Hydrogen bonds form when H is bonded to F, O, or N (small, highly electronegative atoms with lone pairs). Intermolecular H-bonds increase boiling point: H2O (100 C) vs H2S (−60 C), HF (19.5 C) vs HCl (−85 C). Intramolecular H-bonds (within the same molecule) reduce boiling point compared to the para-isomer because they reduce intermolecular association. Example: o-nitrophenol has intramolecular H-bonding (OH to nearby NO2), so it boils lower and is steam-volatile; p-nitrophenol has intermolecular H-bonding, so it has a higher boiling point. H-bond strength order: F-H...F > O-H...O > N-H...N. In proteins, H-bonds between N-H and C=O stabilise alpha-helices and beta-sheets.