Gadolinium ($Gd$, $Z = 64$) has the electronic configuration $[Xe] 4f^7 5d^1 6s^2$. The third ionisation enthalpy involves the removal of the $5d^1$ electron, which results in a highly stable half-filled $4f^7$ core. This extra stability is due to the high exchange enthalpy associated with half-filled $f$ orbitals. Consequently, the third ionisation enthalpy of Gadolinium is abnormally low .