Valence Bonding - The VSEPR model accounts for molecular...

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The VSEPR model accounts for molecular shapes but it does not explain how the shape arise from interactions of atomic orbitals (orbitals are not oriented in those shapes). In addition the shape does not explain the magnetic and spectral properties. For a more complete explanation of chemical bond formation we look to quantum mechanics. In fact, the quantum mechanical study of chemical bonding also provides a means for understanding molecular geometry. At present, two quantum mechanical theories are used to describe covalent bond formation and the electronic structure of molecules. Valence Bond theory Molecular Orbital theory Central themes of Valence Bond Theory 1) What is a covalent bond, and what characteristics give it strength? 2) And how we can explain the molecular shapes? VB theory explains covalent bond in H 2 as the overlap of two orbitals that share a common region in space. Lewis theory treats all covalent bonds the same but in polyatomic molecules VB theory facilitates a satisfactory bonding scheme that accounts for molecular geometry. Now we are going to try to answer the second question and we are going to start stating the VB theory principle: 1. Opposing spins of the electron pair (exclusion principle) 2. Maximum overlap of bonding orbitals (the greater overlap the stronger the bond) Remember the shape of the orbitals therefore the shape and direction is oriented to maximize overlap. 3. Hybridization of atomic orbitals Mathematically result from quantum mechanics that accounts for the molecular shape we observe . To explain this Linus Pauling proposed the valence atomic orbitals are different from those in the isolated atoms. The spatial orientation of new orbitals leads to more stable bonds consistent with the shapes of molecules. Number of hybrid orbitals formed = number of atomic orbitals mixed. Type of hybrid orbitals depends with the types of atomic orbitals mixed. Hybridization(atomic orbitals mix forming hybrid orbitals)→ e-enter with spins parallel (Hund’s rule) → bonds formed. Types of Hybrid Orbitals There are 5 common types of hybridization which corresponds with one of the 5 common e- group arrangements predicted by VSEPR theory. 1. sp Hybridization – Linear shape By mixing 1s and 1p orbital results in 2 sp hybrid orbitals. The orientation of hybrid orbitals extends e- density in the bonding direction and minimizes repulsions
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This note was uploaded on 04/20/2008 for the course CHEM 1300/ taught by Professor Ballester during the Fall '08 term at Nova Southeastern University.

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Valence Bonding - The VSEPR model accounts for molecular...

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