Element identity
Atomic number, periodic identity and reference mass/isotope conventions are established and source-reviewed.
Atomic / electronic model
Atomic configurations and orbital teaching models are reference interpretations; heavy-element electronic structure is more complex than a classical shell picture.
Nuclear / isotope data
Half-lives, isotope identities and decay information are taken from evaluated nuclear-data/reference sources and remain isotope-specific.
Material / phase data
Ordinary bulk and phase values are shown only where defensible; solid-state transitions are kept separate from melting.
Geography / safety boundary
Occurrence, discovery and public research context may be mapped. Operational radioactive-material inventories, access routes and sensitive facility details are intentionally not mapped.
The lens describes evidence status, not confidence theatre. “Unknown” is kept unknown, and teaching schematics are not presented as direct measurements.
Thorium (Th)
Thorium is a naturally occurring radioactive actinide with a very long-lived primordial isotope, distinctive 6d/7s chemistry, an FCC→BCC crystal change before melting, mineral-bound occurrence and evidence-aware nuclear context.
Thorium atomic number, mass, protons, electrons and configuration
Thorium: quick answers
How many protons, neutrons and electrons does thorium have?
Thorium’s atomic number is 90, so every thorium atom has 90 protons, and a neutral atom also has 90 electrons. Its most common natural isotope, thorium-232, has 142 neutrons (other isotopes have different neutron counts).
What is the symbol for thorium?
The chemical symbol for thorium is Th.
Is thorium a solid, liquid or gas at room temperature?
Thorium is a solid at room temperature (about 25 °C).
What family (group) is thorium in?
Thorium is an actinide, in period 7 (the f-block row shown below the main table) of the periodic table.
What is the electron configuration of thorium?
The ground-state electron configuration of thorium is [Rn] 6d² 7s².
From atomic number to chemistry
Read these as a chain of causes, not as isolated facts. Each step links to the concept hub if you want the underlying idea explained.
Ninety protons define thorium; all thorium isotopes share that proton count.
Thorium begins the actinide region with heavy-element d/f-shell chemistry.
Its position connects thorium to the early actinides and heavy-element chemistry.
A long half-life allows primordial thorium to remain in Earth today.
At approximately ambient pressure thorium changes crystal structure while remaining solid.
Thorium is an early actinide in Period 7
Thorium sits after actinium and before protactinium in the actinide series. Its neighbors connect it to both early-actinide chemistry and the broader f-block.
Thorium Visual Lab
Decode the Th tile, rotate a ²³²Th educational nucleus, inspect shell and orbital models, switch between FCC/BCC crystal phases and connect thorium to minerals, materials and research.
Seven facts packed into one square
Thorium in one minute
90 protons define thorium.
²³²Th is extremely long-lived. Its ~14-billion-year half-life explains why primordial thorium remains.
+4 dominates ordinary thorium chemistry.
Thorium changes crystal structure before melting. FCC changes to BCC at high temperature.
Natural occurrence is mineral-bound. Monazite is an important thorium-bearing mineral.
90 electrons across seven principal shells
90 e⁻
Shell totals are 2-8-18-32-18-10-2. Orbital configuration is the more useful chemistry description.
Face-centred cubic (FCC)
Near room temperature thorium adopts a face-centred cubic structure. At high temperature it transforms to a body-centred cubic phase before melting.
Why do 6d and 7s matter?
Thorium is an early actinide whose valence description includes 6d and 7s electrons. Their energies and bonding participation help distinguish thorium chemistry from a simple main-group metal.
Where does thorium matter?
Clickable cards connect thorium to minerals, high-temperature materials, historical uses, geochronology and actinide research.
Thorium is commonly mineral-bound, not native metal
Monazite and related minerals can contain thorium. Natural occurrence and economically recoverable resources are different questions, so the map avoids pretending that occurrence dots are production statistics.
Why is thorium still here if it is radioactive?
A long half-life means slow decay
Thorium-232 has a half-life of roughly 14 billion years. In one half-life, half the nuclei in a large statistical population decay—not all of them at once. Because that timescale is comparable to the age of the universe, primordial thorium can still be present today.
