Question

Assertion (A): Ionization enthalpy increases along each series of 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.

• A. (A) is correct but (R) is not correct
• B. (A) is not correct but (R) is correct
• C. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• D. Both (A) and (R) are correct but (R) is not the correct explanation of (A)

Hint:

\(d\)-electrons shield poorly compared to \(s\)- and \(p\)-electrons. Hence effective nuclear charge increases significantly across transition series.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Ionization enthalpy ($IE$) is the energy required to remove an electron from the outermost shell.
In transition elements, as we move from left to right across a period, electrons are added to the $(n-1)d$ subshell, and protons are added to the nucleus.
Step 2: Detailed Explanation:
Evaluation of Assertion (A):
In any transition series (3d, 4d, or 5d), the first ionization enthalpy generally increases as we move across the period.
However, the increase is not as smooth or steep as in the $s$ and $p$ blocks. This is because the $d$-electrons being added provide an "imperfect shielding" effect for the outer $ns$ electrons.
The small variations occur due to changes in electronic configuration stability (e.g., $d^5$ and $d^{10}$ configurations). Thus, (A) is correct.

Evaluation of Reason (R):
With every unit increase in atomic number, the nuclear charge (number of protons) increases. This increased charge pulls the electrons more strongly toward the nucleus.
Even though electrons are filling the inner $d$-orbitals, they do not fully shield the outer electrons from the growing nuclear pull.
Therefore, the effective nuclear charge ($Z_{eff}$) increases, leading to higher ionization enthalpy. This directly explains the trend mentioned in A.

Conclusion:
The increase in nuclear charge is the fundamental cause for the general increase in ionization enthalpy across a period.
Step 3: Final Answer:
Both Assertion and Reason are correct, and the Reason provides the underlying physical cause for the Assertion.