NEETChemistryClassification of Elements and Periodicity in Properties
Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A) : The first ionisation enthalpies of 5d transition elements are generally higher than those of the corresponding elements in the 3d and 4d series. Reason (R) : The intervening 4f electrons provide strong shielding, which significantly decreases the effective nuclear charge experienced by the outer
Options
- ABoth (A) and (R) are correct and (R) is the correct explanation of (A)
- BBoth (A) and (R) are correct but (R) is not the correct explanation of (A)
- C(A) is correct but (R) is not correct
- D(A) is not correct but (R) is correct
Correct answer
C. (A) is correct but (R) is not correct
Step-by-step solution
The first ionisation enthalpies of 5d transition elements are indeed generally higher than those of the 3d and 4d elements. This is a consequence of the lanthanoid contraction. The intervening 4f electrons provide poor (not strong) shielding of the increasing nuclear charge. As a result, the outer valence electrons in 5d elements experience a much higher effective nuclear charge, making it more difficult to remove an electron. Therefore, Assertion (A) is correct, but Reason (R) is incorrect because it falsely claim