Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Ionisation enthalpy increases along each series of the transition elements from left to right. However, small variations occur. Reason (R): There is a corresponding increase in nuclear charge which accompanies the filling of electrons in the inner d-orbitals. In the light of the above statements, choose the most appropriate answer from the options given below.
Answer: (A) (a) Both (A) and (R) are correct and (R) is the correct explanation of (A). \textbf{Answer:} (a) Both (A) and (R) are correct and (R) is the correct explanation of (A) \textbf{Solution:} Across a transition series, the first ionisation enthalpy generally increases from left to right because the nuclear charge increases steadily.
- A.(a) Both (A) and (R) are correct and (R) is the correct explanation of (A)✓
- B.(b) Both (A) and (R) are correct but (R) is not the correct explanation of (A)
- C.(c) (A) is correct but (R) is not correct
- D.(d) (A) is not correct but (R) is correct
Correct Answer
(A) (a) Both (A) and (R) are correct and (R) is the correct explanation of (A)
Solution & Explanation
\textbf{Answer:} (a) Both (A) and (R) are correct and (R) is the correct explanation of (A) \textbf{Solution:} Across a transition series, the first ionisation enthalpy generally increases from left to right because the nuclear charge increases steadily. The electrons added go into the inner (n-1)d orbitals, which shield the outer electrons only imperfectly, so the effective nuclear charge felt by the valence electrons rises and removing an electron becomes harder. The shielding/exchange-energy effects cause only small, irregular variations in the otherwise rising trend. Thus both (A) and (R) are correct, and (R) correctly explains (A).