and somewhat ductile (can be hammered or rolled). On refinement, it is ductile and can be drawn into wires. Because of its self-heating from alpha decay (anywhere, but ^{232}Th decay energy is low), thorium metal does not require exotic cooling despite its radioactivity.

Key physical constants include:

Density: ~11.7 g/cm³ at 25 °C. (This is slightly lower than copper or iron, even though thorium atoms are larger; the fcc packing and voids make it relatively low for an actinide. For comparison, uranium is ~19.1 g/cm³; thorium’s lower density reflects its less compact crystal.)
Melting point: 1750 °C. This is very high (higher than iron, lower than tungsten). It is nearly the highest among actinides.
Boiling point: 4788 °C. Extremely high; only a few transition metals (W, Ta, Re) have higher boiling points.
Crystal structure: Face-centered cubic (space group Fm3m) in the α-phase. At ambient pressure, one metallic form.
Thermal conductivity: ~54 W·m^−1·K^−1 (at room temperature). This is moderate; lower than copper (~400) or aluminum (~237) but higher than many alloys.
Electrical resistivity: Thorium is a decent conductor (metallic), with resistivity lower than lead but higher than copper (exact numeric not widely reported). It conducts like a typical heavy metal.
Heat capacity: Specific heat ~0.12 J/(g·K) (0.120 J/g·K at room T), so ~120 J/(kg·K).
Thermal expansion: Linear expansion coefficient ~×10^−6/K (moderate). ThO₂ has a larger expansion than UO₂, meaning ThO₂ expands more with heat.

Spectroscopy: Atomic thorium has many spectral lines (especially in the visible and ultraviolet) because of its complex electron structure. In practice, thorium wires or lamps have been used to produce sharp spectral lines for calibration. For example, thorium-argon hollow cathode lamps produce thousands of sharp lines used to calibrate spectrographs in astronomy. Optical properties of ThO₂ (the oxide) include a very high refractive index (~2.21 at visible wavelengths) and good transparency in visible and NIR; this made thorium glass useful in high-quality optical lenses (though its slight radioactivity has been a drawback).

Magnetism: Thorium metal is essentially paramagnetic (because Th^0 has no unpaired electrons, it is weakly Pauli paramagnetic like most metals). The Th^4+ ion has a 5f^0 configuration, so thorium compounds are essentially non-magnetic (with only induced paramagnetism).

Thorium’s radiation can have minor effects: because it emits alpha particles, it can slightly warm its surroundings (alpha decay deposits energy as heat). However, the activity of ^232Th is so low that this heating is negligible at moderate quantities.

Chemical Reactivity and Trends

Thorium is a reactive metal in air and water. Finely divided thorium will ignite in air at elevated temperatures, and even at room temperature it slowly forms a surface oxide film. Bulk thorium gradually tarnishes to thorium dioxide (a passivating layer) in moist air. If you heat thorium metal in oxygen, it vigorously oxidizes to ThO₂. Thorium in steam reacts to form ThO₂ and hydrogen, similar to alkali earth metals.

With water: Thorium metal reacts slowly with cold water, faster with hot water or steam. The net reaction is \[ \text{Th} + 2\,\text{H}2\text{O} \to \text{ThO}2 + 2\,\text{H}_2. \] This means thorium corrodes in steam to the stable oxide, liberating hydrogen gas. In concentrated acids (HCl, H₂SO₄, especially nitric acid), thorium dissolves to form Th^4+ solutions, often requiring an oxidizing acid or fluoride present (e.g. HF dissolves ThO₂ by forming ThF₆^2− complexes).

Thorium halides are readily formed by reaction of thorium metal (or thorium oxide) with the corresponding halogen or halogen acid. For example, thori. um reacts with chlorine gas (especially when heated) to form thorium tetrachloride (ThCl₄). Similarly, with fluorine to make ThF₄. The halides ThCl₄ and ThBr₄ are hygroscopic and dissolve in water, whereas ThF₄ is water-insoluble but soluble in fluoride-forming melts.

Thorium oxides and hydroxides: If you put thorium salt in water at near-neutral pH, Th^4+ hydrolyzes and precipitates as ThO₂·nH₂O (a gelatinous hydrate that dehydrates to the oxide). This behavior is like the +4 lanthanides (Ce^4+, Pr^4+ minimal) and like Zr^4+ or Hf^4+. Thorium oxide itself is very stable and does not dissolve in water or basic solutions. It dissolves only in hydrofluoric acid or in hot concentrated nitric acid (the latter likely because nitrates are soluble).

