Question

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

• A. Fe²⁺, Mn²⁺
• B. O²⁻, F⁻
• C. Na⁺, Mg²⁺
• D. Mn²⁺, Fe³⁺

Answer 1

The Correct Option is A

The question asks which pair of ions is not an iso-electronic pair. Iso-electronic species are atoms or ions that have the same number of electrons. To determine whether the given pairs are iso-electronic, we need to calculate the total number of electrons for each ion in the pair.

1. Fe²⁺ and Mn²⁺:
   • The atomic number of iron (Fe) is 26. Fe²⁺ means it has lost 2 electrons, so the electron count is 26 - 2 = 24.
   • The atomic number of manganese (Mn) is 25. Mn²⁺ means it has lost 2 electrons, so the electron count is 25 - 2 = 23.
   • Since Fe²⁺ has 24 electrons and Mn²⁺ has 23 electrons, they are not iso-electronic.
2. O²⁻ and F⁻:
   • The atomic number of oxygen (O) is 8. O²⁻ means it has gained 2 electrons, so the electron count is 8 + 2 = 10.
   • The atomic number of fluorine (F) is 9. F⁻ means it has gained 1 electron, so the electron count is 9 + 1 = 10.
   • Since both O²⁻ and F⁻ have 10 electrons, they are iso-electronic.
3. Na⁺ and Mg²⁺:
   • The atomic number of sodium (Na) is 11. Na⁺ means it has lost 1 electron, so the electron count is 11 - 1 = 10.
   • The atomic number of magnesium (Mg) is 12. Mg²⁺ means it has lost 2 electrons, so the electron count is 12 - 2 = 10.
   • Since both Na⁺ and Mg²⁺ have 10 electrons, they are iso-electronic.
4. Mn²⁺ and Fe³⁺:
   • We have already calculated Mn²⁺ to have 23 electrons.
   • The atomic number of iron (Fe) is 26. Fe³⁺ means it has lost 3 electrons, so the electron count is 26 - 3 = 23.
   • Since both Mn²⁺ and Fe³⁺ have 23 electrons, they are iso-electronic.

Therefore, the pair that is not iso-electronic is Fe²⁺, Mn²⁺ as they have a different number of electrons (24 and 23 respectively).