What do you understand by isoelectronic species?

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 - $1=10$ electrons), Ne (10 electrons), $\mathrm{O}^{2-}$ ion (8 + 2 = 10 electrons), and $\mathrm{Al}^{3+}$ ion (13-3 $=10$ electrons).

(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 ( $16+2=18$ electrons), $\mathrm{Cl}^{-}$ion ( $17+1=18$ electrons), $\mathrm{K}^{+}$ion (19-1=18 electrons), and $\mathrm{Ca}^{2+}$ ion (20 - 2 = 18 electrons).

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

(iv) $\mathrm{Rb}^{+}$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 $(35+1$ $=36$ electrons $), \mathrm{Kr}$ (36 electrons), and $\mathrm{Sr}^{2+}$ ion (38-2 = 36 electrons).