NEET 2021 · ChemistryAtomic radiusPrevious Year Question

From the following pairs of ions, which one is NOT an isoelectronic pair?

Answer: (B) $Fe X^{2 +}$ , $Mn X^{2 +}$ . \textbf{Answer:} (B) $Fe X^{2 +}$ , $Mn X^{2 +}$ \textbf{Solution:} Isoelectronic species have the same number of electrons.

A. $Mn X^{2 +}$ , $Fe X^{3 +}$
B. $Fe X^{2 +}$ , $Mn X^{2 +}$ ✓
C. $O X^{2 -}$ , $F X^{-}$
D. $Na X^{+}$ , $Mg X^{2 +}$

Correct Answer

(B) $Fe X^{2 +}$ , $Mn X^{2 +}$

Solution & Explanation

\textbf{Answer:} (B) $Fe X^{2 +}$ , $Mn X^{2 +}$ \textbf{Solution:} Isoelectronic species have the same number of electrons. $Fe X^{2 +}$ has $24$ electrons while $Mn X^{2 +}$ has $23$ electrons, so they are NOT isoelectronic. The other pairs ARE isoelectronic: $Mn X^{2 +}$ and $Fe X^{3 +}$ both have $23$ electrons; $O X^{2 -}$ and $F X^{-}$ both have $10$ electrons; $Na X^{+}$ and $Mg X^{2 +}$ both have $10$ electrons.

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