[પરમામાણ્વીય ક્રમાંક ${Gd}=64$ ]
${ }_{64} {Gd}:[{Xe}] 4 {f}^{7} 5 {~d}^{1} 6 {~s}^{2}$
So the electronic configuration of
${ }_{64} {Gd}^{2+}:[{Xe}] 4 {f}^{7} 5 {~d}^{1} 6 {~s}^{0}$
$i.e.$ the number of $4 {f}$ electrons in the ground state electronic configuration of ${Gd}^{2+}$ is $7$