Chemical Bonding and Molecular Structure
Domain 3: Chemical Bonding and Molecular Structure
This domain tests how Lewis structures, VSEPR, valence-bond hybridization, and molecular orbital theory predict shape, polarity, bond order, and magnetic behaviour.
Core rules
- VSEPR counts sigma bonds plus lone pairs on the central atom. Repulsion order: lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Lone pairs occupy equatorial sites in trigonal bipyramidal (SF4 is see-saw) and one axial site in octahedral (BrF5 square pyramidal).
- Hybridization: steric number 2 -> sp, 3 -> sp2, 4 -> sp3; 5 -> sp3d; 6 -> sp3d2. Multiple bonds count as one electron domain.
- Molecular orbital theory: bond order = (bonding − antibonding)/2. O2 has bond order 2 and two unpaired π* electrons, so it is paramagnetic.
- Polarity depends on vector cancellation, not just bond dipoles; CO2 is nonpolar, H2O polar. Hydrogen bonding requires H attached to N, O, or F.
- Formal charge helps choose the best Lewis structure; expanded octets are possible only from period 3 onward.
Worked example
NH3: N has 5 valence electrons; three N–H bonds use three and one lone pair remains. Steric number = 4, so sp3 hybridised with one lone pair; electron geometry tetrahedral, molecular shape trigonal pyramidal, bond angle about 107°, polar. BF3: B has three bonds and no lone pair; steric number 3, sp2, trigonal planar, nonpolar because dipoles cancel.
Traps
- Predicting shape from electron geometry without removing lone-pair positions.
- Assuming all sp3 centres are tetrahedral; lone pairs distort angles.
- Treating O2 as diamagnetic from a simple Lewis structure.
- Using d orbitals in period 2 hybrids.
- Forgetting that CO2 has polar bonds but zero net dipole.