Radioactivity and chemical behavior are separate layers
Thorium’s nuclear instability concerns its nucleus. Its ordinary +4 chemistry and mineral behavior are governed mainly by electrons. One element can therefore have familiar chemical compounds while its nuclei decay on geological timescales.
Thorium beside actinium and uranium
Thorium belongs to the early actinides, where 5f, 6d and 7s energies are close enough that simple one-direction trends are not sufficient. The comparison is a guide to family context, not a claim that every property changes smoothly.
| Family | Actinide |
|---|---|
| Configuration | [Rn] 6d¹ 7s² |
| Family | Actinide |
|---|---|
| Configuration | [Rn] 6d² 7s² |
| Family | Actinide |
|---|---|
| Configuration | [Rn] 5f³ 6d 7s² |
Thorium atomic, physical, thermal, chemical, nuclear and evidence data
Thorium now matches Uranium-family depth while keeping ordinary material properties separate from nuclear evidence and safety context.
| Atomic number | 90 | RSC element reference | Evaluated |
|---|---|---|---|
| Relative atomic mass | 232.0377 | Reference value dominated by ²³²Th | Evaluated |
| Electron configuration | [Rn] 6d² 7s² | RSC reference | Evaluated |
| Block / series | f-block · actinide | Periodic-table classification | Evaluated |
| Density | ≈11.7 g/cm³ | RSC reference value | Evaluated |
|---|---|---|---|
| Room-temperature crystal | FCC | OECD-NEA / materials references | Measured |
| High-temperature solid | BCC | Above the FCC→BCC transition at ambient pressure | Measured |
| Appearance | Silvery metal | Ordinary bulk material context | Reviewed |
| FCC→BCC transition | ≈1630 K | Ambient-pressure reference; literature conventions vary | Evaluated |
|---|---|---|---|
| Melting point | 2023 K | RSC reference value | Evaluated |
| Boiling point | 5058 K | RSC reference value | Evaluated |
| Phase-path scope | FCC → BCC → liquid → gas | Approximate ambient-pressure teaching path | Reviewed |
| Common oxidation state | +4 | Dominant thorium chemistry | Evaluated |
|---|---|---|---|
| Representative oxide | ThO₂ | High-melting refractory oxide; historical/materials context | Reviewed |
| Mineral occurrence | Monazite and related minerals | Occurrence is distinct from resource/economic ranking | Reviewed |
| Dominant natural isotope | ²³²Th | IAEA nuclear-data context | Evaluated |
|---|---|---|---|
| ²³²Th half-life | ≈14.0 billion years | Evaluated nuclear data | Evaluated |
| Primary decay | Alpha decay | Isotope-specific nuclear behavior | Evaluated |
| Stable isotopes | None | All known thorium isotopes are radioactive | Evaluated |
| Geography | Resources / occurrence / research kept separate | No fake production pins | Reviewed |
|---|---|---|---|
| Nuclear safety boundary | Educational, non-operational | No fuel-cycle procedure, separation or handling instructions | Reviewed |
| Structure model | Teaching cell | Not a literal finite material sample or thermal trajectory | Reviewed |
Thorium changes crystal structure before it melts
At approximately standard pressure, this teaching path separates a solid-solid FCC→BCC transition from later melting and boiling. Transition temperatures are reference values, not universal pressure-independent constants.
Solid thorium · FCC
At 293 K thorium is solid in its low-temperature face-centred-cubic phase; the FCC→BCC transition occurs near 1357 °C (≈1630 K) at ambient pressure.
Heating first changes crystal structure, then later changes physical state.
Thorium occurrence, resources and research geography
Thorium is naturally widespread in crustal minerals, but useful resource estimates are concentrated in specific mineral provinces. The map separates mineral occurrence from resource context rather than inventing a current mine-production ranking.
From Thor to radioactivity
Berzelius identifies thorium
Jöns Jacob Berzelius identified the element from a mineral and named it after Thor, the Norse god of thunder.
Radioactivity recognized
Work in the early radioactivity era established that thorium compounds spontaneously emit radiation.
Actinide and nuclear science
Thorium became important in studies of radioactivity, mineral dating, refractory materials and possible nuclear-fuel cycles.
