Chemical Bonding and Intermolecular Forces
Bonding questions test whether you can move from a Lewis structure to a three-dimensional shape, decide polarity, then use intermolecular forces to explain a physical property. Molecular orbital theory appears mainly for diatomics and bond order.
VSEPR in practice
Count electron domains on the central atom, with a lone pair counting as one. Two domains give linear, three trigonal planar, four tetrahedral, five trigonal bipyramidal, six octahedral. Repulsion order lp-lp > lp-bp > bp-bp compresses angles: CH4 109.5°, NH3 107°, H2O 104.5°. In five-coordinate species lone pairs always occupy equatorial sites, so SF4 is see-saw and ClF3 is T-shaped; in six-coordinate species two lone pairs sit trans, making XeF4 square planar.
Polarity and molecular orbitals
A molecule is polar only if the vector sum of bond dipoles and lone-pair dipoles is non-zero. CO2 and XeF4 are nonpolar despite polar bonds; H2O and SF4 are polar. Hybridisation follows the domain count (sp, sp2, sp3, sp3d, sp3d2). Bond order = (bonding - antibonding)/2. O2 has bond order 2 and two unpaired pi-star electrons, hence paramagnetism; N2 has bond order 3; Be2 has bond order 0 and does not exist.
Intermolecular forces
Strength order: ion-dipole > hydrogen bond > dipole-dipole > London dispersion. Hydrogen bonding requires H bonded directly to N, O or F. Dispersion strengthens with polarisability and surface contact, which is why I2 is a solid while Cl2 is a gas, and why n-pentane boils at 36 °C against 10 °C for neopentane at the same molar mass.
Worked example
Predict the shape and polarity of XeF4. Xenon supplies eight valence electrons; four form Xe-F bonds and two remain as lone pairs. Six domains give an octahedral electron geometry, hence a square planar molecular geometry. The four Xe-F dipoles cancel within the plane and the two lone pairs are trans, so the molecule is nonpolar and its only intermolecular force is dispersion.
Traps
Placing lone pairs axial in five-coordinate species; assuming polar bonds guarantee a polar molecule, or the reverse; invoking hydrogen bonding for HCl or H2S; and explaining boiling points by molar mass when the real cause is a change in the dominant intermolecular force.