Observing valence-shell electrons

The valence shell electron pair repulsion (VSEPR) model is based on the observation that the geometrical arrangement of bonds around an atom is influenced by non-bonding electrons present. 

In an SFg molecule we have six valence shell electron pairs and six F atoms surrounding one S atom. Because the valence shell of sulfur contains no lone pairs, the electronic and molecular geometries in SFg are identical. The maximum separation possible for six electron pairs around one S atom is achieved when the electron pairs are at the comers and the S atom is at the center of a regular octahedron. Thus, VSEPR theory is consistent with the observation that SFg molecules are octahedral. 

The observed variations in the N—C bond distances and < C3—N—C angles are not in agreement with the valence-shell electron-pair repulsion model. This predicts that partial removal of the lone-pair electrons on the nitrogen atom would lead to a shortening of the N—C bonds. It would further predict that < C3—N—C should be greater than tetrahedral in the free donor and decrease on complex formation. 

As, and Sb, X is F, Q, Br, and I, and Y is O and S, are summarized in a review paper by Kuchitsu Trends observed in these molecules are well accounted for qualitatively by the valence-shell electron-pair repulsion theory (VSEPR) ... 

For molecules with more than an octet of valence-shell electrons on the central atom, the employment of correlation diagrams has been less systematic. Instead, some individual cases have been treated to see what distortions from assumed idealized geometries might be expected. For example, the T-shaped C1F3 molecule has been treated as indicated in Fig. 4-6, where the results suggest that the angle should be 10° less than 90°, in semi-quantitative agreement with observation. 

Electronic transitions within the valence shell of atoms and molecules appear in the energy-loss spectrum from a few electron volts up to, and somewhat beyond, the first ionization energy. Valence-shell electron spectroscopy employs incident electron energies from the threshold required for excitation up to many kiloelectron volts. The energy resolution is usually sufficient to observe vibrational structure within the Franck-Condon envelope of an electronic transition. The sample in valence-shell electron energy-loss spectroscopy is most often in the gas phase at a sufficiently low pressure to avoid multiple scattering of the... 

The reason for this unusual geometry was the availability of only the stereochemically active electrons for bonding. Thus, a SO modification of the valence shell electron pair repulsion (VSERP) theory was suggested in [130]. Han et al. [129], however, claim that this modification should be applied with caution, since no non-linear II8F2 structure has been detected as a minimum at the HF level of theory. An important observation was made that the fluorides of element 118 will most probably be ionic rather than covalent, as in the case of Xe. This prediction might be useful for future gas-phase chromatography experiments. 

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