Molecular geometry predicting, with VSEPR

AIM theory provides a physical basis for the theory of Lewis electron pairs and the VSEPR model of molecular geometry. Equipped with computers and computer-generated, three-dimensional electron density maps, scientists are able to view molecules and predict molecular phenomena without even having to get off their chairs ... 

If the central atom is surrounded only by single bonds, the process of determining the molecular geometry is fairly simple. We count the number of single bonds emanating from the central atom and use the information in Table 7.3 to correlate that number of electron pairs with the geometry predicted by VSEPR, and this describes the shape of the molecule. 

Explain why it is not necessary to find the Lewis structure with the smallest formal charges to make a successful prediction of molecular geometry in the VSEPR theory. For example, write Lewis structures for SO2 having different formal charges, and predict the molecular geometry based on these structures. 

The VSEPR notation for the Cl2F+ ion is AX2E3. According to Table 11.1, molecules of this type exhibit an angular molecular geometry. Our next task is to select a hybridization scheme that is consistent with the predicted shape. It turns out that the only way we can end up with a tetrahedral array of electron groups is if the central chlorine atom is sp3 hybridized. In this scheme, two of the sp3 hybrid orbitals are filled, while the remaining two are half occupied. 

The Cl—F and Cl—Cl bonds in the cation are then formed by the overlap of the half-filled sp3 hybrid orbitals of the central chlorine atom with the half-filled p-orbitals of the terminal Cl and F atoms. Thus, by using sp3 hybridization, we end up with the same bent molecular geometry for the ion as that predicted by VSEPR theory (when the lone pairs on the central atom are ignored)... 

Valence shell electron pair repulsion theory places the two electron pairs on Be 180° apart, that is, with linear electronic geometry. Both electron pairs are bonding pairs, so VSEPR also predicts a linear atomic arrangement, or linear molecular geometry, for BeCl2. 

With the same kind of reasoning, VSEPR theory predicts that sulfite ion, 803 , has tetrahedral electronic geometry. One of these tetrahedral locations is occupied by the sulfur lone pair, and oxygen atoms are at the other three locations. The molecular geometry of this ion is trigonal pyramidal, the same as for other AB3U species. 

In 1968 Bartell published an article on the use of molecular models in the curriculum. In this paper the qualitative valence shell electron pair repulsion (VSEPR) model and the relative role of bonded and nonbonded interaaion in directed valence is discussed. The author correctly predicted the increasing importance of model force fields for geometry prediction. An early discussion of the use of molecular mechanics in teaching can be found in a paper by Cox. 07 A cursory description of the methodology of force field calculations is presented, along with computational results on the relative energy of the rotamers of butane and the conformers of cyclohexane. 

The VSEPR model, simple as it is, does a surprisingly good job at predicting molecular shape, despite the fact that it has no obvious relationship to the filling and shapes of atomic orbitals. For example, based on the shapes and orientations of the 2s and 2p orbitals on a carbon atom, it is not obvious why a CH4 molecule should have a tetrahedral geometry. How can we reconcile the notion that covalent bonds are formed from overlap of atomic orbitals with the molecular geometries that come from the VSEPR model ... 

Solution. The structure of H2O is shown below, where the labels on the H atoms are imaginary. VSEPR theory predicts tetrahedral electron geometry with a bent molecular geometry and bond angles less than 109.5°. The complete set of symmetry operations is , C2, f7y, and (7 2- The multiplication table is shown below. In this symmetry group, each operation is its own inverse. 

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