UNIT # 4 Introduction Objectives Problems 1 | 2 | 3
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, S2-, Cl-, K+, Ca+2 (ions all have 18 electrons):

• S2- 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. S2- 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- < S2-
(smallest)                    (largest)

 For isoelectronic ions, the radius decreases as the positive nuclear charge increases.

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