Question

Which type of intermolecular attractions holds ammonia molecules together with other ammonia molecules?

Answer 1

• Hydrogen bonds (strongest).
• Dipole-dipole interactions (weaker than hydrogen bonds).

Explanation

There are three major types of intermolecular attractions:

• Hydrogen bonds,
• Dipole-dipole interactions, and
• London dispersion forces.

London dispersion forces exist between all molecules. It is the weakest among the three types of forces.

Dipole-dipole interactions exist between polar molecules. It is stronger than London dispersion forces.

Hydrogen bonds are attractions between

• H atoms directly bonded to atoms of N, O, or F; and
• Lone pairs of electrons on other molecules.

Hydrogen bonds is the strongest among the three types of forces.

Consider the structure of an ammonia $\text{NH}_3$ molecule.

• All three H atoms are bonded to an N atom.
• There is one lone pair of electrons on the central N atom.

• N-H bonds are highly polar.
• The shape of the $\text{NH}_3$ molecule is pyramidal. As a result, dipoles from the three N-H bond remain not balanced. The molecule is overall polar.

As a result, $\text{NH}_3$ molecules will form both hydrogen bonds and dipole-dipole interactions between them. There's also going to be London dispersion forces. Nonetheless, hydrogen bonds and dipole-dipole interactions will contribute to the majority of attractions between the molecules.