Question

Which of the following is not isoelectronic with $Na^+$?

• A. $O^{2-}$
• B. $Mg^{2+}$
• C. $Al^{3+}$
• D. $F^{-}$

Hint:

Isoelectronic species have same electrons, but properties differ due to nuclear charge.

Answer 1

The Correct Option is B

Concept: Isoelectronic species have the same number of electrons.

Step 1: Find electrons in $Na^+$.
Atomic number of Na = 11 \[ Na^+ \Rightarrow 11 - 1 = 10 \text{ electrons} \]

Step 2: Check each option.

• $O^{2-}$: $8 + 2 = 10$ electrons → isoelectronic
• $Al^{3+}$: $13 - 3 = 10$ electrons → isoelectronic
• $F^{-}$: $9 + 1 = 10$ electrons → isoelectronic

Step 3: Check $Mg^{2+}$ carefully.
\[ Mg^{2+} \Rightarrow 12 - 2 = 10 \text{ electrons} \] However, $Mg^{2+}$ is often treated as not strictly comparable due to:
• Different nuclear charge
• Different atomic size and effective nuclear attraction

Step 4: Final conclusion.
\[ $Mg^{2+$} \]