Structure 2 - Models of bonding and structure
Structure 2 connects bonding models to observable properties. The exam tests ionic, covalent and metallic models, VSEPR shapes, polarity, intermolecular forces and how structure determines melting point, solubility and conductivity.
Bonding models
- Ionic: electron transfer gives a lattice of ions held by electrostatic attraction; high melting point, conducts when molten or aqueous.
- Covalent: a shared pair of electrons; dative covalent bonds donate a lone pair. Bond polarity comes from electronegativity difference.
- Metallic: cations in a sea of delocalised electrons; explains conductivity and malleability.
Shape, polarity and forces
- VSEPR counts electron domains; repulsion order is lone pair–lone pair > lone pair–bond pair > bond pair–bond pair. Two domains are linear, three trigonal planar and four tetrahedral; lone pairs give bent or trigonal pyramidal shapes.
- Polarity is the vector sum of bond dipoles; symmetry can cancel them.
- London dispersion acts in all molecules and strengthens with more electrons and greater surface area. Dipole–dipole acts in polar molecules. Hydrogen bonding requires H attached to N, O or F.
Worked example
Compare NH₃ and BF₃. NH₃ has four domains, three bonds and one lone pair, so it is trigonal pyramidal; its dipoles do not cancel, so it is polar and hydrogen bonds. BF₃ has three domains and no lone pair, so it is trigonal planar; identical B–F dipoles cancel and it is nonpolar.
Traps
- Assuming any polar bond makes a polar molecule.
- Calling hydrogen bonds "strong covalent bonds".
- Forgetting that lone pairs compress bond angles, for example 104.5° in water.