Which of the following pair is not isoelectronic species? (At. no. Sm, 62; Er, 68; Yb, 70; Lu, 71; Eu, 63; Tb, 65; Tm, 69)

Question

Which of the following pair is not isoelectronic species?

(At. no. Sm, 62; Er, 68; Yb, 70; Lu, 71; Eu, 63; Tb, 65; Tm, 69)

$\mathrm{Sm}^{2+}$ and $\mathrm{Er}^{3+}$

$\mathrm{Yb}^{2+}$ and $\mathrm{Lu}^{3+}$

$\mathrm{Eu}^{2+}$ and $\mathrm{Tb}^{4+}$

$\mathrm{Tb}^{2+}$ and $\mathrm{Tm}^{4+}$

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