Assertion: Actinoids show greater number of oxidation states than lanthanoids. Reason: Actinoids are radioactive.

Options

A. If both (A) and (R) are correct and (R) is the correct explanation of (A).
B. If both (A) and (R) are correct, but (R) is not the correct explanation of (A).
C. If (A) is correct, but (R) is incorrect.
D. If (A) is incorrect but (R) is correct.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

**Step 1: Understand the Assertion**
The assertion states that "Actinoids show greater number of oxidation states than lanthanoids."
- Actinoids (elements from atomic number 89 onwards) can exhibit multiple oxidation states due to the involvement of 5f, 6d, and 7s orbitals in bonding.
- Lanthanoids (elements from atomic number 57 to 71) primarily show a +3 oxidation state because the 4f orbitals are more stable and less involved in bonding.

**Step 2: Understand the Reason**
The reason states that "Actinoids are radioactive."
- It is true that most actinoids are radioactive due to their high atomic numbers and instability.
- However, the radioactivity of actinoids does not directly explain why they have a greater number of oxidation states compared to lanthanoids.

**Step 3: Evaluate the Relationship Between Assertion and Reason**
- Both the assertion and the reason are correct statements.
- However, the reason does not provide a valid explanation for the assertion. The greater number of oxidation states in actinoids is due to the similar energy levels of their 5f, 6d, and 7s orbitals, not because they are radioactive.

**Step 4: Conclusion**
Based on the analysis:
- The assertion is correct: Actinoids do show a greater number of oxidation states than lanthanoids.
- The reason is also correct: Actinoids are radioactive.
- However, the reason does not explain the assertion.

Thus, the correct answer is:
**Option D: Assertion is correct, Reason is correct, but Reason is not the correct explanation of Assertion.**