Four Electron Groups with Lone Pairs

Four Electron Groups with Lone Pairs 

Notice that although the electron geometry and the molecular geometry are different, the electron geometry is relevant to the molecular geometry. The lone pair exerts its influence on the bonding pairs. 

A FIGURE 10.3 Nonbonding versus Bonding Electron Pairs 

Lone pair-lone pair Lone pair-bonding pair Bonding pair-bonding pair Most repulsive Least repulsive 

Chapter 10 Chemical Bonding II Molecular Shapes, Valence Bond Theory, and Molecular Orbital Theory 

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Four Electron Groups with Lone Pairs... 

In the electron-dot formula of water, H2O, there are also four electron groups, which have minimal repulsion when the electron-group geometry is tetrahedral. However, in H2O, two of the electron groups are lone pairs of electrons. Because the shape of H2O is determined by the two H atoms bonded to the central O atom, the H2O molecule has a bent shape with a bond angle of 109°. Table 10.3 gives the molecular shapes for molecules with two, three, and four bonded atoms. 

In the H2O molecule, two of the four electron groups are bond pairs and two are lone pairs. The molecular shape is obtained by joining the two H nuclei to the O nucleus with straight lines. For H2O, the electron-group geometry is tetrahedral and the molecular geometry is V-shaped, or bent. In the diagram below, the Lewis structure for water is drawn in two ways. 

Ligands with lone pairs such as R, R2N, X, and O groups can use their lone pairs to make 77 bonds to the metal, as in M OR <—> M=OR. (Again, formal charges are usually not drawn.) Thus, these groups can be three- (R, R2N, X), four-(O), or even five-electron donors (RO, X), if the metal needs the extra electron density and it has orbitals of the right symmetry to overlap with the donor orbitals. 

A chemist s-eye view of the structure of liquid water starts with the prediction of the molecular shape of each water molecule. The Lewis structure of water shows that the oxygen atom has four electron-groups around it two covalent bonds and two lone pairs. 

Step 4. Draw and name the molecular shape With four electron groups, one of them a lone pair, PF3 has a trigonal pyramidal shape (AX3E) ... 

Step 4. Shapes around central atoms With four electron groups and no lone pairs, the shapes around the two C atoms in the CH3— groups are tetrahedral (AX4). With three electron groups and no lone pairs, the shape around the middle C atom is trigonal planar (AX3) ... 

When there are four electron groups around the central atom, it is sp hybridized. AB4 molecules and ions with no lone pairs on the central atom have tetrahedral electronic geometry, tetrahedral molecular geometry, and sp hybridization on the central atom. 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

Follow the four-step process described in the flowchart. Begin with the Lewis structure. Use this stracture to determine the steric number, which indicates the electron group geometry. Then take into account any lone pairs to deduce the molecular shape. 

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