Question

Assertion (A) : Actinoids show irregularities in their electronic configurations.
Reason (R) : In actinoids 5f, 6d and 7s orbitals are of comparable energies.

Hint:

Actinoids also show a greater range of oxidation states compared to lanthanoids because the 5f, 6d, and 7s electrons can all participate in bonding due to their close energy.

Answer 1

Step 1: Understanding the Concept:
The f-block elements (lanthanoids and actinoids) have complex electronic configurations because the energy levels of their outer shells are very close.
Step 2: Detailed Explanation:
In the actinoid series, the 5f, 6d, and 7s subshells are very close in energy (comparable energies).
This leads to easy shifting of electrons between these orbitals, resulting in multiple possible configurations and irregularities in the sequence of filling.
For example, the transition from Th to Pa or U doesn't follow a simple 5f-filling pattern.
Hence, the assertion is true and the reason accurately explains the cause of these irregularities.
Step 3: Final Answer:
Code (A) is correct.