What is a hydrogen bond?

• A. A strong covalent bond within a molecule
• B. A weak bond between a hydrogen atom on one molecule and an electronegative atom on another
• C. Ionic bond between metal and nonmetal
• D. Hydrogen-nucleophile interaction as a catalyst

Answer: (B)

Hydrogen bonds are weak attractions that form when a hydrogen atom, already covalently bonded to a strongly electronegative atom (like oxygen, nitrogen, or fluorine), experiences an attractive interaction with a lone pair on a second electronegative atom in another molecule. The electronegative partner pulls electron density away from hydrogen, giving it a partial positive charge, which is then drawn to the lone pair on the other atom. This creates a dipole-dipole type interaction that is much weaker than covalent or ionic bonds, but strong enough to influence properties like water's cohesion, surface tension, and the structure of large biomolecules such as DNA and proteins. The description matches a weak bond between a hydrogen on one molecule and an electronegative atom on another, rather than a covalent, ionic, or catalytic interaction.