Thorium (Th)

Thorium (Th) is an actinide metal with atomic number 90. It sits in period 7 and belongs to the f-block, immediately after actinium and before protactinium.

Natural thorium is predominantly \(^{232}\mathrm{Th}\), which has an extremely long half-life (on the order of \(10^{10}\) years). Because it decays very slowly, the specific activity (radioactivity per gram) is relatively low compared to short-lived radionuclides, hence it is described as slightly radioactive.

Thorium itself is fertile (not fissile). In a neutron field, it captures a neutron and undergoes two beta decays to become fissile uranium-233:

The produced \(^{233}\mathrm{U}\) can then sustain a nuclear chain reaction.

Thorium most commonly forms the +4 oxidation state (\(\mathrm{Th^{4+}}\)), though lower states like +3 are known in special contexts. A ground-state configuration is often given as [Rn] 6d2 7s2 (with f-electron participation context-dependent in bonding for actinides).

Thorium occurs mainly as thoria (\(\mathrm{ThO_2}\)) in monazite sands, a phosphate mineral rich in rare-earth elements. Processing typically involves:

1. Physical concentration of monazite from heavy mineral sands.
2. Chemical digestion (e.g., alkali or acid) to break the matrix.
3. Selective separation and conversion to thorium compounds such as thorium nitrate or thorium oxide.

Thoria has a very high melting point (\(> 3000^{\circ}\mathrm{C}\)), excellent refractoriness, and good ionic conductivity at high temperature. Historically, it was used in gas mantles, and today it finds use in specialized high-temperature ceramics and research applications.

• Availability: Thorium is relatively abundant in Earth’s crust.
• Fuel cycle: Thorium is fertile, requiring conversion to \(^{233}\mathrm{U}\); uranium has native fissile isotopes (\(^{235}\mathrm{U}\)).
• Waste profile: Some thorium fuel cycles can reduce long-lived transuranics, though detailed outcomes depend on reactor design and reprocessing.
• Technology status: Thorium cycles (e.g., molten-salt concepts) remain under active R&D and demonstration rather than broad commercial deployment.

Although only slightly radioactive, thorium is still a radiotoxic heavy metal. Good practice includes:

• Using fume hoods/glove boxes to limit inhalation of dusts (especially \(\mathrm{ThO_2}\) powders).
• Minimizing ingestion and dermal exposure; wearing appropriate PPE.
• Monitoring for external gamma exposure and following licensed handling and waste protocols.

Key compounds include thorium(IV) oxide \(\mathrm{ThO_2}\), thorium(IV) chloride \(\mathrm{ThCl_4}\), and thorium(IV) nitrate \(\mathrm{Th(NO_3)_4}\). Thorium(IV) tends to form hydrolyzed and polymeric species in aqueous solution; \(\mathrm{ThO_2}\) is notably insoluble and chemically robust.

Yes. \(^{232}\mathrm{Th}\) is the head of the thorium decay series, which proceeds via a chain of \(\alpha\)- and \(\beta\)-decays through nuclides such as radium, radon, and polonium, eventually ending at stable \(^{208}\mathrm{Pb}\).