Question

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

• A. ${[Xe]^4f^6 5d^1 6s^2, [Xe]^4f^75d^16s^2}$ and ${[Xe]^4f^9 6s^2}$
• B. ${[Xe]^4f^65d^16s^2, [Xe]^4f^75d^16s^2}$ and ${[Xe]^4f^85d^16s^2}$
• C. ${[Xe]^4f^7 6s^2, [Xe] 4f^75d^16s^2}$ and ${[Xe]4f^96s^2}$
• D. ${[Xe]4f^76s^2, [Xe]4f^86s^2}$ and ${[Xe]4f^85d^16s^2}$

Answer 1

The Correct Option is C

Electron Configuration and Stability of Lanthanides 

Consider the electron configurations of the following lanthanide elements:

• \({_{63}Eu \; \Rightarrow \; [Xe] \; 4f^7 5d^0 6s^2}\): This configuration corresponds to Europium (Eu), which has a half-filled 4f subshell, making it more stable due to the exchange energy stabilization.
• \({_{64}Gd \, \Rightarrow \, [Xe] \; 4f^7 5d^1 6s^2}\): Gadolinium (Gd) has an electron in the 5d subshell in addition to the half-filled 4f subshell. The extra electron in the 5d subshell increases the instability compared to the half-filled 4f subshell.
• \({_{65} Tb \, \Rightarrow \; [Xe] \, 4f^9 \, 5d^0 \, 6s^2}\): Terbium (Tb) has a 4f subshell configuration of 4f⁹, which is not half-filled and thus is less stable compared to the half-filled 4f⁷ configuration.

Explanation:

The half-filled 4f subshell is more stable because of the increased stability due to exchange energy and the symmetry of the electron distribution. The electron configuration of Europium (4f⁷) is more stable compared to Gadolinium (4f⁷ 5d¹) and Terbium (4f⁹), which do not have a half-filled 4f subshell.

Conclusion:

Thus, the half-filled 4f subshell, as seen in Europium (Eu), is more stable than the configurations of Gadolinium (Gd) and Terbium (Tb), which are less stable due to additional electrons in the 5d or incomplete 4f subshells.