<P> In this list the orbitals in parentheses are not occupied in the ground state of the heaviest atom now known (Og, Z = 118). </P> <P> The Aufbau principle can be applied, in a modified form, to the protons and neutrons in the atomic nucleus, as in the shell model of nuclear physics and nuclear chemistry . </P> <P> The form of the periodic table is closely related to the electron configuration of the atoms of the elements . For example, all the elements of group 2 have an electron configuration of (E) ns (where (E) is an inert gas configuration), and have notable similarities in their chemical properties . In general, the periodicity of the periodic table in terms of periodic table blocks is clearly due to the number of electrons (2, 6, 10, 14 ...) needed to fill s, p, d, and f subshells . </P> <P> The outermost electron shell is often referred to as the "valence shell" and (to a first approximation) determines the chemical properties . It should be remembered that the similarities in the chemical properties were remarked on more than a century before the idea of electron configuration . It is not clear how far Madelung's rule explains (rather than simply describes) the periodic table, although some properties (such as the common + 2 oxidation state in the first row of the transition metals) would obviously be different with a different order of orbital filling . </P>

Which list includes species with the same number of electrons