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: Conceptual Overview:
The f-block elements, which include the lanthanoids and actinoids, are known for their complex electronic configurations. This complexity arises because the energy levels of their outermost shells are very close to each other, making electron arrangements less predictable.
Step 2: In-Depth Explanation:
In the actinoid series, the energy levels of the 5f, 6d, and 7s orbitals are quite similar. This closeness in energy allows for the easy transition of electrons between these orbitals. As a result, there are multiple possible electronic configurations, which often lead to irregularities in the expected filling order of these subshells.
For example, the progression from Thorium (Th) to Protactinium (Pa) or Uranium (U) does not follow the simple 5f-filling pattern that would be expected from their positions in the periodic table. The electron configuration of these elements is influenced by the similarity in energy between the 5f, 6d, and 7s subshells, which results in these irregularities.
Therefore, the assertion made is accurate, and the reason provided correctly explains why these irregularities occur in the filling of electron orbitals for actinoids.
Step 3: Final Conclusion:
The correct answer is Code (A), as it accurately describes the cause of the irregularities in the electronic configurations of the actinoid series.