Elements in the extreme right and left portions of the periodic table, whose atoms differ from a closed-shell structure by one or two excess or missing electrons, tend to form stable anions or cations by a gain or loss of one or two electrons. To cite two well-known examples: and , where and both have the closed-shell structure of neon.

The graphic shows the relevant fragments of the periodic table as well as a stylized representation of the electron shell structure. You can select the noble gas configuration and the related ion with either positive or negative charges one or two.