Chemical bonding and structure
Chemical bonding and structure explains why substances have particular melting points, solubilities and electrical behaviour. The exam expects you to build Lewis structures, apply VSEPR to predict shapes, and link intermolecular forces to physical properties.
Bond types and models
- Ionic bonding: electrostatic attraction between oppositely charged ions in a giant lattice. High melting points; conduct only when molten or aqueous because ions must be mobile.
- Covalent bonding: a shared pair of electrons; a dative covalent bond forms when one atom supplies both electrons. Simple molecular substances have weak intermolecular forces, so low melting points.
- Metallic bonding: delocalised electrons attracted to a lattice of cations. Explains conductivity, malleability and generally high melting points.
- Giant covalent structures (diamond, silicon dioxide) have many strong covalent bonds, giving very high melting points; graphite also conducts along its layers because of delocalised electrons.
VSEPR and polarity
Electron domains repel; lone pairs repel more strongly than bonding pairs. Two domains: linear; three: trigonal planar (one lone pair gives bent); four: tetrahedral (one lone pair gives trigonal pyramidal, two give bent); five: trigonal bipyramidal; six: octahedral. A molecule is polar only if its bond dipoles do not cancel by symmetry; lone pairs contribute.
Intermolecular forces
London dispersion forces arise from temporary dipoles and increase with electron number and contact area. Permanent dipole-dipole forces occur between polar molecules. Hydrogen bonds occur only when H is bonded to N, O or F and interacts with a lone pair on N, O or F; they are much stronger than ordinary dipole forces.
Worked example
Predict the shape and polarity of NH3. Nitrogen has five valence electrons; three form N-H bonds and one remains as a lone pair, giving four electron domains. The electron geometry is tetrahedral, so the molecular shape is trigonal pyramidal. The N-H dipoles point towards N, and the lone pair reinforces the net dipole, so NH3 is polar and forms hydrogen bonds.
Traps that cost marks
- Do not count lone pairs as atoms when naming a shape.
- CO2 has polar bonds but is non-polar because the dipoles cancel; H2O is bent and polar.
- Hydrogen bonding requires an H attached to N, O or F; HCl has dipole-dipole forces, not hydrogen bonds.
- Do not give expanded octets to period 2 atoms; they have only four orbitals available.
- Ionic solids do not conduct electricity when solid, because the ions are fixed in the lattice.