Which element from following lanthanoids has half-filled f-orbital in observed and expected electronic configuration?
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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, $4\text{f}^{14} 6\text{s}^2$).
Step 1: Understanding the Question: The question asks for the lanthanoid possessing an exactly half-filled 4f subshell (4f⁷) in both its expected and observed electronic configurations.
Step 2: Key Formula or Approach: A half-filled f-orbital configuration (4f⁷) imparts exceptional stability due to symmetric electron distribution and maximum exchange energy.
Step 3: Detailed Explanation: Europium (Eu, Z=63) has the expected configuration [Xe] 4f⁷ 6s² and retains this exact arrangement in its observed ground state, keeping the 5d orbital vacant to preserve the highly stable half-filled 4f⁷ subshell.
Step 4: Final Answer: The element is Europium (Eu), corresponding to option (A).