Electron-group arrangements

Chlorine pentafluoride and xenon tetrafluoride appear in Figure 9-26. Each has an inner atom with a steric number of 6, but their electron group arrangements include lone pairs. As a result, CIF5 has a square pyramidal shape, whereas XeF4 has a square planar shape. Pictures can help us determine whether or not the bond polarities cancel ... 

The five basic electron-group arrangements and their bond angles... 

In the following ExpressLab, you will make models of the five electron-group arrangements, and measure their bond angles. Afterwards, you will consider some of the variations in molecular shapes that can occur... 

Table 4.2 Common Molecular Shapes and Their Electron Group Arrangements... 

Follow the four-step procedure that helps to predict molecular shape. Use Table 4.2 for names of the electron-group arrangements and molecular shapes. 

In what cases is the name of the molecular shape the same as the name of the electron group arrangement ... 

Electron groups Electron group arrangement Bond electron pairs Lone pairs Geometry of molecule or composite ion Example... 

Valence-Shell Electron-Pair Repulsion (VSEPR) Theory and Molecular Shape Electron-Group Arrangements and Molecular Shapes... 

Figure 10.3 Electron-group repulsions and the five basic molecular shapes. A, As an analogy for electron-group arrangements, two to six attached balloons form five geometric orientations such that each balloon occupies as much space as possible. B, Mutually repelling...

Figure 10.4 The single molecular shape of the linear electron-group arrangement. The key (bottom) for A, X, and E also refers to Figures 10.5, 10.6, 10.8, and 10.9.

Thus, for similar molecules within a given electron-group arrangement, electron-pair repulsions cause deviations from ideal bond angles in the following order ... 

Step 2. Assign an electron-group arrangement by counting all electron groups around the central atom, bonding plus nonbonding. 

Step 3. Predict the ideal bond angle from the electron-group arrangement and the direction of any deviation caused by lone pairs or double bonds. 

Step 3. Predict the bond angle For the tetrahedral electron-group arrangement, the ideal bond angle is 109.5°. There is one lone pair, so the actual bond angle should be less than 109.5°. 

Step 2. Electron-group arrangement With five electron groups, this is the trigonal bipyra-midal arrangement. 

Describe the five electron-group arrangements and associated molecular shapes, predict molecular shapes from Lewis structures, and explain deviations from ideal bond angles ( 10.2) (SPs 10.6-10.8) (EPs 10.25-10.49)... 

Name all the molecular shapes that have a tetrahedral electron-group arrangement. 

What would you expect to be the electron-group arrangement around atom A in each of the following cases For each arrangement, give the ideal bond angle and the direction of any expected deviation ... 

Determine the electron-group arrangement, molecular shape, and ideal bond angle(s) for each of the following ... 

We postulate the presence of a certain type of hybrid orbital after we observe the molecular shape. As we discuss the five common types of hybridization, notice that the spatial orientation of each type of hybrid orbital corresponds with one of the five common electron-group arrangements predicted by VSEPR theory. 

To account for other molecular shapes within a given electron-group arrangement, we postulate that one or more of the hybrid orbitals contains lone pairs. In ozone (O3), for example, the central O is sp hybridized and a lone pair fills one of its three sp orbitals, so ozone has a bent molecular shape. 

Figure 11.5 shows the bonding in other molecular shapes with the tetrahedral electron-group arrangement. The trigonal pyramidal shape of NH3 arises when a lone pair fills one of the four sp orbitals of N, and the bent shape of H2O arises when lone pairs fill two of the sp orbitals of O. 

For the trigonal bipyramidal shape of the PCI5 molecule, for example, the VB model proposes that the one 35, the three 3p, and one of the five 3d orbitals of the central P atom mix and form five sp d hybrid orbitals, which point to the vertices of a trigonal bipyramid (Figure 11.6). Seesaw, T-shaped, and linear molecules have this electron-group arrangement with lone pairs in one, two, or three of the central atom s sp d orbitals, respectively. 

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