Secondary Periodicities Incomplete-Screening and Relativistic Effects

Secondary Periodicities Incomplete-Screening and Relativistic Effects 

So far the nodal structure of the valence s- and p-orbitals themselves has been in our focus, allowing us to explain the special role of the 2p-elements compared to their heavier homologues. The further modulations of chemical and physical properties as we descend to a given group from period 3 on are often summarized under the term secondary periodicity [65, 66]. The main influences here are incomplete screening of nuclear charge by fllled core or semi-core shells and the effects of special relativity. The former reflect shell structure of the atom as a whole and are already important for differences and similarities of the homologous third and fourth period elements, whereas the latter become crucial mainly for the chemistry of the sixth period elements. These aspects have been discussed in detail in various review articles (see, e.g.. Refs [16, 28, 67]), and we, thus, touch them only briefly. 

The most well-known incomplete-screening effect in the periodic table, the lanthanide contraction, is due to the successive Ailing of the 4f-shell by 14 electrons, leading to a contraction of the size of the later lanthanides and of the elements 

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