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Chemistry 121 Chapter 9 MOLECULAR GEOMETRY AND BONDING THEORIES 9.1 Molecular Shapes Lewis structures give atomic connectivity and tell us which atoms are physically connected to which. The shape of a molecule is determined by its bond angles. Consider CCl 4 : o It is experimentally determined that all four angles are equal with a value of 109.5 o o The molecule can therefore not be planar. o All Cl atoms are located at the vertices of a tetrahedron with the C at the center. To predict molecular shape, we assume that the valence electrons repel one another. o The molecule therefore adopts whichever 3D geometry minimizes this repulsion. o We call this model the v alence s hell e lectron 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. o This is a bonding pair of electrons. o Such a region is an electron domain . A nonbonding pair or lone pair of electrons defines an electron domain located principally on one atoms Example: Ammonia, 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. o The arrangement of electron domains about the central atom of an AB n molecule is its electron-domain geometry. o There are five electron-domain geometries: 1 Study material for this chapter: Textbook sections 9.1 through 9.8
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To determine the shape of a molecule we distinguish between nonbonding pairs and bonding pairs of electrons. We use the electron domain geometry to help us predict the molecular geometry o Draw the Lewis structure. o Count the number of electron pairs around the central atom. o Arrange the electron pairs in one of the above geometries to minimize electron-electron repulsion. Next, determine the 3D structure of the molecule: o Ignore lone pairs in the molecular geometry. o Describe the molecular geometry in terms of the angular arrangement of the bonded atoms. o Multiple bonds count as one electron domain. The Effects 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. o By experiment , the H-X-H bond angle decreases from C (109.5 o in CH 4 ), to N(107 o in NH 3 ) to O (104.5 o in H 2 O). o A bonding pair of electrons is attracted by two nuclei. They do not repel as much as nonbonding pairs, which are primarily attracted by only one nucleus. o Electron domains for nonbonding electron pairs thus exert greater repulsive forces on adjacent electron domains. They tend to compress the bond angles.
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121lec9 - <?xml version="1.0"...

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