Which one of the following ions is not iso-electronic with F⁻?
- (a)O²⁻
- (b)Na⁺
- (c)Ne
- (d)N⁻
Correct — D, N⁻. Isoelectronic species have the same number of electrons. Fluorine's atomic number is 9, so F⁻ has 9 + 1 = 10 electrons. Checking each option: O²⁻ = 8 + 2 = 10, Na⁺ = 11 − 1 = 10, and Ne = 10 — all match F⁻. But N⁻ has only 7 + 1 = 8 electrons, so it is not isoelectronic with F⁻. (The nitrogen species that would be isoelectronic with F⁻ is N³⁻, not N⁻.)
- (a)O²⁻ — Oxygen (Z = 8) plus 2 extra electrons gives 10 electrons — isoelectronic with F⁻, so not the exception.
- (b)Na⁺ — Sodium (Z = 11) minus 1 electron gives 10 electrons — isoelectronic with F⁻, so not the exception.
- (c)Ne — Neon has 10 electrons already — isoelectronic with F⁻, hence not the answer.
Isoelectronic species are atoms and ions that have the same total number of electrons (and hence the same electron configuration). To test them, count electrons as the atomic number Z adjusted for charge: add one electron for each unit of negative charge, subtract one for each unit of positive charge.
F⁻ carries 10 electrons — the neon configuration. Three of the options are built around that 10-electron set, so the odd one out is the species whose electron count differs. N⁻ has 8 electrons, so it belongs with the oxygen-atom (O) 8-electron family, not the fluoride family.
- Isoelectronic = same number of electrons.
- Electron count = atomic number Z, plus the negative charge or minus the positive charge.
- F⁻ has 10 electrons (the neon, 2,8 configuration).
- N⁻ has 8 electrons; the nitrogen ion isoelectronic with F⁻ would be N³⁻ (10 electrons).
F⁻ has 10 electrons; only N⁻ (8 electrons) differs, so the answer is (d).
- Counting protons instead of electrons, or ignoring the ionic charge.
- Assuming N⁻ (rather than N³⁻) reaches the neon configuration.
Asked as which ion is NOT isoelectronic with a given ion — solved by counting electrons as Z adjusted for charge.
No directly related past PYQ was found.
- practice — not a real PYQ
Which one of the following is isoelectronic with the Na⁺ ion?
- (a)K⁺
- (b)Mg²⁺
- (c)Cl⁻
- (d)Ca²⁺
Answer(b) Mg²⁺ — Na⁺ has 10 electrons (11 − 1), and Mg²⁺ has 10 electrons (12 − 2).
- practice — not a real PYQ
The number of electrons in the fluoride ion (F⁻) is
- (a)8
- (b)9
- (c)10
- (d)11
Answer(c) 10 — fluorine (Z = 9) gains one electron, giving the 10-electron neon configuration.