termed polar molecules. Figure 2.14 is a schematic representation of a hydrogen
chloride molecule; a permanent dipole moment arises from net positive and negative
charges that are respectively associated with the hydrogen and chlorine ends
of the HCl molecule.
Polar molecules can also induce dipoles in adjacent nonpolar molecules, and
a bond will form as a result of attractive forces between the two molecules. Furthermore,
the magnitude of this bond will be greater than for fluctuating induced dipoles.
Permanent Dipole Bonds
Van der Waals forces will also exist between adjacent polar molecules.The associated
bonding energies are significantly greater than for bonds involving induced dipoles.
The strongest secondary bonding type, the hydrogen bond, is a special case of polar
molecule bonding. It occurs between molecules in which hydrogen is covalently
bonded to fluorine (as in HF), oxygen (as in ), and nitrogen (as in ). For
each H—F, H—O, or H—N bond, the single hydrogen electron is shared with the
other atom. Thus, the hydrogen end of the bond is essentially a positively charged
bare proton that is unscreened by any electrons. This highly positively charged end
of the molecule is capable of a strong attractive force with the negative end of an
adjacent molecule, as demonstrated in Figure 2.15 for HF. In essence, this single proton
forms a bridge between two negatively charged atoms.The magnitude of the hydrogen
bond is generally greater than that of the other types of secondary bonds
and may be as high as 51 kJ/mol (0.52 eV/molecule), as shown in Table 2.3. Melting
and boiling temperatures for hydrogen fluoride and water are abnormally high in
light of their low molecular weights, as a consequence of hydrogen bonding.