NEET 2021 Chemistry Question 88 Solution

Isoelectronic ions are ions of different chemical elements that possess the identical total number of electrons and identical ground-state electronic configurations.

- Pair 1 ($O^{2-}, F^-$): - Oxygen ($Z=8$): $8 - (-2) = 10\text{ electrons}$ - Fluorine ($Z=9$): $9 - (-1) = 10\text{ electrons}$ (Isoelectronic, Neon configuration $[Ne]$) - Pair 2 ($Na^+, Mg^{2+}$): - Sodium ($Z=11$): $11 - 1 = 10\text{ electrons}$ - Magnesium ($Z=12$): $12 - 2 = 10\text{ electrons}$ (Isoelectronic, Neon configuration $[Ne]$) - Pair 3 ($Mn^{2+}, Fe^{3+}$): - Manganese ($Z=25$): $25 - 2 = 23\text{ electrons}$ ($[Ar]3d^5$) - Iron ($Z=26$): $26 - 3 = 23\text{ electrons}$ ($[Ar]3d^5$) (Isoelectronic, both half-filled $d^5$)

Calculate electrons for iron(II) and manganese(II): - $Fe^{2+}$ ($Z = 26$): $26 - 2 = 24\text{ electrons}$ ($[Ar]3d^6$) - $Mn^{2+}$ ($Z = 25$): $25 - 2 = 23\text{ electrons}$ ($[Ar]3d^5$) Because $24 \neq 23$, this pair does not have the same number of electrons and is NOT an isoelectronic pair.