JEE Chemistry 2023 Question with Solution

Given: Assertion A says that $5f$ electrons participate in bonding to a greater extent than $4f$ electrons. Reason R says that $5f$ orbitals are less buried than $4f$ orbitals.

Find: Which option correctly describes the truth of A and R, and whether R explains A.

Identify principle: In f-block chemistry, orbitals that are more spatially extended and less buried can overlap more effectively with ligand orbitals, so they participate more in bonding.

Apply: The statement about $5f$ electrons is true because $5f$ orbitals are more spatially extended than $4f$ orbitals. Hence they interact more with ligand orbitals and contribute more to bonding.

The reason is also true because $5f$ orbitals are not as buried as $4f$ orbitals. Therefore, they are more available for overlap and bonding.

Since the lesser burial of $5f$ orbitals directly causes their greater participation in bonding, R is the correct explanation of A.

Therefore, both A and R are true, and R is the correct explanation of A. The correct option is B.

The solution labels the option as A, but its written conclusion and the listed correct answer state (2), which corresponds to B. Hence B is the defensible answer.

Given: Comparison of bonding behavior of $4f$ and $5f$ orbitals.

Find: Whether the assertion-reason pair is correct.

Detailed reasoning:

1. $4f$ orbitals in lanthanoids are highly buried inside the atom.
2. Because they are buried, their overlap with ligand orbitals is weak.
3. $5f$ orbitals in actinoids are relatively more extended in space.
4. Because they are less buried, they can overlap more effectively with ligand orbitals.
5. Better overlap means greater participation in bond formation.

So, the assertion is true, the reason is true, and the reason correctly explains the assertion.

Therefore, the correct option is B.