Question

Which element from following lanthanoids has half-filled f-orbital in observed and expected electronic configuration?

• A. Eu
• B. Sm
• C. Ce
• D. Pm

Hint:

In the 4f-series (lanthanoids), remember that the two elements showcasing exceptional stability through a half-filled ($4\text{f}^7$) or completely filled ($4\text{f}^{14}$) f-subshell while keeping a standard $6\text{s}^2$ pair are Europium (Eu, $4\text{f}^7 6\text{s}^2$) and Ytterbium (Yb, $4\text{f}^{14} 6\text{s}^2$).

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify which lanthanoid element has an exactly half-filled 4f subshell ($4\text{f}^7$) in both its expected (theoretical) and observed (experimental) electronic ground-state configurations.

Step 2: Key Formula or Approach:
The general electronic configuration for the lanthanoid series is $[\text{Xe}]\,4\text{f}^n 5\text{d}^m 6\text{s}^2$, where $n$ ranges from 1 to 14 and $m$ can be 0 or 1. A completely half-filled f-orbital contains exactly 7 electrons ($4\text{f}^7$).

Step 3: Detailed Explanation:
Let's look at the atomic numbers and configurations of the options provided:

Ce (Cerium, $Z=58$): Expected: $[\text{Xe}]\,4\text{f}^2 6\text{s}^2$. Observed: $[\text{Xe}]\,4\text{f}^1 5\text{d}^1 6\text{s}^2$. Not half-filled.

Pm (Promethium, $Z=61$): Expected and Observed: $[\text{Xe}]\,4\text{f}^5 6\text{s}^2$. Not half-filled.

Sm (Samarium, $Z=62$): Expected and Observed: $[\text{Xe}]\,4\text{f}^6 6\text{s}^2$. Not half-filled.

Eu (Europium, $Z=63$): Expected: $[\text{Xe}]\,4\text{f}^7 6\text{s}^2$. Observed: $[\text{Xe}]\,4\text{f}^7 6\text{s}^2$. Because the $4\text{f}^7$ arrangement provides extra stability due to structural symmetry and high exchange energy, Europium keeps its 5d orbital empty, ensuring its configuration contains a perfectly half-filled f-orbital in both cases.

Step 4: Final Answer:
The element with the half-filled f-orbital is Europium (Eu), corresponding to option (A).