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 \((13 - 3 = 10 \;\text{electrons)}\).
(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 \((20 - 2 = 18 \;\text{electrons)}\).
(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 \((13 - 3 = 10\;\text{ electrons)}\).
(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 \((38 - 2 = 36\;\text{ electrons)}\).