Thorium oxidation states in redox chemistry: The +4 state dominates. It is very difficult to oxidize thorium beyond +4 (no stable Th(V) or Th(VI) oxide is known), and easier to reduce it to +3 or +2 under strong reducing conditions. Thus Th(IV) ⇌ Th(III) + 1e⁻ is a redox pair (E° moderately negative, so Th^4+ is a moderately strong oxidizing agent in acid). Th^4+ can be reduced by active metals (e.g. sodium amalgam) to Th^3+. Th^4+ is so strongly positive that even carbonate or hydroxide precipitates form readily: Th^4+ in solution is very acidic (Lewis) and will drive pH up and precipitate if not buffered.

Passivation and safety: Because ThO₂ is quite refractory, a passivating oxide film can protect bulk thorium metal under many conditions. However, if the metal surface is scratched or dust is present, the reaction with air (and especially moisture) resumes. Thorium dust can be pyrophoric. In industrial contexts, thorium-containing materials (monazite ore, separated Th) require dust control and radiological safety procedures.

Compared to the usual reactivity series, thorium is fairly low in potential: it is more electropositive than many transition metals, comparable to calcium-magnesium group in reactivity (though much heavier). It forms stable compounds with oxygen and halogens, and does not form an easily oxidizable low-oxidation-passivation layer like aluminum does (Al forms Al₂O₃, inert). Th forms ThO₂ which also effectively passivates, but its very high melting point means it does not alloy easily, etc.

Thorium compounds are mostly not acidic or basic in the conventional sense. ThO₂ itself is basic (like other metal oxides) and reacts with acids. In solution, Th^4+ is highly polarizing and acts as a Lewis acid; it will strongly bind anions and ligands. For example, thorium nitrate (Th(NO₃)₄) is acidic in water and protonates water to some extent.

In summary, thorium’s chemical reactivity is characterized by: ease of oxidation to +4, strong affinity for electronegative ligands (oxygen, fluorine), and formation of stable insoluble Th^4+ compounds (hydroxide, phosphate, carbonate) on contact with common anions. It behaves like a heavy analog of zirconium/hafnium or the +4 lanthanides (Ce^4+ in part).

Occurrence and Production

Thorium is a primordial element produced by stellar nucleosynthesis (r-process) and present on Earth since its formation. It never participated in significant chemical or biological cycles, but remains locked in minerals. Its cosmic abundance is low, but relative to Earth’s crust it is moderate. Estimates place thorium at roughly 7–10 parts per million (ppm) by mass in the crust. This makes it one of the more common heavy elements; for comparison lead is ~13 ppm, uranium ~2–3 ppm. Because thorium tends to form oxides and phosphates, it did not sink into Earth’s core during differentiation and remains in the lithosphere.

Minerals: The primary natural hosts of thorium are:

Monazite: (Ce,La,Nd,Th)(PO₄). This rare-earth phosphate mineral is the most important commercial thorium source. Monazite sand deposits are found in Brazil, India (Kerala, Orissa states), Australia, South Africa, UK (Cornwall), Malaysia, and other places. Monazite typically contains ~2–7% ThO₂ by mass (some high-grade deposits up to ~20%). The monazite often occurs in placer deposits (heavy mineral sands) along with zircon and ilmenite.
Thorite: ThSiO₄ and its hydrated form thorianite (The mineral thorite is structurally similar to zircon). Thorite is a more Th-rich mineral (closer to pure ThSiO₄) but is rare compared to monazite. Some pegmatites and granites contain thorite.
Allanite: (Ca,Ce,La,Th)₂(Al,Fe)₃(SiO₄)₃(OH). This epidote-related mineral can have 0.1–2% Th but is not usually mined for thorium.
Thorianite: an uncommon mineral consisting of nearly pure ThO₂ (often mixed with uranium as (Th,U)O₂). Thorianite represents a direct source of ThO₂ but is scarce.
Zircon and Xenotime: ZrSiO₄ and YPO₄ often contain a few percent Th/U by substitution. In some regions (e.g. Australia), zircon recovered from sand may contain trace thorium.

