Which statement best describes molecules capable of forming hydrogen bonds?

• A. Any molecule that has a hydrogen atom attached directly to an oxygen or a nitrogen
• B. Any molecule that contains carbon and hydrogen
• C. Any polar molecule
• D. Any molecule that forms ionic bonds

Answer: (A)

Hydrogen bonding is a specific dipole-dipole interaction that happens when a hydrogen atom is covalently bonded to a highly electronegative atom, giving hydrogen a partial positive charge. That positively charged hydrogen can be attracted to lone pair electrons on another electronegative atom in a neighboring molecule. Because of this, a molecule must have a hydrogen attached directly to oxygen or nitrogen to act as a donor for hydrogen bonding. This is why substances like water and alcohols can form hydrogen bonds—they have O–H or N–H groups that provide the necessary hydrogen donor, and the accepting atom provides lone pairs. A molecule that is simply carbon-hydrogen is not sufficiently electronegative to create that strong donor site, and a polar molecule without an O–H or N–H bond may be polar but can’t form hydrogen bonds in the same way. Ionic bonds, on the other hand, involve full transfer of electrons and are not hydrogen bonds.