Question

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

• A. $\left[Xe\right]4f^{6}5d^{1}6s^2,\left[Xe\right]4f^{7}5f^1\text{and}\left[Xe\right]4f^{9}6s^2$
• B. $\left[Xe\right]4f^{6}5d^{1}6s^2,\left[Xe\right]4f^{7}5d^{1}6s^2\text{and}\left[Xe\right]4f^{8}5d^{1}6s^2$
• C. $\left[Xe\right]4f^{7}6s^2,\left[Xe\right]4f^{7}5d^{1}6s^2\text{and}\left[Xe\right]4f^{9}6s^2$
• D. $\left[Xe\right]4f^{7}6s^2,\left[Xe\right]4f^{8}6s^2\text{and}\left[Xe\right]4f^{8}5d^{1}6s^2$

Answer 1

The correct option is C

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

Penultimate and anti-penultimate shells are partially filled in f-block elements/inner transition elements as this is a more stable configuration.