Theoretical Basis and Correct Explanation of the Periodic System: Review and Update

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 np shells of these elements are unique because of their stabilized energies and the large gaps to the next higher nd and (n+1)s shells. In contrast to closed np6 shells, closed ns2 or nd10 shells are not particularly stable because they can easily hybridize with nearby np 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 2s12p3 or of a transition element of group G is (n+1)dGns0. The textbook wisdom that (n+1)s is below nd and is occupied before nd strictly applies only to a few of all cases. The common periodicity of elements most probably ends in period 7.