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: Recall the pattern of filling.
In lanthanoids and actinoids the last electron should enter the f subshell. The expected pattern is a steady rise in f electrons across the series.

Step 2: Test the assertion with examples.
Actinium has $6d^1$ and no 5f electron. Thorium has $6d^2$ with no 5f electron. Curium has $5f^7 6d^1$ instead of $5f^8$. Such jumps break the simple order, so actinoids do show irregularities. The assertion is correct.

Step 3: Test the reason.
A $5f$ set that is empty, half-filled or full is stable. Curium holds on to the stable $5f^7$ half-filled shell and puts the next electron in 6d.
The small energy gap between 5f and 6d also lets electrons move easily. The reason is correct.

Step 4: Decide on the link.
The stability of $f^0$, $f^7$ and $f^{14}$ is what causes the odd configurations. So the reason gives the explanation of the assertion.

Final Answer:
Both are true and R is the right explanation. \[ \boxed{\text{Option (A)}} \]