Valence-shell occupancy

In order to write correct Lewis structures, two more concepts are needed. First, consider the total number of electrons in the immediate neighborhood of each atom. This number is called the valence-shell occupancy of the atom, and to find it, all unshared electrons around the atom and all electrons in bonds leading... 

Write the core symbols for the atoms and fill in the number of electrons determined in Step 1. The electrons should be added so as to make the valence-shell occupancy of hydrogen 2 and the valence-shell occupancy of other atoms not less than 8 wherever possible. 

Valence-shell occupancy must not exceed 2 for hydrogen and 8 for a first-row atom for a second-row atom it may be 10 or 12. 

It is because the lowest-energy structures are most important that we specified in the rules for writing Lewis structures that the number of bonds should be maximum and the valence-shell occupancy not less than 8 whenever possible. Structures that violate these stipulations, such as 11 and 12, represent high-energy forms and hence do not contribute significantly to the structural pictures, which... 

Valence Shell Occupancy 8 electrons 10 electrons 12 electrons... 

It is noteworthy that Rydberg orbital occupancies on the central atom (rY, final column of Table 3.29) are relatively negligible (0.01-0.03e), showing that d-orbital participation or other expansion of the valence shell is a relatively insignificant feature of hyperbonded species. However, the case of HLiH- is somewhat paradoxical in this respect. The cationic central Li is found to use conventional sp linear hybrids to form the hydride bonds, and thus seems to represent a genuine case of expansion of the valence shell (i.e., to the 2p subshell) to form two bonding hybrids. However, the two hydride bonds are both so strongly polarized toward H (93%) as to have practically no contribution from Li orbitals, so the actual occupancy of extra-valent 2pu orbitals ( 0.03< ) remains quite small in this case. 

Table 4.5. The NBO descriptors of group 6-11 MH metal hydrides of the third transition series, showing the percentage accuracy (%fy) of the Lewis-like description (valence shell and total), metal hybrid (Iim), percentage polarization toward M (100cm2), and occupancy of bonding ctmh NBOs (see Fig. 4.8)...

Where p (r) is the electron density of each pseudo atom, Pcore(r) and pvai ( r) are the core and spherical densities of the valence electron shells, Pvai and Pim (multipoles) describe the electron shell occupations, k and k denote the spherical deformation and y (r/r) is a geometrical function. The parameters K, k , Pvai and Pim are refined during adjustment of the experimental and models structure amplitudes. 

Ab initio HFS calculations have shown that, consistent with the predictions of Banister the S3N3" anion is a fully delocalized lOre-electron system The valence shell energy levels of the anion are reproduced in Fig. 10. Although the 7c-orbital pattern is reminiscent of that found for benzene, the occupancy of the... 

The chart in Figure 13.1 provides a handy reference for the electron count in the transition metals in their M(0) configurations. Occupancy occurs in the n d and n + s orbitals. The n + 1 p orbitals are also counted as part of the valence level and are used in hybridization of the metal center by mixing with the n d and n + 1 s orbitals, giving a total valence shell capacity of 18 electrons. 

CV and valence calculations leads to almost insuperable linear dependence problems in the resulting basis set [97], We should note that these problems axe perhaps at their worst for the first row. For heavier elements the separation between the valence shell and the core (meaning the next innermost shell here) is not as great, so the disparities in correlating orbital occupation number are less. Suitable ANO sets can often be obtained by correlating all the desired electrons [48, 98]. 

The chemical reactions of nitrogen and phosphorus are similar because they share the same number of electrons in their outer shell (five). The reactivity of oxygen resembles the reactivity of sulfur because of their shared outer-shell occupancy (six). This outer-shell occupancy of an atom is called its valence. Carbon has a valence of four (with four electrons in its outer shell), and its chemistry shares some similarities with silicon, which also has a valence of four. Silicon, germanium, tin, and lead, which have the same valence, have all been used in various proportions to form semiconductors, interesting and important materials that we will investigate later when we discuss chemical bonding. 

---