reader2 - Interactions in Molecules • Attractive forces...

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Unformatted text preview: 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 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 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 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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This note was uploaded on 12/04/2009 for the course CHE 15940 taught by Professor Madsen during the Spring '09 term at UC Davis.

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reader2 - Interactions in Molecules • Attractive forces...

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