Structural Theory – Structure Determines Properties

1. Structural Theory – Structure Determines Properties • Structure = how atoms are bonded together in a molecule • Bonding Theories – from simplest to most complex • Lewis Theory • Valence Bond Theory • Orbital Hybridization • Resonance • Molecular Orbital Theory

2. Lewis Theory of Bonding • Only the valence electrons are involved in bonding – represent these with “dots” • Atoms participate in bonding so as to achieve a noble gas electron configuration (a complete valence shell) • Octet for 2nd period and below, duet for hydrogen • B and Al often settle for six • Period 3 and lower can exceed octet • Covalent Bond = shared electron pair

3. Figure 1.2 A periodic table of the common elements seen in organic chemistry

4. So let’s write some Lewis Structures CCl4 NH3 H2O CO2 CH3Br CH4O CO2 PI3 N2H4 H2CO3 C3H8 C3H6

5. Figure 1.3 Summary: The usual number of bonds of common neutral atoms

6. Formal Charge • Formal Charge can be assigned to each atom in a Lewis structure. It is the charge the atom would have if its bonding electrons were being shared even steven. • FC = the # of e-s needed to balance nuclear charge minus the # of e-s owned by the atom • FC = group # - # nonbonding e-s – ½# bonding e-s