To begin our discussion of the trend in atomic radii lets consider the electron configuration for the elements in the third period, sodium through argon.

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To develop the next portion of the table we need to discuss two new terms; valence electrons and inner core electrons.

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So lets determine the number of valence electrons and inner core electrons for each of the elements in our table.

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Inner core electrons shield valence electrons from the nucleus. If we are on a valence electron looking back at the nucleus, the inner core electrons shield a portion of the nuclear charge from us. So the valence electron does not feel the attraction of all of the protons in the nucleus, but the attraction of an effective nuclear charge which is less than the total charge on the nucleus. The effective nuclear charge is determined by subtracting from the number of protons in the nucleus (Z), the number of inner core (I.C.) electrons that shield the valence electron from the nucleus.

Z_eff = Z - I.C.

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So lets consider the valence electrons in magnesium and consider the shielding effect of its inner core electrons and whether electrons in the same shell shield the electron we are considering.

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So here is a question to see whether the concept of effective nuclear charge is clear.

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Let's review what we've discussed and extend the idea of shielding and effective nuclear charge. You have to be able to determine the effective nuclear charge on any electron in an atom in the A groups.

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