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