che106_Lecture20

che106_Lecture20 - Chemistry 106 Lecture 20 Topics...

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Chemistry 106 Lecture 20 Topics: Molecular Shapes VESPR Model Text Reading: 9.1-9.4
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Bond Theory In this chapter we will discuss the geometries of molecules in terms of their electronic structure. We will also explore two theories of chemical bonding: valence bond theory and molecular orbital theory . Molecular geometry is the general shape of a molecule, as determined by the relative positions of the atomic nuclei.
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Molecular Shape The shape of molecules is critical to their function. Shape is important in both small (e.g. Vitamin C) and large (e.g. proteins) molecules. C2952H4664N812O832S8Fe4 Hemoglobin C6H8O6 Vitamin C
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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.
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Molecular Models of BF3 and PF3 BF3 and PF3 have the same atom count, but why different shapes? According to the Lewis structures, BF3 does not have a lone pair on the central atom, PF3 does have a lone pair on the central atom.
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What Determines the Shape of a Molecule? 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.
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Electron Domains (Regions) We can refer to the electron pairs as electron domains or regions . 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.
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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.
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The Valence-Shell Electron Pair Repulsion (VSEPR) Model The valence-shell electron pair repulsion (VSEPR) model predicts the shapes of molecules and ions by assuming that the valence shell electron pairs are arranged as far from one another as possible . To predict the relative positions of atoms around a given atom using the VSEPR model, you first note the arrangement of the electron pairs around that central atom.
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Predicting Molecular Geometry The following rules and figures will help determine electron pair arrangements.
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