Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Ionisation enthalpies of early actinoids are lower than for early lanthanoids. Reason (R): Electrons are entering 5f orbitals in actinoids which experience greater shielding from nuclear charge. In the light of the above statements, choose the correct answer from the options given below:
Answer: (A) (a) Both (A) and (R) are true and (R) is the correct explanation of (A).. \textbf{Answer:} (a) Both (A) and (R) are true and (R) is the correct explanation of (A).
- A.(a) Both (A) and (R) are true and (R) is the correct explanation of (A).✓
- B.(b) Both (A) and (R) are true but (R) is not the correct explanation of (A).
- C.(c) (A) is true but (R) is false.
- D.(d) (A) is false but (R) is true.
Correct Answer
(A) (a) Both (A) and (R) are true and (R) is the correct explanation of (A).
Solution & Explanation
\textbf{Answer:} (a) Both (A) and (R) are true and (R) is the correct explanation of (A). \textbf{Solution:} The early actinoids do have lower ionisation enthalpies than the early lanthanoids, so (A) is true. In actinoids the differentiating electrons enter the 5f orbitals. The 5f orbitals are more diffuse and penetrate less, so they shield the outer electrons poorly themselves but the electrons in them experience greater shielding from the nuclear charge (they are held less tightly than 4f electrons in lanthanoids). Because these 5f electrons are less firmly bound, they are removed more easily, lowering the ionisation enthalpy. Hence (R) is true and correctly explains (A).