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

To determine which pair of ions is not iso-electronic, we need to understand the term "iso-electronic." Iso-electronic species are atoms, molecules, or ions that have the same number of electrons.

Let's analyze each pair:

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

Conclusion: The pair Fe²⁺ and Mn²⁺ is not iso-electronic because they do not have the same number of electrons. Therefore, the correct answer is Fe²⁺, Mn²⁺.