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

To decide if a pair of ions is iso‑electronic, we count the total electrons each ion possesses after accounting for the charge.

1. $Fe$ has atomic number 26, so a neutral Fe atom contains 26 electrons.
The ion $Fe^\{2+\}$ has lost 2 electrons, leaving it with 26 − 2 = 24 electrons.

2. $Mn$ has atomic number 25, so a neutral Mn atom contains 25 electrons.
The ion $Mn^\{2+\}$ has lost 2 electrons, leaving it with 25 − 2 = 23 electrons.

Since 24 ≠ 23, the ions $Fe^\{2+\}$ and $Mn^\{2+\}$ do not have the same number of electrons. Therefore they are **not** an iso‑electronic pair.

For comparison, the other options are iso‑electronic:

• $Mn^\{2+\}$ (23 e⁻) and $Fe^\{3+\}$ (26 − 3 = 23 e⁻) both have 23 electrons.

• $O^\{2-\}$ (8 + 2 = 10 e⁻) and $F^\{-\}$ (9 + 1 = 10 e⁻) both have 10 electrons.

• $Na^\{+\}$ (11 − 1 = 10 e⁻) and $Mg^\{2+\}$ (12 − 2 = 10 e⁻) both have 10 electrons.

Thus the only pair that is not iso‑electronic is Fe²⁺ and Mn²⁺.