THE PERIODIC TABLE

4.9 - Ionic Size

4.9.3 - Ionic Size for Isoelectronic Series

Isoelectronic ions are ions that have the same number of electrons. There are many ions that are isoelectronic.

For example:

Isoelectronic ions	number of electrons
1. Na+, F-	10 electrons
2. S2-, Cl-, K+, Ca+2	18 electrons

Let's look at the first example of isoelctronic ions, Na⁺, F^- (ions both have 10 electrons):

• Na⁺ has 11 protons, a nuclear charge of +11.
• F^- has 9 protons, a nuclear charge of +9.

We expect the Na⁺, with its greater nuclear charge will have a greater attraction of its electrons than F^-.

Na⁺ has a smaller ionic radius than F^-

Let's look at the second example of isoelctronic ions, S^2-, Cl^-, K⁺, Ca⁺² (ions all have 18 electrons):

• S^2- has 16 protons, a nuclear charge of +16.
• Cl^- has 17 protons, a nuclear charge of +17.
• K⁺ has 19 protons, a nuclear charge of +19.
• Ca⁺² has 20 protons, a nuclear charge of +20.

We expect the Ca⁺², with the greatest nuclear charge will have a greatest attraction of its electrons. S^2- has the smallest nuclear charge will not have as much pull of its electrons toward the nucleus. Therefore, the ionic size:

Ca⁺² < K⁺ < Cl^- < S^2-
(smallest)                    (largest)

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