VSEPR formula

To help us predict the shapes of molecules, we use the generic VSEPR formula ... 

Sei e-Tfst 3.3A (a) Give the VSEPR formula of an NH3 molecule. Predict (b) its electron arrangement and (c) its shape. 

FIGURE 3.7 The arrangements of atoms that give rise to polar and nonpolar molecules. In these VSEPR formulas, A stands for a central atom, X for an attached atom, and E for a lone pair. Identical atoms are... 

Using Lewis structures and VSEPR, give the VSEPR formula for each of the following species and predict its shape (a) sulfur tetrachloride (b) iodine trichloride (c) 1F4 ... 

Give the VSEPR formula, molecular shape, and bond angles for each of the following species (a) I, (b) SbCl5 ... 

Write the Lewis structure and the VSEPR formula, list the shape, and predict the approximate bond angles for (a) CF,C1 ... 

Lone pairs on the central atom of a molecule also affect the shape of the molecule. To help us predict the shapes of molecules, we use a generic VSEPR formula AX Em to identify the different combinations of atoms and lone pairs attached to the central atom. We let A represent a central atom, X an attached atom, and E a lone pair. For example, the BF3 molecule, with three attached fluorine atoms and no lone pairs on B, is an example of an AX3 species. The sulfite ion, S()32- (18), is an example of an AX3E species. 

Self-Test 3.3A Predict (a) the electron arrangement and (b) the shape of an NHj molecule, (c) Give its VSEPR formula. 

STRATEGY The existence of residual entropy at T = 0 suggests that the molecules are disordered. From the shape of the molecule (which can be obtained by using VSEPR theory), we need to determine how many orientations, W, it is likely to be able to adopt in a crystal then we can use the Boltzmann formula to see whether that number of orientations leads to the observed value of S. 

C09-0109. Species with chemical formula X I4 can have the following shapes. For each, name the molecular geometry, identify the ideal VSEPR bond angles, tell how many lone pairs are present in the structure, and give a specific example. 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

The final cu-bonded formulas (3.213), (3.214), and (3.219)-(3.221) bear an obvious resemblance to the usual VSEPR representations of these hypervalent species. Indeed, each cu-bonded structure has the same number of formal bond pairs (bp) and lone pairs (lp) as the VSEPR representation. Furthermore, the predicted angular geometries of the two models are essentially identical, with the linear (or near-linear) cu-bonded ligands occupying axial positions in the SN2-like trigonal bipyramidal motif. 

Use the VSEPR model to predict the electron arrangement and shape of a molecule or polyatomic ion from its formula, giving each bond angle approximately, Examples 3.1, 3.2, and 3.3. 

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... 

Obviously, the formulas CO2 and SO2 do not provide any information about the shapes of these molecules. However, there is a model that can be used to predict the shape of a molecule. This model is based on the valence shell electron pair repulsion (VSEPR) theory. Using this model, you can predict the shape of a molecule by examining the Lewis structure of the molecule. 

It is time to reflect on how well VSEPR satisfies the criteria for a successful bonding theory. Since it combines the features of Lewis dot structures (molecular formula) with a geometric prediction, we fulfill two of the three requirements, but we still have not answered the major question, that is, What is the rationale for bonding Our search must continue. 

What advantages does the VSEPR model of chemical bonding have compared with Lewis formulas ... 

Write a Lewis formula for the anion SiFg - that would be produced from the weak acid HySiEg. Use the VSEPR theory to predict the shape of SiEg . 

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