Because thorium is usually found mixed with rare earths (phosphates of Ce, La, etc.), the mining of thorium is typically associated with rare-earth element mining or titanium mining (as a co-product). Unlike uranium, thorium has no large-scale dedicated mining industry. Many monazite deposits were exploited for rare-earth content or titanium minerals (in beach sands); thorium was separated out only when processing monazite for rare earths. If the demand is low, monazite concentrates may simply be stockpiled or waste-discharged (as thorium gives long-term radioactivity problems).

Abundance comparison: The Earth’s upper crust contains on the order of 8.1 g of thorium per tonne of rock (8.1 ppm) according to one standard estimate. This is slightly lower than lead (13 ppm) and higher than tin (~2 ppm). So thorium is fairly common for a heavy element. It is not found in measurable quantity in seawater (Th^4+ is insoluble: any Th in seawater quickly falls out as particle coatings). Plants and animals do not bioconcentrate thorium (it is chemically disfavored in biological systems).

Production and refining: There is no significant world production of thorium metal or compounds independent of other industries. Instead, thorium is produced as a by-product of processing monazite or other REE minerals. The typical flow is:

Mining: Extract apatite-heavy mining of beach sands, alluvial deposits, or hard rock veins that contain monazite or thorium minerals.
Concentration: Use gravity and magnetic/turbulent separation to concentrate heavy minerals (monazite, zircon, ilmenite) from lighter sand.
Monazite Processing: Treat the monazite concentrate (30–55% THO₂ in an oxide form) with hot concentrated acids (usually sulfuric or hydrochloric) or alkali (NaOH) to break down the phosphate matrix. In an acid route, the phosphate is converted into soluble sulfates (REE sulphates stay in solution at low pH, while thorium can be precipitated). In a caustic route (NaOH), monazite is decomposed to rare-earth hydroxides and disodium thorate.
Thorium separation: After leaching, thorium is separated from rare earths because it has different solubilities. For example, adjusting pH to around 1.3 causes Th to precipitate as thorium phosphate or hydroxide, while REEs remain in solution. Techincally, ammonium parathorium (NH₄Th(SO₄)₂·12H₂O) or hydrated thorium oxide is precipitated and calcined to ThO₂.
Purification to metal: Thorium oxide (ThO₂) is the common purified intermediate. To get metal, ThO₂ is typically reduced with calcium metal (or sometimes lanthanum) at about 1200 °C in a sealed vessel, giving thorium metal (and a slag of CaO or La₂O₃). This calcium reduction step produces brittle metal that must be remelted carefully (often in a vacuum arc or electron beam furnace) to ensure purity and density.
Metal forging/finishing: The pure thorium metal can be cast into ingots and worked.

Because thorium supply by-product far exceeds demand, most monazite-derived Th is stockpiled or wasted. For example, during 1980–95, about 160,000 tonnes of monazite concentrate was shipped out of Western Australia most of which went to France for REE recovery. A processing plant shut down in the 1990s due to the problem of disposing of thorium-laden waste. In many modern rare-earth operations (e.g. in China), thorium is again accumulating as waste because the main focus is on scandium/yttrium/lighter REEs and thorium is a nuisance. This means that if a strong market for thorium came back, there is plenty of known resource locked in existing monazite deposits and mine tailings.

Major producers: Historically, India (monazite sands) has had large thorium reserves and has long studied thorium fuel. Australia, Brazil, and Malaysia also have significant monazite. In the United States, deposits in Idaho and New Mexico were mined in the mid-20th century. Today no country “mines thorium,” but rare-earth operations in China, India, and Australia generate thorium-bearing materials. (India’s uniquely large thorium reserves have spurred a long-term program to use ^232Th in their nuclear program.)

Applications and Technology

The primary modern interest in thorium is nuclear, but it has had a variety of other uses:

Nuclear fuel: Thorium-232 is a fertile material that can breed fissile uranium-233. This underlies the thorium fuel cycle. Thorium has been used experimentally in nuclear reactors. For example, the U.S. Shippingport reactor (1977–1982) demonstrated a thorium-uranium fuel (a ^233U core with ^232Th blanket). India’s three-stage nuclear program envisions a future with thorium-based reactors (since India has scant uranium but vast thorium). Indeed, the unique Kamini reactor (Kalpakkam, India) is a research reactor using U-233 fuel bred from thorium. Interest in molten-salt reactors (which dissolve fuel in fluoride salts) has revived thorium-thorium fuel concepts because ThF₄ is soluble and breeding can be efficient. Thorium fuel produces very little plutonium (less non-fissile actinides) and U-233 has proliferation-sensitive U-232 contamination (making weaponization harder, due to intense gamma from U-232 decay). Thorium’s long-term waste signature is argued to be somewhat more benign (mostly shorter-lived isotopes plus a small amount of long-lived Pa-231). Several countries and research groups (India, China, Norway, Canada, the US, and EU researchers) are exploring thorium reactors now.

