Question

What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions.

1. F-
2. Ar
3. Mg2+
4. Rb+

Answer 1

Atoms and ions having the same number of electrons are called isoelectronic species.

(i) $F ^{-}$ ion has $9 + 1 = 10$ electrons. Thus, the species isoelectronic with it will also have $10$ electrons. Some of its isoelectronic species are
$Na ^+$ ion $(11 - 1 = 10 \,\text{electrons})$, $Ne (10\; \text{electrons}), O^{2-} \text{ion} (8 + 2 = 10 \;\text{electrons})$, and $Al ^{3+}$ion $(13 - 3 = 10 \;\text{electrons)}$.

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(ii) $Ar$ has $18$ electrons. Thus, the species isoelectronic with it will also have $18$ electrons. Some of its isoelectronic species are
$S ^{2-}$ ion $(16 + 2 = 18 \;\text{electrons)}$, $Cl ^-$ ion $(17 + 1 = 18 \;\text{electrons)}$, $K ^+$ ion $(19 - 1 = 18 \;\text{electrons)}$, and $Ca ^{2+}$ ion $(20 - 2 = 18 \;\text{electrons)}$.

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(iii) $Mg ^{2+}$ ion has $12 - 2 = 10$ electrons.
Thus, the species isoelectronic with it will also have $10$ electrons. Some of its isoelectronic species are $F ^-$ ion $(9 + 1 = 10 \;\text{electrons)}$, Ne $(10\;\text{ electrons)}$, $O ^{2-}$ ion $(8 + 2 = 10\;\text{ electrons)}$, and $Al ^{3+}$ ion $(13 - 3 = 10\;\text{ electrons)}$.

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(iv) $Rb ^+$ ion has $37 - 1 = 36$ electrons.
Thus, the species isoelectronic with it will also have $36$ electrons. Some of its isoelectronic species are $Br ^-$ion $(35 + 1 = 36 \;\text{electrons)}$, $Kr$ $(36 \;\text{electrons)}$, and $Sr ^{2+}$ ion $(38 - 2 = 36\;\text{ electrons)}$.