From the following pairs of ions which one is not an iso-electronic pair ?

The correct answer is: Fe²⁺, Mn²⁺

Key Concept: Isoelectronic species are atoms or ions that have the same number of electrons. To check, we calculate the electron count for each ion by starting with the atomic number (Z) and adjusting for the charge.

Let us verify each pair:

Option 1: O²⁻ and F⁻
Oxygen (Z = 8): O²⁻ → 8 + 2 = 10 electrons
Fluorine (Z = 9): F⁻ → 9 + 1 = 10 electrons
Both have 10 electrons → Isoelectronic ✓

Option 2: Na⁺ and Mg²⁺
Sodium (Z = 11): Na⁺ → 11 − 1 = 10 electrons
Magnesium (Z = 12): Mg²⁺ → 12 − 2 = 10 electrons
Both have 10 electrons ��� Isoelectronic ✓

Option 3: Mn²⁺ and Fe³⁺
Manganese (Z = 25): Mn²⁺ → 25 − 2 = 23 electrons
Iron (Z = 26): Fe³⁺ → 26 − 3 = 23 electrons
Both have 23 electrons → Isoelectronic ✓

Option 4: Fe²⁺ and Mn²⁺
Iron (Z = 26): Fe²⁺ → 26 − 2 = 24 electrons
Manganese (Z = 25): Mn²⁺ → 25 − 2 = 23 electrons
They have different electron counts (24 ≠ 23) → Not Isoelectronic ✗

Conclusion: Although Fe²⁺ and Mn²⁺ carry the same charge (+2), they have different atomic numbers (26 and 25), so removing two electrons from each still leaves them with different electron counts. Having the same charge does not guarantee isoelectronic nature — what matters is the total number of electrons being equal. Therefore, Fe²⁺ and Mn²⁺ is the pair that is not isoelectronic.