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

The correct option is Fe²⁺, Mn²⁺.

Iso-electronic species are atoms, molecules, or ions that share the same number of electrons. To determine which pair is not iso-electronic, we calculate the total number of electrons for each ion using the atomic number (Z) of the corresponding neutral element.

The number of electrons in a monoatomic ion is calculated as:

$\text{Number of electrons}=\text{Atomic number (Z)}-\text{Charge of the ion}$

Let us calculate the number of electrons for each pair:

Option 1: O²⁻ and F⁻
For O²⁻: Atomic number of Oxygen (O) is 8. Number of electrons = 8 - (-2) = 10 electrons.
For F⁻: Atomic number of Fluorine (F) is 9. Number of electrons = 9 - (-1) = 10 electrons.
Since both ions have 10 electrons, they form an iso-electronic pair.

Option 2: Na⁺ and Mg²⁺
For Na⁺: Atomic number of Sodium (Na) is 11. Number of electrons = 11 - 1 = 10 electrons.
For Mg²⁺: Atomic number of Magnesium (Mg) is 12. Number of electrons = 12 - 2 = 10 electrons.
Since both ions have 10 electrons, they form an iso-electronic pair.

Option 3: Mn²⁺ and Fe³⁺
For Mn²⁺: Atomic number of Manganese (Mn) is 25. Number of electrons = 25 - 2 = 23 electrons.
For Fe³⁺: Atomic number of Iron (Fe) is 26. Number of electrons = 26 - 3 = 23 electrons.
Since both ions have 23 electrons, they form an iso-electronic pair.

Option 4: Fe²⁺ and Mn²⁺
For Fe²⁺: Atomic number of Iron (Fe) is 26. Number of electrons = 26 - 2 = 24 electrons.
For Mn²⁺: Atomic number of Manganese (Mn) is 25. Number of electrons = 25 - 2 = 23 electrons.
Since Fe²⁺ has 24 electrons and Mn²⁺ has 23 electrons, their electron counts are not equal:

$24\ne 23$

Thus, the pair Fe²⁺ and Mn²⁺ is not an iso-electronic pair.