Readout
CH₄Methane
Molar mass16.04 g/molBoiling pt−161.5 °CMelting pt−182.5 °CDipole moment0 D (nonpolar)ΔHvap8.19 kJ/molIn water2.2 mg/100 mL Four polar C-H bonds arranged tetrahedrally, so the bond dipoles cancel exactly. The molecule is nonpolar.
What to notice
Cohesion is a competition between thermal energy and intermolecular attraction. Below the boiling point the attraction wins and the sample condenses.
Forces at work
LDFLondon dispersion0.05–40 kJ/mol
Electrons in any molecule are constantly redistributing. A momentary imbalance creates an instantaneous dipole, which induces a matching dipole in a neighbour. Strength scales with polarisability, so it climbs with electron count and with how much surface two molecules can lie along.
Present in every substance, including the polar and hydrogen bonded ones.
D-DDipole-dipole5–25 kJ/mol
A permanent dipole needs polar bonds that do not cancel by symmetry. Molecules line up delta-plus to delta-minus, which costs energy to pull apart.
CH₄ and CO₂ have polar bonds and no dipole. Check the geometry, not just the bonds.
H-BHydrogen bonding10–40 kJ/mol
A special case of dipole-dipole. An H covalently bonded to N, O or F is left almost bare of electron density and is drawn into a lone pair on the N, O or F of a neighbour.
Both halves are required. An O with lone pairs can accept, but with no O-H it cannot donate.
I-DIon-dipole40–600 kJ/mol
A full ionic charge attracts the partial charge on a polar molecule. Water turns its oxygen toward cations and its hydrogens toward anions, building a hydration shell around each ion.
This is why ionic solids dissolve. It is not what holds the dry crystal together.
Energy scale
0.11101001k
London dispersion0.05–40
Dipole-dipole5–25
Hydrogen bonding10–40
Ion-dipole40–600
Covalent bond150–1100
Ionic lattice600–4000
kJ/mol, log scale. The ranges overlap on purpose: dispersion in a large molecule beats hydrogen bonding in a small one, which is why Br₂ is a liquid at room temperature and HF is not far off. Bonds are shown for scale and are an order of magnitude above every force here.