Covalent bonding bonds VSEPR model

Vinylic substituent (Section 6.1) Refers to a substituent on a carbon atom that participates in a carbon-carbon double bond. VSEPR model (valence shell electron pair replusion) (Section 1.16) A method of predicting the geometry at a covalently bonded atom by considering the optimum geometric separation between groups of bonding and non-bonding electrons around the atom... 

The electron-dot structures described in Sections 7.6 and 7.7 provide a simple way to predict the distribution of valence electrons in a molecule, and the VSEPR model discussed in Section 7.9 provides a simple way to predict molecular shapes. Neither model, however, says anything about the detailed electronic nature of covalent bonds. To describe bonding, a quantum mechanical model called valence bond theory has been developed. 

Of the 20th century s development of structural chemistry, we mention the discovery of the electron-pair covalent bond by Lewis [22] which remains a fundamental tenet. It is remembered in every line we have drawn to represent a linkage and is present in most models of molecular structure, such as, for example, the valence shell electron pair repulsion (VSEPR) model [23]. 

MO models may be used for molecules that have covalent bonds. Ionic bonds having little orbital overlap between the bonded atoms are less influenced by the stereochemical guidance of the valence orbitals. The geometry of ionic compounds is mainly determined by the electronic repulsion between the nonbonded atoms. A bonding model that considers the latter interaction as the dominant factor for determining molecular geometries is the VSEPR approach, which is discussed further below. 

All the Group 5A elements except nitrogen can form molecules with five covalent bonds (of general formula MX5). Nitrogen cannot form such molecules because of its small size. The MX5 molecules have a trigonal bipyrami-dal shape (see Fig. 19.2) as predicted by the VSEPR model, and the central atom can be described as dsp3 hybridized. The MX5 molecules can accept an additional electron pair to form ionic species containing six covalent bonds. An example is... 

This chapter provides a substantial introduction to molecular structure by coupling experimental observation with interpretation through simple classical models. Today, the tools of classical bonding theory—covalent bonds, ionic bonds, polar covalent bonds, electronegativity, Lewis electron dot diagrams, and VSEPR Theory—have all been explained by quantum mechanics. It is a matter of taste whether to present the classical theory first and then gain deeper insight from the... 

Valence bond theory is one of the two quantum mechanical approaches that explain bonding in molecules. It accounts, at least qualitatively, for the stability of the covalent bond in terms of overlapping atomic orbitals. Using the concept of hybridization, valence bond theory can explain molecular geometries predicted by the VSEPR model. However, the assumption that electrons in a molecule occupy atomic orbitals of the individual atoms can only be an approximation, since each bonding electron in a molecule must be in an orbital that is characteristic of the molecule as a whole. 

Xe is in group 18 and possesses eight electrons in its valence shell. F is in group 17, has seven valence electrons and forms one covalent single bond. Before applying the VSEPR model, decide which is the central atom in the molecule. In each of Xep2 and [XeF5] , Xe is the central atom. 

The Group 5A elements can form molecules or ions that involve three, five, or six covalent bonds to the Group 5A atom. Examples involving three single bonds are NH3, PH3, NF3, and ASCI3. Each of these molecules has a lone pair of electrons (and thus can behave as a Eewis base) and a pyramidal shape as predicted by the VSEPR model (see Fig. 20.8). 

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