Thorium (Th): Properties, Reactions, and Importance

Introduction: Why Thorium Matters

Thorium (Th) is a naturally occurring radioactive metallic element, primarily found in monazite sands. It is the second member of the actinide series. Thorium is gaining significant attention as a potential cleaner and more abundant nuclear fuel alternative to uranium, particularly in the Thorium-Uranium fuel cycle, which produces fissile Uranium-233. Its unique nuclear properties and relatively high terrestrial abundance make it a subject of ongoing research and strategic importance.

CBSE/JEE Quick Revision Notes

• Symbol: Th
• Atomic Number (Z): 90
• Atomic Mass (A): 232.038 u
• Block: f-block (Actinide series)
• Period: 7
• Group: Not assigned to a specific group in the main periodic table, part of the actinides.
• Common Oxidation State: +4 (most stable and prevalent)
• Nature: Silvery-white radioactive metal, tarnishes to black on exposure to air.
• Radioactivity: All isotopes are radioactive; ^\{232\}Th is the most stable with a very long half-life (1.4 x 10^\{10\} years), making it primordial.

Electron Configuration & Bonding Behavior

• Ground State Electron Configuration: [Rn] 6d^2 7s^2
  • Note: The 5f orbitals are empty in the ground state of thorium, making its configuration somewhat atypical for the start of the f-block.
• Oxidation State Explanation: Thorium predominantly exhibits a +4 oxidation state. This is achieved by the loss of the two 7s electrons and the two 6d electrons. The empty 5f orbitals become available for bonding in certain compounds, but the +4 state is highly stable due to a pseudo-noble gas configuration and favorable lattice energies in ionic compounds.
• Bonding: Thorium forms primarily ionic compounds in its +4 state, although some covalent character can be observed, especially with highly electronegative elements.

Crucial Chemical Reactions

1. Reaction with Air/Oxygen (Oxidation): Thorium metal tarnishes slowly in air, forming thorium dioxide. When heated, it burns vigorously in air. Th(s) + O_2(g) \rightarrow ThO_2(s) (slow at room temperature, rapid on heating)

2. Reaction with Halogens: Thorium reacts with all halogens to form tetrahalides.

   • With Chlorine: Th(s) + 2Cl_2(g) \rightarrow ThCl_4(s)
   • With Fluorine: Th(s) + 2F_2(g) \rightarrow ThF_4(s)

3. Reaction with Acids:

   • Dilute Hydrochloric Acid (HCl): Thorium reacts slowly with non-oxidizing acids. Th(s) + 4HCl(aq) \rightarrow ThCl_4(aq) + 2H_2(g)
   • Concentrated Nitric Acid (HNO₃): Thorium is generally resistant to concentrated nitric acid due to passivation, though hot nitric acid can dissolve it slowly. A mixture of HNO_3 and HF is effective for dissolution.
   • Sulphuric Acid (H₂SO₄): Reacts slowly. Th(s) + 2H_2SO_4(aq) \rightarrow Th(SO_4)_2(aq) + 2H_2(g)

4. Reaction with Water (Steam): Thorium reacts slowly with hot water or steam to form thorium dioxide and hydrogen gas. Th(s) + 2H_2O(g) \rightarrow ThO_2(s) + 2H_2(g)

Industrial and Biological Importance

Industrial Importance

• Nuclear Fuel Cycle: Thorium-232 is fertile, meaning it can absorb a neutron to become Uranium-233 (^\{232\}Th + n \rightarrow ^\{233\}Th \rightarrow ^\{233\}Pa \rightarrow ^\{233\}U). Uranium-233 is a fissile isotope, meaning it can sustain a nuclear chain reaction. This Th-U fuel cycle offers significant advantages including greater abundance than uranium, reduced long-lived radioactive waste, and potential for proliferation resistance. India’s nuclear program heavily emphasizes this cycle.
• Alloys: Small additions of thorium to magnesium alloys improve their strength and creep resistance at high temperatures.
• Catalysis: Thorium dioxide (ThO_2) is used as a catalyst in various organic reactions, such as the production of nitric acid and the cracking of petroleum.
• Historical Uses: Thorium dioxide was historically used in gas mantles for portable gas lamps due to its incandescence when heated, providing a bright white light. It was also used in vacuum tubes, camera lenses, and as an alloying agent in welding electrodes.

Biological Importance

• No Known Biological Role: Thorium has no known biological role in any living organism.
• Toxicity: Thorium and its compounds are radioactive and toxic. Inhalation or ingestion can lead to internal radiation exposure, increasing the risk of cancer (especially lung and bone cancer) and other radiation-induced diseases. Its long half-life means it persists in the environment and body for extended periods.