Isoelectronic species are defined as those species which belong to different atoms or ions which possess same number of electrons but different magnitude of nuclear charge.

A positive charged ion denotes the loss of an electron & a negative charged ion represents the gain of an electron by a species.

The isoelectronic ions with greater nuclear charge will possess small size as compared to the ion with smaller nuclear charges.

[i] F^- ion has 9+1 = 10 electrons. The isoelectronic species of Fluoride will also have 10 electrons. Some examples of isoelectronic species are Na⁺ ion [11-1 = 10 electrons], Ne [10 electrons], O^2- ion [8+2 =10 electrons], and AL³⁺ ion [13-3 = 10 electrons].

[ii] Ar has 18 electrons. The isoelectronic species of Argon will also have 18 electrons. Some examples of isoelectronic species are S^2- ion [16+2 = 18 electrons], CL^- ion [17+1 = 18 electrons], K⁺ ion [19-1 = 18 electrons], and Ca⁺ ion [20-2 = 18 electrons].

[iii] Mg²⁺ ion has 12 – 2 = 10 electrons. The isoelectronic species of Magnesium will also have 10 electrons. Some examples of isoelectronic species are F^- ion [9+1 = 10 electrons], ne [10 electrons’, O^2- ion [8+2 = 10 electrons], and AL³⁺ ion [13-3 = 10 electrons].

[iv] Rb⁺ ion has 37 – 1 = 36 electrons. The isoelectronic species of Rubidium will also have 36 electrons. Some examples of isoelectronic species are Br^- ion [35+1 = 36 electrons], Kr [36 electrons], and Sr²⁺ ion [38-2 = 36 electrons].