1. Rules for Writing & Evaluating Resonance Structures1
Rule #1: No first-row element (C, N, O, etc.) can have > 8 electrons in its valence shell. H can only share 2 valence electrons.
Rule #2: Resonance structures in which all atoms have a full octet (i.e. share 8 valence electrons) are lower in energy than those in which atoms are electron-deficient.
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Rule #3: If there are electron deficiencies, these should preferably be on electropositive elements (e.g. B, C) not electronegative elements (e.g. O, N). Other factors that stabilize positive charge also help.
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Rule #4: Resonance structures with charge separation are higher in energy than those where charge is neutralized.
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Rule #5: If charge is separated, then electronegative elements should preferably have the negative charge, and electropositive elements should have the positive charge.
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Chemistry 335 Supplemental Slides: Chapter 1

2. Hybridization & Frontier Molecular Orbital Theory2
Chemistry 335 Supplemental Slides: Chapter 1

3. Hybridization & Frontier Molecular Orbital Theory3
Chemistry 335 Supplemental Slides: Chapter 1

4. Hybridization & Frontier Molecular Orbital Theory4
Chemistry 335 Supplemental Slides: Chapter 1

5. Hybridization & Frontier Molecular Orbital Theory5
We can use these hybrid orbitals to explain bonding, shape, and lots of other properties!
Chemistry 335 Supplemental Slides: Chapter 1

6. Hybridization & Frontier Molecular Orbital Theory6
To create an orbital picture for ethane, we first combine an sp3-hybridized carbon atom with three hydrogens, to make a methyl radical...
Chemistry 335 Supplemental Slides: Chapter 1

7. Hybridization & Frontier Molecular Orbital Theory7
... and then we combine two methyl radicals to generate a not-totally-accurate (but still very useful!) model of the electronic structure of ethane.
Chemistry 335 Supplemental Slides: Chapter 1

8. Hybridization & Frontier Molecular Orbital Theory8
To create an analogous picture for ethene (ethylene), we combine an sp2-hybridized carbon atom with two hydrogens...
Chemistry 335 Supplemental Slides: Chapter 1

9. Hybridization & Frontier Molecular Orbital Theory9
... and then combine two fragments to generate a not-totally-accurate (but still very useful!) model of the electronic structure of ethene.
Chemistry 335 Supplemental Slides: Chapter 1

10. Molecular Orbitals for Benzene10
Chemistry 335 Supplemental Slides: Chapter 1