© 2009, Prentice-Hall, Inc. Chapter 9 Molecular Geometries and Bonding Theories Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten John D. Bookstaver St. Charles Community College Cottleville, MO

© 2009, Prentice-Hall, Inc. Molecular Shapes The shape of a molecule plays an important role in its reactivity. By noting the number of bonding and nonbonding electron pairs we can easily predict the shape of the molecule.

© 2009, Prentice-Hall, Inc. What Determines the Shape of a Molecule? Simply put, electron pairs, whether they be bonding or nonbonding, repel each other. By assuming the electron pairs are placed as far as possible from each other, we can predict the shape of the molecule.

© 2009, Prentice-Hall, Inc. Electron Domains We can refer to the electron pairs as electron domains. In a double or triple bond, all electrons shared between those two atoms are on the same side of the central atom; therefore, they count as one electron domain. The central atom in this molecule, A, has four electron domains.

© 2009, Prentice-Hall, Inc. Valence Shell Electron Pair Repulsion Theory (VSEPR) “The best arrangement of a given number of electron domains is the one that minimizes the repulsions among them.”

© 2009, Prentice-Hall, Inc. Electron-Domain Geometries These are the electron-domain geometries for two through six electron domains around a central atom.

© 2009, Prentice-Hall, Inc. Electron-Domain Geometries All one must do is count the number of electron domains in the Lewis structure. The geometry will be that which corresponds to the number of electron domains.

© 2009, Prentice-Hall, Inc. Molecular Geometries The electron-domain geometry is often not the shape of the molecule, however. The molecular geometry is that defined by the positions of only the atoms in the molecules, not the nonbonding pairs.

© 2009, Prentice-Hall, Inc. Molecular Geometries Within each electron domain, then, there might be more than one molecular geometry.

© 2009, Prentice-Hall, Inc. Linear Electron Domain In the linear domain, there is only one molecular geometry: linear. NOTE: If there are only two atoms in the molecule, the molecule will be linear no matter what the electron domain is.

© 2009, Prentice-Hall, Inc. Trigonal Planar Electron Domain There are two molecular geometries: – Trigonal planar, if all the electron domains are bonding, – Bent, if one of the domains is a nonbonding pair.

© 2009, Prentice-Hall, Inc. Nonbonding Pairs and Bond Angle Nonbonding pairs are physically larger than bonding pairs. Therefore, their repulsions are greater; this tends to decrease bond angles in a molecule.

© 2009, Prentice-Hall, Inc. Multiple Bonds and Bond Angles Double and triple bonds place greater electron density on one side of the central atom than do single bonds. Therefore, they also affect bond angles.

© 2009, Prentice-Hall, Inc. Tetrahedral Electron Domain There are three molecular geometries: – Tetrahedral, if all are bonding pairs, – Trigonal pyramidal if one is a nonbonding pair, – Bent if there are two nonbonding pairs.

© 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain There are two distinct positions in this geometry: – Axial – Equatorial

© 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain Lower-energy conformations result from having nonbonding electron pairs in equatorial, rather than axial, positions in this geometry.

© 2009, Prentice-Hall, Inc. Trigonal Bipyramidal Electron Domain There are four distinct molecular geometries in this domain: – Trigonal bipyramidal – Seesaw – T-shaped – Linear

© 2009, Prentice-Hall, Inc. Octahedral Electron Domain All positions are equivalent in the octahedral domain. There are three molecular geometries: – Octahedral – Square pyramidal – Square planar

© 2009, Prentice-Hall, Inc. Larger Molecules In larger molecules, it makes more sense to talk about the geometry about a particular atom rather than the geometry of the molecule as a whole.

© 2009, Prentice-Hall, Inc. Larger Molecules This approach makes sense, especially because larger molecules tend to react at a particular site in the molecule.

Sample Exercise 9.1 Using the VSEPR Model Use the VSEPR model to predict the molecular geometry of (a) O 3, (b) SnCl 3 –. Solution 1.Draw Lewis structure 2.Count the number of electron domains around the central atom (includes nonbonding pairs, single, double, and triple bonds 3.Arrange the domains of electrons as far away in 3-D space as possible (minimize repulsions). And determine molecular geometry.

Sample Exercise 9.1 Using the VSEPR Model Predict the electron-domain geometry and the molecular geometry for (a) SeCl 2, (b) CO 3 2–. Answer: (a) tetrahedral, bent; (b) trigonal planar, trigonal planar Practice Exercise Solution (continued) 1.Draw Lewis structure 2.Count the number of electron domains around the central atom (includes nonbonding pairs, single, double, and triple bonds 3.Arrange the domains of electrons as far away in 3-D space as possible (minimize repulsions). And determine molecular geometry.

Sample Exercise 9.2 Molecular Geometries of Molecules with Expanded Valance Shells Use the VSEPR model to predict the molecular geometry of (a) SF 4, (b) IF 5. Solution Seesaw shaped (distorted tetrahedron) Comment: The experimentally observed structure is shown on the right. We can infer that the nonbonding electron domain occupies an equatorial position, as predicted. The axial and equatorial S—F bonds are slightly bent back away from the nonbonding domain, suggesting that the bonding domains are “pushed” by the nonbonding domain, which is larger and has greater repulsion (Figure 9.7).

Sample Exercise 9.2 Molecular Geometries of Molecules with Expanded Valance Shells Predict the electron-domain geometry and molecular geometry of (a) ClF 3, (b) ICl 4 –. Answer: (a) trigonal bipyramidal, T-shaped; (b) octahedral, square planar Practice Exercise Comment: Because the domain for the nonbonding pair is larger than the other domains, the four F atoms in the base of the pyramid are tipped up slightly toward the F atom on top. Experimentally, we find that the angle between the base and top F atoms is 82°, smaller than the ideal 90°angle of an octahedron. Solution (continued) Square pyramidal (Table 9.3):

Sample Exercise 9.3 Predicting Bond Angles Predict the H—C—H and C—C—C bond angles in the following molecule, called propyne: Answer: 109.5°, 180° Practice Exercise

© 2009, Prentice-Hall, Inc. Polarity In Chapter 8 we discussed bond dipoles. But just because a molecule possesses polar bonds does not mean the molecule as a whole will be polar.

© 2009, Prentice-Hall, Inc. Polarity By adding the individual bond dipoles, one can determine the overall dipole moment for the molecule.

© 2009, Prentice-Hall, Inc. Polarity

Sample Exercise 9.4 Polarity of Molecules Predict whether the following molecules are polar or nonpolar: (a) BrCl, (b) SO 2, (c) SF 6. Solution

Sample Exercise 9.4 Polarity of Molecules Determine whether the following molecules are polar or nonpolar: (a) NF 3, (b) BCl 3. Answer: (a) polar because polar bonds are arranged in a trigonal-pyramidal geometry, (b) nonpolar because polar bonds are arranged in a trigonal-planar geometry Practice Exercise Solution (continued)