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Chapter 9 Molecular Geometry and Bonding Theories
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Trigonal Bipyramidal Electron Domain –Trigonal bipyramidal –Seesaw –T-shaped –Linear Table 9.3
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Shapes of Larger Molecules Consider the geometry about a particular atom rather than the geometry of the molecule as a whole
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Larger molecules tend to react at a particular site in the molecule Called a functional group Shapes of Larger Molecules acetic acid
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Molecular Shape and Molecular Polarity A molecule possessing polar bonds does not imply that the molecule as a whole will be polar Fig 9.11 CO 2, a nonpolar molecule
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To determine the overall dipole moment for the molecule, add the individual bond dipoles vectorially Molecular Shape and Molecular Polarity Fig 9.12
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Molecular Shape and Molecular Polarity Fig 9.13 Molecules containing polar bonds Polar Nonpolar Polar
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Valence bond theory – bonds are formed by sharing of e − from overlapping atomic orbitals (AOs) Overlap of:2 1s orbitals How does Lewis theory explain the bonds in H 2 and HCl? “Sharing of two electrons between the two atoms ” Covalent Bonding and Orbital Overlap 1s orbital and 3p orbital
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Fig 9.15 Formation of the H 2 molecule 74 pm
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Hybrid Orbitals VSEPR theory allows prediction of molecular shapes How can tetrahedral, trigonal bipyramidal, and other geometries arising from the atomic orbitals we recognize?
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Hybridization – mixing of two or more atomic orbitals to form a new set of hybrid orbitals. 1.Mix at least 2 nonequivalent atomic orbitals (e.g. s and p). Hybrid orbitals have very different shape from original atomic orbitals. 2.Number of hybrid orbitals = number of pure atomic orbitals used in the hybridization process. 3.Covalent bonds are formed by: a)Overlap of hybrid orbitals with atomic orbitals b)Overlap of hybrid orbitals with other hybrid orbitals
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sp Hybrid Orbitals F F Be VSEPR predicts: Linear, 180° Be Assume Be absorbs the small amount of energy needed to promote an electron from the 2s to the 2p orbital: it can form two bonds.
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sp Hybrid Orbitals F F Be VSEPR predicts: Linear, 180° Mixing the s and p orbitals yields two degenerate orbitals that are hybrids of the two orbitals: –These sp hybrid orbitals have two lobes like a p orbital. –One of the lobes is larger and more rounded as is the s orbital. Fig 9.16 formation of sp hybrid orbitals
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These two degenerate orbitals would align themselves 180 from each other This is consistent with the observed geometry of beryllium compounds: linear sp Hybrid Orbitals Fig 9.17 Formation of two equivalent Be-F bonds in BeF 2
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Using a similar model for boron leads to… sp 2 Hybrid Orbitals Fig 9.18
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With carbon we get… sp 3 Hybrid Orbitals Fig 9.19
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For geometries involving expanded octets on the central atom, we must use d orbitals in our hybrids : Hybridization Involving d Orbitals
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This leads to five degenerate sp 3 d orbitals… …or six degenerate sp 3 d 2 orbitals. Hybridization Involving d Orbitals
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# of Lone Pairs + # of Bonded Atoms HybridizationExamples 2 3 4 5 6 sp sp 2 sp 3 sp 3 d sp 3 d 2 BeCl 2 BF 3 CH 4, NH 3, H 2 O PCl 5 SF 6 How do I predict the hybridization of the central atom? Count the number of lone pairs AND the number of atoms bonded to the central atom
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Sigma () Bonds Characterized by: Head-to-head overlap Single bonds are always bonds Fig 9.14
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Pi () Bonds Pi bonds are characterized by: Side-to-side overlap In a multiple bond: one of the bonds is a bond and the rest are bonds Fig 9.22
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Multiple bonds in ethylene Fig 9.23 Molecular geometry of ethylene Fig 9.24 The σ bonds in ethylene sp 2
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Multiple Bonds
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The π bond in ethylene Fig 9.25
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