Which one of the following sets of ions represents a collection of isoelectronic species? (Atomic numbers: F = 9, Cl =…

🧠 The Periodic Table Scan The question gives you atomic numbers as clues. We are looking for a set of species where the sum of (Atomic Number - Charge) is constant for all.

🗺️ Testing the Suspects Let's check Option (d):

• $\ce{K^+}$: $19 - 1 = 18 \text{ e}^-$
• $\ce{Cl^-}$: $17 - (-1) = 18 \text{ e}^-$
• $\ce{Ca^2+}$: $20 - 2 = 18 \text{ e}^-$
• $\ce{Sc^3+}$: $21 - 3 = 18 \text{ e}^-$ Since all possess 18 electrons (making them isoelectronic with Argon), this is our winner.

⚡ The Group Mismatch Filter Look at Option (a): $\ce{Li^+, Na^+, Mg^2+, Ca^2+}$. These are all cations, but they belong to different shells. $\ce{Li^+}$ has 2 electrons, while $\ce{Na^+}$ has 10. They can't possibly be isoelectronic. You can eliminate options like this in seconds by noticing "shell jumping."

⚠️ Common Traps Watch out for Option (c): $\ce{N^3-, O^2-, F^-}$ ($10 \text{ e}^-$) and $\ce{S^2-}$ ($18 \text{ e}^-$). One heavy ion like $\ce{S^2-}$ ruins the entire set. Always check every member, not just the first three!

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