Long-standing questions on the theoretical basis of the periodic system have been answered in recent years. A specific type of periodicity is imposed on all elements by the main groups just before and after the noble gasses. The upper "n"p shells of these elements are unique because of their stabilized energies and the large gaps to the next higher "n"d and ("n"+1)s shells. In contrast to closed "n"p[superscript 6] shells, closed "n"s[superscript 2] or "n"d[superscript 10] shells are not particularly stable because they can easily hybridize with nearby "n"p or ("n"+1)s shells, respectively. It is misleading to consider the electron configurations of the ground states of "free" neutral atoms as the dominant configurations of the chemical elements, that is, of "bound" atoms in chemical substances. It blurs the fact that the dominant valence configuration of, for example, chemically bound carbon is 2s[superscript 1]2p[superscript 3] or of a transition element of group "G" is ("n"+1)d[superscript G]"n"s[superscript 0]. The textbook wisdom that ("n"+1)s is below "n"d and is occupied before "n"d strictly applies only to a few of all cases. The common periodicity of elements most probably ends in period 7. (Contains 3 tables, 1 figure and 12 notes.)