Electron configuration of ions in the third period.

Main group, s and p block, ions always resemble the nearest noble gas.

Why doesn't Na form Na²⁺?

The electrons which are removed from Na and Mg are 3s electrons. The electrons which are removed from Al are 3s and 3p electrons. The electrons in the 3^rd shell are shielded from much of the charge of the nucleus by the 10 electrons in the 1^st and 2^nd shells.

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By comparison, the electrons in the 2^nd shell are shileded from the nucleus by the 2 electrons in the 1s shell. Therefore, the electrons in the second shell are MUCH more strongly attracted to the nucleus. Since the electrons are strongly attracted to the nucleus they are too difficult to remove.

Why doesn't Cl form Cl^2-?

When an electon is added to a Cl atom it is added to the third shell. The electron which is being added is shielded from the nucleus by the 10 electrons in the 1^st and 2^nd shells. If a second electron is added to a Cl^- ion the electron must go into the 4s orbital. If the electron is put in the 4^th shell it is shielded from the nucleus by 18 electrons. Essentially, the electron in the 4^th shell cannot "see" any positive charge from the nucleus. Since there is little or no attraction between the nucleus and the 2 additional electron the Cl^2- ion releases the electron.