Which of the following pair is not isoelectronic? (Atomic numbers: Sm = 62, Er = 68, Yb = 70, Lu = 71, Eu = 63, Tb =…

🧠 The Lanthanoid Challenge This question tests your knowledge of f-block elements and their typical $2+, 3+, 4+$ states. We need to find the pair where the (Atomic Number - Charge) mismatch identifies them as NOT isoelectronic.

🗺️ The Electronic Audit Let's check the options one by one:

• Option (b): $\ce{Yb^2+} (70 - 2 = 68)$ and $\ce{Lu^3+} (71 - 3 = 68)$. These match!
• Option (c): $\ce{Eu^2+} (63 - 2 = 61)$ and $\ce{Tb^4+} (65 - 4 = 61)$. These match too!
• Option (d): $\ce{Tb^2+} (65 - 2 = 63)$ and $\ce{Tm^4+} (69 - 4 = 65)$. Here, $63 \neq 65$. They are NOT isoelectronic.

⚡ The Quick Delta Check Instead of counting electrons, look at the difference in atomic numbers vs. the difference in charges. For Option (b): $\Delta Z = 1$, $\Delta \text{Charge} = 1$. Mismatch in $Z$ is compensated by mismatch in charge. Isoelectronic! For Option (d): $\Delta Z = 69 - 65 = 4$. $\Delta \text{Charge} = 4 - 2 = 2$. The charge shift ($2$) doesn't cover the $Z$ gap ($4$). Instant rejection!

⚠️ Common Traps Rare earth elements (Lanthanoids) can be scary because of high atomic numbers. Don't panic. The logic is identical to simple ions: Proton count minus charge. You don't need to write the full $4f$ configuration to find the electron count.

$\boxed{\text{Answer: (d)}}$