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

1. [Xe]4f7 6s2, [Xe]4f8 6s2 and [Xe]4f9 6s2
2. [Xe]4f7 5d1 6s2, [Xe]4f7 5d1 6s2 and [Xe]4f9 6s2
3. [Xe]4f7 6s1 6s2, [Xe]4f7 5d1 6s2 and [Xe]4f8 5d1 6s2
4. [Xe]4f7 6s2, [Xe]4f7 5d1 6s2 and [Xe]4f9 6s2

Correct answer: [Xe]4f7 6s2, [Xe]4f7 5d1 6s2 and [Xe]4f9 6s2

Solution

Eu has a stable half-filled 4f7 configuration. Gd gains stability with 4f7 and 5d1. Tb continues filling the 4f subshell.

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