7. Lewis Structures: model to describe
bonding in covalent molecules
- Valence electrons represented by dots
- Bonding pair represented by line
- Valence e- so 2 for H and 8 for 2nd row elements
8. Formal Charge: way of keeping track* of
where charge is within a molecule
Electrons that contribute to atom’s charge:
1. All of unshared electrons
2. Half of shared electrons
FC = group number – nonbonding e- – ½ shared e-
10. Resonance “rules of thumb”
• Must be valid Lewis structure
• Move electrons, not nuclei
• Number of unpaired electrons (if any) remain
the same
• Major contributor has lowest energy (see
other rules)
• Resonance – most important when charge is
delocalized
11. Comparing Resonance Structures
• As many octets as possible
• As many bonds as possible
• Any negative charges on electronegative
atoms
• As little charge separation as possible