Question

Identify the element having highest ionisation enthalpy ( $IE_1$ ) from following.

• A. Ce
• B. La
• C. Gd
• D. Yb

Hint:

In lanthanides, $IE_1$ generally increases with increasing atomic number due to lanthanide contraction.

Answer 1

The Correct Option is D

Step 1: Concept
Ionisation enthalpy generally increases across the lanthanide series due to the increase in effective nuclear charge (Lanthanide Contraction).
Step 2: Electronic Configuration
- $\text{La}$ ($Z=57$): $[Xe] 5d^1 6s^2$
- $\text{Ce}$ ($Z=58$): $[Xe] 4f^1 5d^1 6s^2$
- $\text{Gd}$ ($Z=64$): $[Xe] 4f^7 5d^1 6s^2$
- $\text{Yb}$ ($Z=70$): $[Xe] 4f^{14} 6s^2$

Step 3: Analysis
Yb has a completely filled $4f$ orbital ($4f^{14}$) and $6s$ orbital ($6s^2$), making it exceptionally stable. Additionally, it is located furthest to the right among the given options.
Final Answer: (D)