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Oct_24 - angles of 90° • The observed H – P – H bond...

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Chapter 7 Valence-Bond Theory (Localized Bonding Model) Goal: to be able to describe the bonding in organic molecules Molecular Orbital Theory (Delocalized bonding Model) Goal: to explain the conduction properties of metals, semiconductors, and insulators
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Localized Bonding Lewis theory gives us a pattern to apply to predict stable electron configurations in molecules It does NOT explain why they are stable, why electons are paired, why an octet is special This requires consideration of whats happening when a bond forms
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Valence-bond Method + H H H H F F + F F 2 orbitals + 2 electrons in those orbitals = covalent bond the orbitals must overlap in space with the same phase electrons pair up 1s F + F → F 2 F: 1s 2 2s 2 2p 5 Examples: H + H → H 2 H: 1s 1 2s 2p
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Example: PH 3 P: 3s 2 3p 3 3s 3p H: 1s 1 1s Three H – P bonds are formed by the overlap of half-filled P 3p orbitals with each of the half filled H 1s orbitals The the resulting bonds will have bond
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Unformatted text preview: angles of 90° • The observed H – P – H bond angle is 94° Valence Bond Theory - Promotion CH 4 would pose problems using the previous approach. Problem 1: C needs to make 4 bonds but has a 2s 2 2p 2 valence electron configuration Solution: excite the paired electrons to get 4 half-filled orbitals for bonding Can make 4 bonds but they will not all be the same (three 2p+1s overlaps and one 2s+1s overlap) ↑ ↓ ↑ ↑ 2s 2p ↑ ↑ ↑ ↑ 2s 2p Valence Bond Theory - Hybridization CH 4 is known to have four equivalent C – H bonds with tetrahedral bond angles. How can we explain this with valence bond theory? Algebraically combine 2 s and 2 p orbitals to give hybrid orbitals (sp 3 ) which have the correct geometry and are equivalent This is possible because we are treating the orbitals as waves. Combining the orbitals is equivalent to an interference pattern for waves sp 3 hybridization...
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