Which intermolecular force is typically strongest when hydrogen is bonded to N, O, or F?

• A. Hydrogen bonding
• B. Dipole-dipole
• C. London dispersion
• D. Ionic

Answer: (A)

When hydrogen is bonded to nitrogen, oxygen, or fluorine, the strongest intermolecular force that can form between molecules is hydrogen bonding. This happens because the H–X bond is highly polar, giving hydrogen a substantial positive partial charge while the lone pairs on a nearby electronegative atom create a strong electrostatic attraction. The small size of hydrogen and the high electronegativity of N, O, or F allow a very close, nearly linear arrangement, producing a particularly strong and directional interaction. This makes hydrogen bonds stronger than ordinary dipole–dipole interactions or London dispersion forces, which rely on weaker or more transient charge effects. Ionic forces, on the other hand, arise from full charges on ions and aren’t present in neutral H–N, H–O, or H–F molecules. Water, for example, exhibits many hydrogen bonds between molecules, which explains its high boiling point and unique properties, and HF/NH3 exhibit hydrogen bonding as well, though to different extents.