Question

Assertion (A) : Actinoids show irregularities in their electronic configurations.

Reason (R) : Due to varying stabilities of \( f^0 \), \( f^7 \) and \( f^{14} \) occupancies of the 5f orbitals.

• A. Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
• B. Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A).
• C. Assertion (A) is true, but Reason (R) is false.
• D. Assertion (A) is false, but Reason (R) is true.

Hint:

Actinoids prefer empty, half-filled or full 5f subshells, which causes irregular configurations, so R explains A.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Actinoids are the elements from thorium (Z = 90) to lawrencium (Z = 103), where the 5f orbitals are being filled. Their configurations do not always follow a smooth order.

Step 2: Check the Assertion (A).
Some actinoids put an electron in the 6d orbital instead of the 5f orbital. For example, Th is \(\mathrm{[Rn]\,6d^2\,7s^2}\), Pa is \(\mathrm{[Rn]\,5f^2\,6d^1\,7s^2}\) and U is \(\mathrm{[Rn]\,5f^3\,6d^1\,7s^2}\). Cm is \(\mathrm{[Rn]\,5f^7\,6d^1\,7s^2}\). These are irregular, so (A) is TRUE.

Step 3: Check the Reason (R).
An empty (\(f^0\)), half-filled (\(f^7\)) or fully filled (\(f^{14}\)) subshell is extra stable. In Cm, the \(5f^7\) half-filled set is kept, and the next electron goes to 6d.
The 5f and 6d energies are also close, so electrons shift between them. So (R) is TRUE.

Step 4: Link R to A.
The extra stability of \(f^0\), \(f^7\) and \(f^{14}\) is the very reason for the irregular filling. So (R) correctly explains (A).

Final Answer:
Both statements are true and R explains A, so the code is (A). \[ \boxed{\text{Option (A)}} \]