10229_n_23391

10229_n_23391 - Lecture 17 Valence bond theory involves...

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Lecture 17 Valence bond theory involves combining orbitals to generate new sets of orbitals. + 2 sp orbitals s orbital p orbital + + sp __ __ __ 2p __ 2s hybridize  __ __ sp __ __ 2p The energies of these hybrid orbitals are the average of the orbitals that are mixed. Note that these hybrid atomic orbitals (sp, sp 2 , sp 3 ) are formed from atomic orbitals all on the same atom. Molecular Orbital Theory The basis of molecular orbital theory is that the atomic orbitals on different atoms can be combined to form molecular orbitals (MO’s), which cover the entire molecule . H 2 H-H H H
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+ + + + Atomic Orbitals 1s 1s Molecular Orbital + - + - 1s 1s node If one combines two orbitals with the same phase, one get a favorable interaction or bonding orbital . If one combines two orbitals with different phases, one gets an unfavorable interaction or antibonding orbital . There is also an energy associated with each molecular orbital. A bonding orbital is lower in energy, while an antibonding orbital is higher in energy. + + + 1s + + - 1s σ 1 s σ 1 s Energy When an orbital is formed from the direct interaction of two orbitals, it is called a sigma ( ) orbital. Two important points 1) The average energy of the bonding and antibonding MO’s must be the same as the average of the atomic orbitals they are formed from. 2) The number bonding and antibonding orbitals equals the number of atomic orbitals.
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Two 1s orbitals --> one bonding + one antibonding 1s 1s σ 1 s σ 1 s Energy E Bond Energy = 2 E The energy of the bond can be understood in terms of the drop in energy of electrons going from atomic orbitals to molecular orbitals. So MO picture can help you to understand the stability of a bond. One can also form MO’s from p atomic orbitals. Example N 2 N N There are two types of interactions for p-orbitals. One where the p orbitals are directed at one another (sigma bond). + 2p
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10229_n_23391 - Lecture 17 Valence bond theory involves...

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