: 220
Intermolecular forces (in increasing strength): London dispersion < dipole-dipole < hydrogen bonding < ion-dipole. London forces are present in ALL molecules and increase with molecular size and polarisability. Dipole-dipole forces exist only in polar molecules. Hydrogen bonding requires H bonded to F, O, or N (small, highly electronegative atoms with lone pairs). H-bond strength: F-H...F > O-H...O > N-H...N. Water has an anomalously high boiling point because each molecule can form 4 H-bonds (2 donors + 2 acceptors), creating a 3D tetrahedral network. HF forms linear chains (1 donor, 3 acceptors but only 1 effective). Intramolecular H-bonding (within a molecule, e.g., o-nitrophenol) reduces boiling point compared to intermolecular H-bonding (p-nitrophenol). n-Pentane boils higher than neopentane despite identical formula because its elongated shape provides more surface contact for London forces. For JEE: when comparing boiling points, first check for H-bonding, then dipole-dipole, then London forces. Among similar molecules (same force type), larger molecular weight generally means higher boiling point.