The electronic configuration of Eu (Atomic no.63), Gd (Atomic no. 64) and Tb (Atomic no. 65) are
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[Xe]4f6 5d1 6s2, [Xe] 4f7 5d1 6s2 and [Xe] 4f9 6s2
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[Xe] 4f6 5d1 6s2 , [Xe] 4f7 5d1 6s2 and [Xe] 4f8 5d1 6s2
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[Xe] 4f7 6s2, [Xe] 4f7 5d1 6s2 and [Xe] 4f9 6s2
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[Xe] 4f7 6s2,[Xe] 4f8 6s2 and [Xe] 4f8 5d1 6s2
