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Answer: 3. high exchange enthalpy

Key idea: exchange enthalpy stabilises half-filled subshells.

Gd is \([\mathrm{Xe}]4f^75d^16s^2\). Removing the third electron (the 5d electron) leaves \(4f^7\), a stable half-filled configuration with high exchange energy. So the third ionisation enthalpy is low, because of high exchange enthalpy: option 3.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report
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