151chap9Guo%20%5bCompatibility%20Mode%5d

151chap9Guo%20%5bCompatibility%20Mode%5d - Molecular Shapes...

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Unformatted text preview: Molecular Shapes Molecular Shapes In order to predict molecular shape, we assume the valence electrons repel each other. Therefore, the molecule adopts whichever 3D geometry minimized this repulsion. We call this process Valence Shell Electron Pair call this process Repulsion (VSEPR) theory. The VSEPR Model – common Mol. Geo. Mol. Geo. ElectronElectron-Domain Geo. Geo. The VSEPR Model The VSEPR Model Predicting Molecular Geometries Predicting Molecular Geometries 1 The VSEPR Model The VSEPR Model Predicting Molecular Geometries Molecules with Expanded Valence Shells The VSEPR Model The VSEPR Model Molecules with Expanded Valence Shells The The VSEPR Model The VSEPR Model The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles By experiment, the H-X-H bond angle decreases on moving from C to N to O: H H CH H 109.5O HNH H 107O •We determine the electron domain geometry by looking at electrons around the central atom. •We name the molecular geometry by the positions of atoms. •We ignore lone pairs in the molecular geometry. O H H 104.5O Since electrons in a bond are attracted by two nuclei, they do not repel as much as lone pairs. Therefore, the bond angle decreases as the number of lone pairs increase. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles Similarly, electrons in multiple bonds repel more than electrons in single bonds. Cl 111.4o Cl CO 124.3o 2 The VSEPR Model Molecules with More than One Central Atom In acetic acid, CH3COOH, there are three central atoms. We assign the geometry about each central atom separately. Polarity of Molecules Dipole Moments of Polyatomic Molecules Polarity of Molecules Polar molecules interact with electric fields. If the centers of negative and positive charge do not coincide, then the molecule is polar. Polarity of Molecules Dipole Moments of Polyatomic Molecules Example: in CO2, each C-O dipole is canceled because the molecule is linear. In H2O, the H-O dipoles do not cancel because the molecule is bent. Covalent Bonding and Orbital Overlap Covalent Bonding and Orbital Overlap Lewis structures and VSEPR do not explain why a bond forms. How do we account for shape in terms of quantum mechanics? What are the orbitals that are involved in bonding? We use Valence Bond Theory: •Bonds form when orbitals on atoms overlap. •There are two electrons of opposite spin in the orbital overlap. 3 Covalent Bonding and Orbital Overlap Hybrid Orbitals sp Hybrid Orbitals Consider the BeF2 molecule (experimentally known to exist): •Be has a 1s22s2 electron configuration. •There is no unpaired electron available for bonding. •We conclude that the atomic orbitals are not adequate to describe orbitals in molecules. We know that the F-Be-F bond angle is 180° (VSEPR theory). We also know that one electron from Be is shared with each one of the unpaired electrons from F. FG09_013.JPG FG09_015.JPG Formation of sp2 Orbitals Orbitals Formation of sp Hybrid Orbital FG09_016.JPG FG09_017.JPG Formation of sp3 Orbitals Orbitals Bonding in H2O 4 Hybridization in Ethylene FG09_021.JPG TB09_005.JPG Table 9.4 p 366 Hybrid Orbitals Pi Bond Formation in Ethylene FG09_022.JPG Summary To assign hybridization: •draw a Lewis structure; •assign the electron pair geometry using VSEPR theory; •from the electron pair geometry, determine the hybridization; and •name the molecular geometry by the positions of the atoms. the molecular geometry by the positions of the atoms Triple Bond in Acetylene FG09_023.JPG FG09_028.JPG Bonding in Benzene 5 FG09_029.JPG Orbitals of Benzene Molecular Orbitals Some aspects of bonding are not explained by Lewis structures, VSEPR theory and hybridization. (E.g. why does O2 interact with a magnetic field?; Why are some molecules colored?) For these molecules, we use Molecular Orbital (MO) Theory. Just as electrons in atoms are found in atomic orbitals, electrons in molecules are found in molecular orbitals. Molecular orbitals: •each contain a maximum of two electrons; •have definite energies; •can be visualized with contour diagrams; •are associated with an entire molecule. Molecular Orbitals FG09_039.JPG MO MO Electron Configurations The Hydrogen Molecule 6 ...
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