Chemical Bonding and Molecular Structure
Chemical Bonding and Molecular Structure tests Lewis structures, VSEPR geometry, valence-bond hybridization, molecular orbital theory, bond order, polarity, and intermolecular forces. You must predict shape from electron domains and then explain properties.
Core rules
- VSEPR counts a multiple bond as one electron domain; lone-pair repulsion is stronger than bond-pair repulsion.
- Common shapes: two domains linear; three trigonal planar; four tetrahedral; five trigonal bipyramidal; six octahedral. Lone pairs give bent, pyramidal, seesaw, T-shaped, or square-planar derivatives.
- Hybridization from domains: 2 → sp; 3 → sp2; 4 → sp3; 5 → sp3d; 6 → sp3d2.
- MO theory gives bond order = (bonding − antibonding)/2 and predicts magnetism.
- Polar bonds do not always give a polar molecule; vector cancellation matters.
Worked example
For O2, the MO filling is σ2s2 σ2s2 σ2p2 π2p4 π2p2. Bonding electrons = 8, antibonding = 4, so bond order = 2. Two electrons occupy the degenerate π* orbitals singly, so O2 has two unpaired electrons and is paramagnetic. This is a key success of MO theory over a simple Lewis structure.
Traps
Do not count lone pairs as geometry domains without considering repulsion order. Do not assume sp3 always means tetrahedral when lone pairs are present. In CO2, the two C=O dipoles cancel, so the molecule is nonpolar despite polar bonds. Do not confuse sigma and pi bond counts; a double bond has one sigma and one pi bond.