Secondary periodicity

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. 

In this chapter, we have tried to emphasize general aspects of main-group chemical bonding, with particular emphasis on periodic trends. The periodic table remains the most important classification tool in chemistry, and it is crucial to understand even subtle secondary periodicities if one is to make efficient use of the various elements for different chemical applications. The radial nodal structure of the valence orbitals has been pointed out to account for more of the known trends than most practitioners of chemistry are aware of. For example, the inversion barriers of phosphines or silyl anions, the dependence of the inert-pair effect on the electronegativity of the substituents, the stability of carbene- or carbyne-type species or of multiple bonds between heavy main-group elements are aU intricately linked to hybridization defects of s- and p-valence orbitals of disparate sizes. Even the question of hypervalency is closely connected to the effects of primogenic repulsion . 

Ralph Sanderson, an author who has published extensively on the periodic system, has listed some further interesting examples of secondary periodicity ... 

Table re-drawn from Chistyakov, but using lUPAC group numbering. E.V. Chistyakov, Biron s Secondary Periodicity of the Side d-Subgroups of Mendeleev s Short Table, Zhurnal Ohshchei Khimii (Engl. Ed.), 38,213-214,1968. 

Finally, several authors who seek to provide group-theoretical explanations for the periodic system have claimed that their approaches also predict secondary periodicity. ... 

Admittedly, Newlands s grouping of all these elements together makes sense in terms of secondary periodicity relationships as embodied in many short-form periodic tables. E.g., each of these elements shows a valence of 1. 

AsClj has now actually been prepared, but the difficulty in doing so is shown by the fact that this feat was achieved only in 1977, compared with PCl and SbCl, which have been known since 1834. On a separate point, the screening explanations have been confirmed by relativistic quantum mechanical calculations carried out by P. Pyykko, On the Interpretation of Secondary Periodicity in the Periodic System,Jourua/ of Chemical Research (Sweden), (S), 380-381,1979. 

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