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reader2 - Interactions in Molecules Attractive forces...

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Interactions in Molecules Attractive forces between positive nuclei and electrons Electron-electron repulsion Minimized by maximizing spherical symmetry Repulsion between positive nu- clei Minimized by placing electrons between nuclei to screen charge Molecular Bonding Theories Two models that describe covalent bonding Valence-bond theory * Electrons are in localized bonds that arise from overlap of atomic orbitals Molecular orbital theory * Electrons occupy orbitals that are delocalized over entire molecule and made up of atomic orbitals Valence Bond Theory Valence bond theory gives a localized picture of bonding where chemical bonds are formed as a consequence of overlapping atomic orbitals
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Super simplistic view of VB theory: Unpaired electron in one atomic orbitals on different atoms com- bine to form bonding pair Hybridization It is pretty obvious that the picture does not reflect reality if we restrict ourselves to unmodified atomic orbitals The atomic orbitals are modified by the bonding To account for this we introduce hybridization Atomic orbitals that are involved in binding are mixed to form a symmetric set of new orbitals Bonding is overlap of (hy- bridized) orbitals sp Hybridization Mixing of s orbital and one of the p orbitals results in two identical sp orbitals The two p orbitals that do not contribute to bonding stay unchanged The overall geometry is linear 2
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s + p x + ( p y + p z ) = sp + ( p y + p z ) Example: H–Be–H Be ([He] 2 s 2 ) H( 1 s 1 ) sp 2 Hybridization Mixing of s orbital and two of the p orbitals results in three identical sp 2 orbitals One p orbital stays unchanged (normal to sp 2 orbitals) The overall geometry is trigonal planar s + p x + p y (+ p z ) = sp 2 (+ p z ) Example: BH 3 B ([He] 2 s 2 2 p 1 ) H( 1 s 1 ) sp 3 Hybridization Mixing of s orbital and the three p orbitals results in four identical sp 3 orbitals The overall geometry is tetrahedral 3
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s + p x + p y + p z = sp 3 Historically molecules with expanded octets has been described in VB theory by taking into account the involvement of d orbitals. So molecules with bipyramidal electronic structure like PF 5 and I 3 has been considered having sp 3 d hybridiza- tion, and molecules with octahedral electronic structure like XeF 4 and SF 6 has been considered to be the result of sp 3 d 2 hybridization. However, detailed calcu- lations (and experiments) have shown that the d orbital involvement is negligible, mostly because of the very different sizes of the s / p orbitals and the d orbitals. Despite this knowledge many general textbooks (including Petrucci) and general chemistry classes (including Chem 2A) because it gives an easy (albeit incorrect!) picture of these multivalent molecules.
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