The electronic configurations of Eu (Atomic No. 63), Gd (Atomic No. 64) and Tb (Atomic No. 65) are:

$\displaystyle [\mathrm{Xe}] 4 f^7 6 \mathrm{~s}^2,[\mathrm{Xe}] 4 f^7 5 \mathrm{~d}^1 6 \mathrm{~s}^2$ and $\displaystyle [\mathrm{Xe}] 4 f^9 6 \mathrm{~s}^2$

$\displaystyle [\mathrm{Xe}] 4 f^7 6 \mathrm{~s}^2,[\mathrm{Xe}] 4 f^8 6 \mathrm{~s}^2$ and $\displaystyle [\mathrm{Xe}] 4 f^8 5 \mathrm{~d}^1 6 \mathrm{~s}^2$

$\displaystyle [\mathrm{Xe}] 4 f^6 5 d^1 6 s^2,[\mathrm{Xe}] 4 f^7 5 d^1 6 s^2$ and $\displaystyle [\mathrm{Xe}] 4 f^9 6 s^2$

$\displaystyle [\mathrm{Xe}] 4 f^6 5 \mathrm{~d}^1 6 \mathrm{~s}^2,[\mathrm{Xe}] 4 f^7 5 \mathrm{~d}^1 6 \mathrm{~s}^2$ and $\displaystyle [\mathrm{Xe}] 4 f^8 5 \mathrm{~d}^1 6 \mathrm{~s}^2$