Chapter 9 Skieletons

Chapter 9 Skieletons - Chapter 9 Molecular Geometry and...

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Chapter 9. Molecular Geometry and Bonding Theories Lecture Outline 9.1 Molecular Shapes Lewis structures give atomic connectivity: they tell us which atoms are physically connected to which atoms. The shape of a molecule is determined by its bond angles . The angles made by the lines joining the nuclei of the atoms in a molecule are the bond angles. Consider CCl 4 : Experimentally we find all Cl C Cl bond angles are 109.5 . Therefore, the molecule cannot be planar. All Cl atoms are located at the vertices of a tetrahedron with the C at its center. In order to predict molecular shape, we assume that the valence electrons repel each other. Therefore, the molecule adopts the three-dimensional geometry that minimizes this repulsion. We call this model the V alence S hell E lectron P air R epulsion ( VSEPR ) model. 9.2 The VSEPR Model A covalent bond forms between two atoms when a pair of electrons occupies the space between the atoms. This is a bonding pair of electrons. Such a region is an electron domain . A nonbonding pair or lone pair of electrons defines an electron domain located principally on one atom. Example: NH 3 has three bonding pairs and one lone pair. VSEPR predicts that the best arrangement of electron domains is the one that minimizes the repulsions among them. The arrangement of electron domains about the central atom of an AB n molecule is its electron- domain geometry . There are five different electron-domain geometries: linear (two electron domains), trigonal planar (three domains), tetrahedral (four domains), trigonal bipyramidal (five domains) and octahedral (six domains). The molecular geometry is the arrangement of the atoms in space. To determine the shape of a molecule we must distinguish between lone pairs and bonding pairs. We use the electron-domain geometry to help us predict the molecular geometry. Draw the Lewis structure. Count the total number of electron domains around the central atom. Arrange the electron domains in one of the above geometries to minimize electron-electron repulsion. Next, determine the three-dimensional structure of the molecule. We ignore lone pairs in the molecular geometry. Describe the molecular geometry in terms of the angular arrangement of the bonded atoms. Multiple bonds are counted as one electron domain. The Effect of Nonbonding Electrons and Multiple Bonds on Bond Angles We refine VSEPR to predict and explain slight distortions from “ideal” geometries. Consider three molecules with tetrahedral electron domain geometries: CH 4 , NH 3 , and H 2 O.
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Molecular Geometry and Bonding Theories 103 By experiment, the H X H bond angle decreases from C (109.5 in CH 4 ) to N (107 in NH 3 ) to O (104.5 in H 2 O).
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