INTERVIEWER: Hydrogen bonding occurs when a hydrogen atom is bonded to a small, highly electronegative atom, such as nitrogen, oxygen, or fluorine. In this example, its combination of high electronegativity and small radius enables fluorine to draw electron density away from the hydrogen atom. The HF bond is highly polar. Because hydrogen has no core electrons, its nucleus is left practically unshielded. This gives the hydrogen atom a relatively high partial positive charge, which is powerfully attracted to the partial negative charge on the fluorine atom of a neighboring HF molecule. Although hydrogen bonding is a type of dipole-dipole interaction, some hydrogen bonds more closely resemble ion dipole attractions in magnitude. In a series of similar compounds, such as these group 4a hydrides, boiling point typically decreases with decreasing molecular mass. This reflects the smaller London dispersion forces that exist between smaller molecules. Note, however, that in the series of group 6A hydrides, although the same boiling point trend is observed for the heaviest members of the series, the boiling point of water is anomalously high because more energy in the form of heat is required to break the hydrogen bonds and separate the molecules. The difference between the expected boiling point of water and its observed boiling point is due to hydrogen bonding.