Fissionable isotopes for science: ^229Th (and its low-energy isomer) is studied for ultra-precise clocks. And small amounts of ^232Th from reactors can be a source of alpha-emitting isotopes for industrial or medical uses. (For instance, ^228Th and its decay daughters can be used in alpha sources, and intriguing ideas exist for ^225Ac production from irradiated thorium for cancer therapy.)

Gas lamp mantles: From about 1885 to mid-20th century, thorium dioxide was widely used in incandescent gas mantles (invented by Carl Auer von Welsbach) because ThO₂ glowed brightly when heated. These mantles (for camping lamps and some street lights) produced a brilliant white light. Today thorium has been phased out of such consumer products due to radiation concerns, replaced by yttrium or cerium oxides.

Welding and lighting: Thorium oxide is mixed with tungsten to make thoriated tungsten electrodes for TIG (gas tungsten arc) welding. The ThO₂ dopant lowers the work function of tungsten and allows a more stable arc. Similarly, thoriated tungsten filaments have been used in high-quality vacuum tubes and light bulbs (though now largely replaced by non-radioactive alternatives).

Alloys: Additions of thorium improve high-temperature strength of alloys. For example, magnesium-thorium alloys (5–10% Th) were used in early jet engines and aerospace (they have higher creep resistance than pure magnesium). Some nickel-based superalloys include thorium historically to improve grain structure (though thorium today is mostly avoided in superalloys due to radioactivity).

Ceramics and refractories: ThO₂ (thoria) is used in specialized refractory ceramics (e.g. rocket nozzles, furnace materials) because of its very high melting point and good thermal shock resistance. Monolithic thoria crucibles once were used for aluminum smelting. ThO₂ powder is also studied as an advanced nuclear fuel pellet material because it remains stable at very high temperatures (higher thermal conductivity than UO₂, and does not swell or disintegrate as easily under irradiation).

Catalyst support: Although not common now, thorium oxide has been used as a catalyst or catalyst support. Historically, thoria was tested as a catalyst in hydrogenation and in the Fischer–Tropsch process. More recently, thorium-doped catalysts have been investigated for reforming natural gas or water-gas shift, owing to the stability of ThO₂.

Optical materials: Thorium compounds were used in high-index, low-dispersion glass. Thorium-doped glasses (2–3% ThO₂) were praised for optical clarity, high refractive index, and negligible ultraviolet absorption. For example, certain WWII-era photographic lenses (Kodak Aero-Ektar) contained thoria in the glass. Thorium fluoride (ThF₄) is used in certain optical coatings (like infrared telescope mirrors coating) and in specialized lenses. However, thorium glass is slightly radioactive (can cause a yellow tint over decades) and is no longer used in new consumer optics.

Electronics and magnetrons: Thoriated tungsten is used in thermionic cathodes (electron emitters) in vacuum tubes and magnetrons (e.g. microwave ovens). The thorium lowers the work function, improving electron emission at lower temperatures. As electronics have moved to semiconductors, this application is niche but still in some aerospace or high-power tubes.

Instrumentation: Thorium is used in small amounts in some instruments and reference standards. For example, borosilicate glass containing thorium has a high internal gain for photomultiplier tubes. Scientists have used thorium spectral lamps (mixed with argon) to calibrate astronomical spectrographs due to their sharp lines.

Energy-related research: The potential of thorium for energy has spurred experiments in molten-salt reactors, accelerator-driven subcritical reactors, and pebble-bed reactors. Thorium oxide microspheres have been tested as a nuclear fuel form (ThO₂-UO₂ mixed oxide, MOX) in existing reactors to substitute part of the uranium fuel.

In summary, key modern uses of thorium are in the nuclear field (fuel and research), while historical uses include gas mantles, welding electrodes, and optical materials. Because thorium is radioactive, many consumer uses have been discontinued or limited by regulations. Nevertheless, thorium compounds’ excellent high-temperature properties and radioluminescence once made them important in industry.

