Question

if a solid line represents a covalent bond and a dotted line represents intermolecular attraction, which of the choices shows a hydrogen bond?
□ $\ce{h4c}$......$\ce{h-f}$
□ $\ce{h-h}$
□ $\ce{h3n}$......$\ce{h-o-h}$
□ $\ce{h2o}$......$\ce{h-ch3}$

Explanation:

A hydrogen bond requires a hydrogen atom covalently bonded to a highly electronegative atom (N, O, F) and attracted to another electronegative atom (N, O, F) in a different molecule.

1. $\text{H}_4\text{C}\cdots\cdots\text{H-F}$: The H in $\text{H}_4\text{C}$ is bonded to low-electronegativity C, so no hydrogen bond.
2. $\text{H-H}$: This is a covalent bond, not an intermolecular attraction.
3. $\text{H}_3\text{N}\cdots\cdots\text{H-O-H}$: The H is covalently bonded to electronegative O in $\text{H-O-H}$, and attracted to electronegative N in $\text{H}_3\text{N}$, fitting hydrogen bond criteria.
4. $\text{H}_2\text{O}\cdots\cdots\text{H-CH}_3$: The H in $\text{H-CH}_3$ is bonded to low-electronegativity C, so no hydrogen bond.

Answer:

$\text{H}_3\text{N}\cdots\cdots\text{H-O-H}$