The outer electronic configuration of Gadolinium (, ) is exceptional compared to the standard Aufbau — Atomic Structure Chemistry Question

💡 Solution & Explanation

Gadolinium ($Z=64$) is a lanthanide. Following the general Aufbau principle, the $4f$ subshell is expected to be filled after $6s$. However, to achieve the extra stability associated with an exactly half-filled $4f$ subshell ($4f^7$), the next incoming electron enters the $5d$ orbital instead of pairing up in the $4f$ orbital. Thus, the anomalous configuration is $[Xe] 4f^7 5d^1 6s^2$.