Biology, Environment, and Safety

Thorium is not known to have any useful biological role. Living organisms have no need for thorium, and it is generally considered a toxic heavy metal and a radiological hazard. Exposure to thorium (mainly through inhaling dust or ingesting contaminated materials) poses health risks primarily due to its radioactivity. The alpha particles it emits can damage lung tissue or bones if internalized. Thorium dioxide (the oxide) is classified as a Group 1 carcinogen by several agencies when inhaled chronically, and ThO₂ is listed in reports on carcinogens. Long-term exposure to thorium dust increases the risk of lung cancer.

In occupational settings (workers in mining, rare-earth processing or thorium refining), strict controls are necessary. Recommended exposure limits for airborne thorium compounds (like ThO₂) are very low (on the order of micrograms per cubic meter) and radiation safety rules (air monitoring, lung counting of alpha activity) are applied. If thorium is ingested, it is poorly absorbed by the gut (most passes through), but whatever is absorbed tends to deposit in bone and liver for long periods, increasing cancer risk there. On skin contact, thorium compounds are not very dangerous unless there are wounds. The main hazard is inhalation of fine dust.

Thorium is environmentally persistent. It does not readily migrate in groundwater because Th^4+ hydrolyzes and sticks to soils. Thorium-bearing minerals in sand (such as monazite beaches in India and Brazil) cause localized natural “hot spots” of background radiation – in Kerala, India, some beaches have thorium-rich sand with radiation ~50 times higher than average. However, because the main radiation is alpha particles (stopped by a few cm of air or a thin layer of water), these natural sites pose hazard only if the dust is inhaled.

In the nuclear fuel context, thorium’s environmental impact is often cited as somewhat lower than uranium’s: spent thorium-based fuel contains fewer long-lived transuranics (mainly, U-232 comes with gamma emitters and Pa-231 long-lived), but this distinction is complex. What is clear is that thorium is less toxic chemically (non-radioactive heaviness) than heavy metals like lead, but its radiotoxicity is similar to uranium (alpha emitter with very long-lived components). If thorium is released into the environment (e.g. as mining tailings or reactor waste), it remains locally concentrated for geologic time rather than dispersing widely.

Safety handling: Thorium metal and compounds should be handled with care. Protective gear (gloves, respirators) is used to prevent inhalation or ingestion. Thorium-bearing items (like older camera lenses with ThO₂ glass or welding rods) are treated as low‐level radioactive material; regulations often require documenting and limiting their spread. In many countries, use of thorium in consumer products has been banned or restricted. For example, thoriated mantles and thorium welding rods are subject to licensing or replaced by thorium-free alternatives.

Medically, thorium is not used (though in the past, soluble thorium salts were injected in experimental cancer treatments, a practice long abandoned due to toxicity). Environmental cleanup: Because thorium is long-lived, contaminated sites need to be managed over centuries, if soil is disturbed with thorium content (though typically not an urban pollutant except near old uranium/thorium mines).

In summary, thorium is primarily an environmental and health concern due to its radioactivity. Its chemistry does not make it particularly dangerous (it is relatively insoluble and binds to soil), but its alpha radiation can cause cell damage if ingested or inhaled. Safety guidelines focus on minimizing inhalation of Thorium-containing dust, controlling waste, and monitoring exposures.

History and Etymology

Thorium’s story began in 1828 when the Norwegian mineralogist Morten Thrane Esmark collected a dark mineral (later called thorite) from a beach near Raudsand, Norway. Swedish chemist Jöns Jacob Berzelius analyzed Esmark’s sample in 1829 and identified a new element, which he named thorium in honor of Thor, the Norse god of thunder – reflecting its “heavy” nature perhaps akin to thunderous power. Berzelius isolated it as an oxide (thoria, ThO₂) and published the discovery. Thorium was among the earliest elements discovered after uranium and the rare earths. Its name derives from mythology (Thorin the god, Latin Thorium), as was common in that era (uranium was named for a planet, etc.).

For decades thorium was of purely chemical interest. In 1898, shortly after Becquerel’s discovery of uranium radiation, chemists Gerhard Carl Schmidt (a German) and Marie Curie independently found that thorium compounds are radioactive. It was the second new radioactive element identified (after uranium) – Schmidt noticed thorium’s faint activity from a solution of thorium nitrate, and Curie confirmed it that same year. Thorium’s radioactivity helped establish the new field of nuclear physics.

