This recurrence of chemical properties whenever the same outer electron-shell arrangement is encountered is called periodicity of behavior, and is most easily understood with the help of a periodic table. The beginning of such a table is shown above, with each electron shell represented by a horizontal row of elements.

Finally, in Chapters 7 and 8 we shall see the theoretical reasons for shell structure, the periodic repetition of chemical properties, and the structure of the periodic table. Although the full story will turn out to be more complicated, eight is a "magic number" for these simple atoms because of their electronic structures.