Isoelectronic species are those which have same number of electrons. (i) $\mathrm{F}^{-}$ has 10 electrons. Therefore, the species $\mathrm{N}^{3-}, \mathrm{O}^{2-}, \mathrm{Ne}, \mathrm{Na}^{+}, \mathrm{Mg}^{2+}$, etc., are isoelectronic with $\mathrm{F}^{-}$. (ii) Ar has 18 electrons. Therefore, the species $\mathrm{P}^{3-}, \mathrm{S}^{2-}, \mathrm{Cl}^{-}, \mathrm{K}^{+}, \mathrm{Ca}^{2+}$, etc., are isoelectronic with Ar. (iii) $\mathrm{Mg}^{2+}$ has 10 electrons. Therefore, the species $\mathrm{N}^{3-}, \mathrm{O}^{2-}, \mathrm{Ne}, \mathrm{Na}^{+}$, etc., are isoelectronic with $\mathrm{Mg}^{2+}$. (iv) $\mathrm{Rb}^{+}$ has 36 electrons. Therefore, the species $\mathrm{Br}^{-}$, Kr, $\mathrm{Sr}^{2+}$, etc., are isoelectronic with $\mathrm{Rb}^{+}$.