The intense research in radioactivity in the early 20th century extended to thorium. Ernest Rutherford and Frederick Soddy studied thorium’s decay chain, showing the series of daughter isotopes (Ra-228, Ac-228, etc.). They identified thorium as the parent of a series that leads to stable lead-208, which was instrumental in understanding the nature of radioactive decay series.

In parallel, compounds of thorium found practical use. Notably, in 1885 Austrian chemist Carl Auer von Welsbach developed the gas mantle (incandescent mantle) containing thorium oxide and cerium oxide. This mantle, used in gas lamps, revolutionized lighting by producing a bright white glow from burning gas — a major application of thorium. Thorium nitrate and oxalate were also used in chemical research and early fluorescent paints (though often radium later overshadowed those uses).

In the 20th century, thorium played a role in the emerging nuclear industry. In 1927, researchers discovered that thorium, like uranium, can undergo induced radioactivity when bombarded by particles. In the 1940s, scientists realized ^232Th could be converted to ^233U. Glenn Seaborg, when organizing the periodic table’s actinides, counted thorium as the first actinide (though thorium’s electron configuration makes it a borderline case as an f-block element).

The Manhattan Project era saw thorium used in reactors. At Oak Ridge National Lab in the 1940s, chemists demonstrated production of ^233U from thorium. In the 1950s–60s, the U.S. Shippingport reactor and the Indian Kamini reactor both tested thorium fuel. In India, thorium’s history is central: early geological surveys (1950s) identified the vast reserve of beach sands. The Indian Atomic Energy Commission then began a multi-stage program, planning eventual commercial reactors fueled by India’s thorium.

In material science, further uses turned up: Thorium oxide was used in camera lenses around WWII because it gave excellent optical clarity. Some famous old lenses (Kodak’s Aero-Ektar 1939 design, for example) contain thorium glass, and these are mildly radioactive – an artifact of history. Similarly, thorium dioxide in welding electrodes was developed in the 1920s to improve electron emission. These uses have since been limited for safety, but are part of thorium’s tale.

Discovery of isotopes: The first accurate atomic weight of thorium (232) was determined by radiometric methods in the early 1900s. The technique of radiochemistry was refined by Rutherford, Soddy, and later scientists. For a long time thorium was thought mononuclidic; only in the 21st century did IUPAC note measurable ^230Th in deep seawater. The “Thorium series” of radioactive decay (starting at ^232Th) was one of the four classic decay series (uranium, neptunium, thorium, actinium series) known pre-nuclear-structure.

Etymology note: As mentioned, the name honors Thor. Largely, thorium’s name entered general knowledge through chemistry, not mythology. Over time “thorium” gave rise to terms like “thorite” (the mineral ThSiO₄), “thorianite” (ThO₂ mineral), and “thorite series” minerals. The term “thoriated” (as in thoriated tungsten) comes from thorium doping.

Thorium has left a mild cultural trace: It appears in the rock band Veruca Salt’s song “Thor,” was eyed as a storyline in science fiction (some nuclear scenarios), and occasionally as a collector’s note (radioactive camera lens glass). But overall it remains a specialized chemical element in most minds.

Data Table

Property	Value
Symbol	Th
Atomic number (Z)	90
Atomic weight	232.0377 (standard of ^232Th)
Period	7
Block	f-block (Actinide series)
Electron configuration	[Rn] 6d² 7s²
Valence electrons	4 (in 6d and 7s orbitals)
Common oxidation states	+4 (dominant), +3, +2
Phase at STP	Solid (metal)
Density (20 °C)	~11.7 g/cm³
Melting point	1750 °C (2023 K)
Boiling point	4788 °C (5061 K)
Crystal structure (α-phase)	Face-centered cubic (fcc)
Electronegativity (Pauling)	1.3
First ionization energy	~6.3 eV
Atomic radius (metallic)	~180 pm
Covalent radius	~165 pm (single-bond)
Isotopic composition (natural)	^232Th (~100%)
Half-life (longest-lived isotope)	1.40×10^10 years (^232Th)
Crustal abundance	~7–10 ppm (8.1 g/tonne)
Category	Actinide metal (electropositive)
Discovered	1828 by J. J. Berzelius
Name origin	Named for Thor, Norse god