s20 2011 week 1 day 2

s20 2011 week 1 day 2 - Chemistry S-20ab Week 1 Chemistry...

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Molecular Orbitals: H 2 and He 2 We originally showed that the overlap of atomic orbitals forms a single covalent bond. However, the mathematics of quantum mechanics requires that the number of orbitals must be conserved. Thus: when two orbitals are combined, two new combinations must result. How does this work in forming H 2 ? This molecular orbital treatment can explain why H 2 exists but He 2 does not. We can define the bond order of a species as: Bond order = 1 2 (electrons in bonding orbitals – electrons in antibonding orbitals) Reading : Section 1.8 Chemistry S-20ab Week 1 23
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Phases and Overlap of Orbitals Why do both bonding and antibonding orbitals form from the combination of atomic orbitals? What is an antibonding orbital, anyway? Reading : Section 1.8 Chemistry S-20ab Week 1 24
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Forming ! -bonds From Hybridized Orbitals Construct an MO diagram for the C–H ! -bond in methane (CH 4 ). Construct an MO diagram for the C–C ! -bond in ethane (C 2 H 6 ). Reading : Supplemental Handout, Section 2.2 Chemistry S-20ab Week 1 25
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Forming ! -bonds From “Leftover” p -Orbitals Construct an MO diagram for the C=C ! -bond in ethene (H 2 C=CH 2 ). Draw a complete MO diagram for all the bonds in ethene. What can we say, at this point, about the relative energy levels of the orbitals in this molecule? Reading : Supplemental Handout, Section 2.2 Chemistry S-20ab Week 1 26
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Energies of Atomic Orbitals The following chart shows the approximate energies of orbitals in several of the second- period elements. Can we make any generalizations about the energies of orbitals based on these observations? Approximate energies (in kcal mol –1 ) of atomic orbitals C N O F 2s 2p 2s 2p 2s 2p 2s 2p 0 100 200 300 400 500 600 700 800 900 1000 1100 Ne 2s 2p 0 100 200 300 400 500 600 700 800 900 1000 1100 How would the charge on an atom affect the energies of its orbitals? To summarize:
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s20 2011 week 1 day 2 - Chemistry S-20ab Week 1 Chemistry...

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