Chemical Bonding and Molecular Shape
Bond types
Ionic bonding transfers electrons between a metal and a non-metal, producing a lattice held by electrostatic attraction; melting points are high and molten or aqueous forms conduct. Covalent bonding shares electron pairs, giving discrete molecules or giant covalent networks. Metallic bonding is a lattice of cations in a delocalised electron sea, explaining conductivity and malleability.
Polarity and shape
Bond polarity depends on electronegativity difference. Molecular polarity depends on both bond dipoles and geometry: a molecule with polar bonds can be non-polar if dipoles cancel. Use VSEPR to predict shape from electron-pair repulsion, counting lone pairs as regions of electron density.
| Regions | Lone pairs | Shape | Angle |
|---|---|---|---|
| 2 | 0 | linear | 180° |
| 3 | 0 | trigonal planar | 120° |
| 4 | 0 | tetrahedral | 109.5° |
| 4 | 1 | trigonal pyramidal | ~107° |
| 4 | 2 | bent | ~104.5° |
Intermolecular forces
Dispersion forces act between all molecules and strengthen with electron count. Dipole–dipole forces require permanent dipoles. Hydrogen bonding requires H bonded to N, O or F, and explains water's anomalously high boiling point and ice's low density. Intermolecular forces are much weaker than covalent bonds, so melting or boiling breaks the former, not the latter.
Traps
- Saying ionic compounds conduct as solids.
- Claiming a molecule with polar bonds must be polar (CO₂ is linear and non-polar).
- Forgetting lone pairs reduce bond angles.
- Confusing intramolecular bonds broken during boiling with intermolecular forces.
Rule of thumb
Predict shape first, then add dipoles as vectors. Symmetry cancels dipoles.
Worked example
Water has four electron regions (two bonding, two lone pairs), giving a bent shape near 104.5°. The O–H dipoles do not cancel, so water is polar and hydrogen bonds, raising its boiling point well above that of hydrogen sulfide.