Question
Write the isoelectronic pair among Na+, Cl-, F-, Li+.
The Correct Answer and Explanation is:
Correct Answer:
The isoelectronic pair among Na⁺, Cl⁻, F⁻, and Li⁺ is Na⁺ and F⁻.
Explanation
Isoelectronic species are atoms or ions that have the same number of electrons and hence the same electron configuration, though they may have different nuclear charges.
Let’s examine each species:
- Na⁺ (Sodium ion):
- Atomic number of Na = 11
- Loses 1 electron → electrons = 11 – 1 = 10
- Cl⁻ (Chloride ion):
- Atomic number of Cl = 17
- Gains 1 electron → electrons = 17 + 1 = 18
- F⁻ (Fluoride ion):
- Atomic number of F = 9
- Gains 1 electron → electrons = 9 + 1 = 10
- Li⁺ (Lithium ion):
- Atomic number of Li = 3
- Loses 1 electron → electrons = 3 – 1 = 2
Now compare the number of electrons:
- Na⁺ = 10
- F⁻ = 10
- Cl⁻ = 18
- Li⁺ = 2
Clearly, Na⁺ and F⁻ both have 10 electrons and are therefore isoelectronic. Despite having different nuclear charges (Na has 11 protons, F has 9), the electron configuration is the same:
1s² 2s² 2p⁶ — the configuration of Neon (Ne).
It’s important to note that although isoelectronic species have the same electron configuration, they can have different chemical and physical properties due to differences in nuclear charge (Z). A higher nuclear charge pulls electrons in more tightly, resulting in a smaller ionic radius. For example, Na⁺ is smaller than F⁻ even though both have 10 electrons, because Na⁺ has more protons pulling on the same number of electrons.
Conclusion:
Na⁺ and F⁻ are isoelectronic because they both have 10 electrons and share the same electron configuration.
