1. [Xe]4f⁷6s² , [Xe]4f⁸6s² and [Xe]4f⁸5d¹6s² 2. [Xe]4f⁶5d¹6s² , [Xe]4f⁷5d¹6s² and [Xe]4f⁹6s² 3. [Xe]4f⁶5d¹6s² , [Xe]4f⁷5d¹6s² and [Xe]4f⁸5d¹6s² 4. [Xe]4f⁷6s² , [Xe]4f⁷5d¹6s² and [Xe]4f⁹6s²

Answer: 4. [Xe]4f⁷6s² , [Xe]4f⁷5d¹6s² and [Xe]4f⁹6s² Key idea: half-filled 4f⁷ is extra stable, and Gd and Tb follow from it. Eu (63): [Xe]4f⁷ 6s² . Gd (64): [Xe]4f⁷ 5d¹ 6s² (the extra electron goes to 5d to keep 4f⁷ ). Tb (65): [Xe]4f⁹ 6s² . This matches option 4. Options 1-3 give wrong Eu, Gd or Tb configurations.

Topic: Lanthanoids and lanthanoid contraction· d- and f-Block Elements

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

1. \([Xe]4f^{7}6s^{2}\), \([Xe]4f^{8}6s^{2}\) and \([Xe]4f^{8}5d^{1}6s^{2}\)

2. \([Xe]4f^{6}5d^{1}6s^{2}\), \([Xe]4f^{7}5d^{1}6s^{2}\) and \([Xe]4f^{9}6s^{2}\)

3. \([Xe]4f^{6}5d^{1}6s^{2}\), \([Xe]4f^{7}5d^{1}6s^{2}\) and \([Xe]4f^{8}5d^{1}6s^{2}\)

4. \([Xe]4f^{7}6s^{2}\), \([Xe]4f^{7}5d^{1}6s^{2}\) and \([Xe]4f^{9}6s^{2}\)

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Answer: 4. \([Xe]4f^{7}6s^{2}\), \([Xe]4f^{7}5d^{1}6s^{2}\) and \([Xe]4f^{9}6s^{2}\)

Key idea: half-filled \(4f^7\) is extra stable, and Gd and Tb follow from it.

Eu (63): \([Xe]4f^7 6s^2\).
Gd (64): \([Xe]4f^7 5d^1 6s^2\) (the extra electron goes to 5d to keep \(4f^7\)).
Tb (65): \([Xe]4f^9 6s^2\).

This matches option 4. Options 1-3 give wrong Eu, Gd or Tb configurations.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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

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