JEE Main · 2011 · 10 Apr 2011 · Offline · d and f Block Elements

Gadolinium, $Z=64$, would naively be $[\mathrm{Xe}]4f^{8}6s^2$, but the extra stability of a half-filled $4f$ subshell pulls one electron into the $5d$ orbital, giving the actual outer configuration $4f^{7}5d^{1}6s^2$, an exception much like chromium and copper in the $d$ block.