bound mainly by this type of attraction have rather soft crystals, are easily
deformed, and vaporize easily. Because of the low intermolecular forces, the
melting points are low and evaporation takes place so easily that it may occur at
room temperature. Examples of such solids are iodine crystals and moth balls
(paradichlorobenzene and naphthalene).
Hydrogen Bonds
A proton or hydrogen nucleus has a high concentration of positive charge. When a
hydrogen atom is bonded to a highly electronegative atom, its positive charge will
have an attraction for neighboring electron pairs. This special kind of dipole-
dipole attraction is called a hydrogen bond. The more strongly polar the
molecule is, the more effective the hydrogen bonding is in binding the molecules
into a larger unit. As a result the boiling points of such molecules are higher than
those of similar nonpolar molecules. Good examples are water and hydrogen
fluoride.
Studying Figure 14 shows that in the series of compounds consisting of H 2 O,
H 2 S, H 2 Se, and H 2 Te an unusual rise in the boiling point of H 2 O occurs that is not
in keeping with the typical slow increase of boiling point as molecular mass
increases. Instead of the expected slope of the line between H 2 O and H 2 S, which
is shown in Figure 14 as a dashed line, the actual boiling point of H 2 O is quite a
bit higher—100°C. The explanation is that hydrogen bonding occurs in H 2 O but
not to any significant degree in the other compounds.
TIP
Notice how hydrogen bonding elevates the boiling point of H 2 O above the expected slope.Figure 14. Boiling Points of Hydrogen Compounds with Similar Electron Dot
StructuresThis same phenomenon occurs with the hydrogen halides (HF, HCl, HBr, and
HI). Note in Figure 14 that hydrogen fluoride, HF, which has strong hydrogen
bonding, shows an unexpectedly high boiling point.
Hydrogen bonding also explains why some substances have unexpectedly