What causes the anomalous boiling points of ammonia, water, and hydrogen fluoride?

The anomalous boiling points of ammonia, water, and hydrogen fluoride can be attributed to hydrogen bonding, which is a specific and particularly strong type of dipole-dipole interaction that occurs between molecules. In these substances, hydrogen atoms are bonded to highly electronegative atoms (nitrogen in ammonia, oxygen in water, and fluorine in hydrogen fluoride), leading to significant polarity in each molecule.

The hydrogen bond occurs because the positive charge associated with the hydrogen atom is attracted to the negative charge of the electronegative atom in a neighboring molecule. This results in a network of strong intermolecular attractions, significantly increasing the boiling points compared to other molecules that may only exhibit weaker London dispersion forces or dipole-dipole interactions.

For example, while simple molecules such as methane have weaker intermolecular forces that result in low boiling points, the hydrogen bonding in water leads to its high boiling point relative to its molecular weight. This phenomenon can also be observed in ammonia and hydrogen fluoride, where the presence of hydrogen bonding contributes to their higher than expected boiling points.