Lanthanoid ions with $4 \mathrm{f}^{7}$ configuration are : (A) $\mathrm{Eu}^{2+}$ (B) $\mathrm{Gd}^{3+}$ (C) $\mathrm{Eu}^{3+}$ (D) $\mathrm{Tb}^{3+}$ (E) $\mathrm{Sm}^{2+}$ Choose the correct answer from the options given below :

JEE Main · 2025 · 22 Jan 2025 · Shift 1 · d and f Block Elements

Lanthanoid ions with

4 f^{7}

configuration are :
(A)

Eu^{2 +}

(B)

Gd^{3 +}

(C)

Eu^{3 +}

(D)

Tb^{3 +}

(E)

Sm^{2 +}

Choose the correct answer from the options given below :

Answer: (A)

Ground-state configurations:

_{62} S m : [X e] 4 f^{6} 6 s^{2}

_{63} E u : [X e] 4 f^{7} 6 s^{2}

_{64} G d : [X e] 4 f^{7} 5 d^{1} 6 s^{2}

_{65} T b : [X e] 4 f^{9} 6 s^{2}

.

Ions form by removing the outermost

6 s

(and

5 d

) electrons first:

E u^{2 +} : [X e] 4 f^{7}

— matches.

G d^{3 +}

: remove

6 s^{2}

and

5 d^{1}

, leaving

[X e] 4 f^{7}

— matches.

E u^{3 +} : [X e] 4 f^{6}

— does not match.

T b^{3 +} : [X e] 4 f^{8}

— does not match.

S m^{2 +} : [X e] 4 f^{6}

— does not match.

Only

E u^{2 +}

and

G d^{3 +}

carry the

4 f^{7}

half-filled configuration, which also accounts for their extra stability.

JEE Main · 2025 · 22 Jan 2025 · Shift 1 · Single correct option