First row anomaly

Generally speaking, the mechanisms of p-block element reactions are not particularly consistent with the rules outlined above. The reason for this boils down to the so-called first-row anomaly, where both first- and second-period elements (H-Ne) are all somewhat unreasonably lumped together as first row. The expression means that the chemical properties of first-row elements are anomalous relative to those of their heavier congeners. Let us go through the above four rules one by one and see how well they hold up in a main-group inorganic context. 

Periodicity (Continued) of elements, 27-29 first- and second-row anomalies, 858-861... 

First- and Second-Row Anomalies 858 The Use of p Orbitals in Pi Bonding 861 The Use (or Not) of d Orbitals by Nonmetals 86ft Reactivity and d Orbital Participation 875... 

This ionic model provides a good basis for explaining anomalies in the variation of lattice energies and hydration energies across the first-row d-block for octahedral metal ions. As electrons are added to the t... 

The structures, energetics and properties of molecules formed from elements in the first row of the periodic table, Li to Ne, can be dramatically different from those formed from elements in the subsequent rows—the so-called row anomaly. The anomaly manifests itself in a number of ways, such as the inability of the first row p-block elements to form hypervalent species. N and P are an example of this anomaly P is able to form hypervalent molecules such as PF5 and PCI5, while N only forms NF3 and NCI3. Another manifestation of the difference between N and P has drawn quite a bit of attention. The ground states of NH3, NF3, PH3 and PF3 are all pyramidal, as expected. However, NH3, NF3 and PH3 invert through a transition state with D3h symmetry, while the transition state for inversion in PF3 is T-shaped with Cav symmetry. 

Fig. 109. Schematic illustration of phase relationships, structural transitions and anomalies appearing in the nonstoichiometric range of 123-0,. First row EM investigations of Beyers et al. (1989). Second row XRD and NPD on poly- and single-crystalline samples, after Plakhty et al. (1994, 1995). Third row Hard X-ray single-crystal refinements of superstructures, after von Zimmermann et al. (1999). Fourth and fifth rows under normal and higher pressures, measured in equilibrium samples. Sixth to tenth rows phase relationships deduced from lattice parameters of slowly cooled samples (preparation methods described in sect. 3.1.2.2). Eleventh row EXAFS results after Rohler et al. (1997a,b, 1998).

One striking exception was the very early discovery of I decay to Xe (Jeffery and Reynolds 1961). This discovery reflects the particular properties of rare gases which are nearly absent in telluric planetary bodies. Because they are not diluted by high abrmdances of isotopically normal noble gases, anomalies in rare noble gas components were the first to be detected. This is also the reason for the Xe record of the fission of Pu (Rowe and Kuroda 1965). From the available data on short-lived nuclides at that time, it was concluded that the last nucleosynthetic input into the protosolar cloud predated the formation of the planets by 100-200 Ma. 

---