How thorium is encountered industrially: high-level only
Thorium can occur as a co-product or residue in mineral-sands and rare-earth processing. Any radioactive-material handling requires professional controls; this page intentionally avoids operational extraction instructions.
Identify thorium-bearing minerals
Monazite and related deposits are characterized geologically and mineralogically.
Separate mineral concentrates
Industrial mineral processing can concentrate heavy minerals under site-specific controls.
Regulated chemical processing
Where thorium is recovered, radioactive-material and environmental rules govern processing and waste management.
What is thorium used for or studied for?
Nuclear research
Thorium-232 is studied as a fertile nuclear material and in fuel-cycle research, discussed here only at a conceptual level.
Refractory oxide
Thorium dioxide has a very high melting point and has had specialized historical materials uses.
Geochronology
Thorium and uranium decay systems help constrain geological ages and Earth processes.
Historic gas mantles
Thorium compounds were formerly used in incandescent gas mantles; modern use is limited by radiological concerns.
Thorium(IV) dominates common chemistry
Thorium dioxide
A highly refractory oxide and a central compound in thorium materials and nuclear-fuel research.
Thorium(IV)
+4 is the dominant oxidation state in ordinary thorium chemistry.
Mineral incorporation
Thorium can substitute into phosphate and silicate mineral structures, helping explain its natural occurrence.
Thorium-232 and shorter-lived thorium isotopes
Thorium-232 dominates natural thorium. Thorium-230 and thorium-228 occur in radioactive decay chains and have much shorter half-lives.
90 protons · 142 neutrons · half-life ≈14.0 billion years
Its exceptionally long half-life is why primordial thorium remains abundant enough to occur naturally today.
Five-question Thorium check
What is thorium’s atomic number?
Which isotope dominates natural thorium?
Why can radioactive thorium still occur naturally?
What happens before thorium melts?
What is the dominant thorium oxidation state?
Thorium questions students commonly ask
Start with the direct answer, then use the explanation to connect the fact to isotopes, phases and natural occurrence.
Why is thorium still found in nature if it is radioactive?
Short answer: Because ²³²Th decays extremely slowly.
Thorium-232 has a half-life of about 14 billion years, comparable with the age of the universe. Radioactive does not mean short-lived: when a half-life is this long, a substantial fraction of primordial thorium can remain in Earth materials today.
Key point: Radioactivity describes decay; half-life tells you how quickly that decay occurs.
Does thorium have stable isotopes?
Short answer: No. Thorium-232 is very long-lived but radioactive.
All known thorium isotopes are radioactive. Natural thorium is dominated by ²³²Th, whose enormous half-life makes it persistent on geological timescales even though it is not stable in the strict nuclear sense.
Key point: “Very long-lived” and “stable” are not the same nuclear-data label.
Does thorium change crystal structure before melting?
Short answer: Yes. At approximately standard pressure it changes from FCC to BCC while still solid.
This is a solid–solid phase transition: the atoms rearrange into a different crystal lattice before the material reaches its melting point. The temperature explorer therefore treats the two solid regions separately rather than showing one generic solid state.
Key point: A material can change crystal structure without becoming liquid.
Where is thorium found?
Short answer: Thorium occurs mainly in minerals such as monazite rather than as native metallic thorium.
Natural occurrence is therefore a mineral/geological question, not a map of pieces of thorium metal. Resource, processing and research geography are separate contexts and should not be confused with natural occurrence.
Key point: Elemental thorium metal and thorium-bearing minerals are different material forms.
Where the Thorium information comes from
- Royal Society of Chemistry — Thorium element data and history
- IAEA Nuclear Data Services — isotope and decay context
- USGS Mineral Commodity Summaries 2026 — thorium resources and commodity context
- OECD Nuclear Energy Agency — evaluated thorium thermodynamics; FCC at 298 K and FCC→BCC transition near 1630 K
- Materials (2023) / PMC — ambient-pressure FCC→BCC thorium transition near 1357 °C
Questions to ask next about Thorium
A good element lesson should lead to the next useful question, not end after a list of facts.
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