Page 69 - Materials Science and Engineering An Introduction

2.7 Secondary Bonding or van der Waals Bonding • 41

Figure 2.22 Schematic
H representations of (a) a
+ Cl
– hydrogen chloride molecule
(dipole) and (b) how an
(a) HCl molecule induces an
electrically symmetric atom/
Electrically symmetric
atom/molecule molecule to become a dipole—
Induced dipole also the van der Waals bond
between these dipoles.
H Cl + + H Cl + – –
+ +
– –
– van der Waals
bond
(b)
Polar molecules can also induce dipoles in adjacent nonpolar molecules, and a bond
forms as a result of attractive forces between the two molecules; this bonding scheme is
represented schematically in Figure 2.22b. Furthermore, the magnitude of this bond is
greater than for fluctuating induced dipoles.
Permanent Dipole Bonds
Coulombic forces also exist between adjacent polar molecules as in Figure 2.20. The asso-
ciated 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 H 2 O), or nitrogen (as in NH 3 ). For each HOF, HOO,
Tutorial Video: or HON bond, the single hydrogen electron is shared with the other atom. Thus, the hy-
Bonding drogen end of the bond is essentially a positively charged bare proton unscreened by any
What are the electrons. This highly positively charged end of the molecule is capable of a strong attrac-
Differences between tive force with the negative end of an adjacent molecule, as demonstrated in Figure 2.23 for
Ionic, Covalent,
Metallic, and van der HF. In essence, this single proton forms a bridge between two negatively charged atoms.
Waals Types The magnitude of the hydrogen bond is generally greater than that of the other types of
of Bonding? secondary bonds and may be as high as 51 kJ/mol, as shown in Table 2.3. Melting and boil-
ing temperatures for hydrogen fluoride, ammonia, and water are abnormally high in light
of their low molecular weights, as a consequence of hydrogen bonding.

In spite of the small energies associated with secondary bonds, they nevertheless are
involved in a number of natural phenomena and many products that we use on a daily basis.
Examples of physical phenomena include the solubility of one substance in another, surface
tension and capillary action, vapor pressure, volatility, and viscosity. Common applications
that make use of these phenomena include adhesives—van der Waals bonds form between
two surfaces so that they adhere to one another (as discussed in the chapter opener for this
chapter); surfactants—compounds that lower the surface tension of a liquid, and are found
in soaps, detergents, and foaming agents; emulsifiers—substances that, when added to two
immiscible materials (usually liquids), allow particles of one material to be suspended in
another (common emulsions include sunscreens, salad dressings, milk, and mayonnaise);
and desiccants—materials that form hydrogen bonds with water molecules (and remove
moisture from closed containers—e.g., small packets that are often found in cartons of pack-
aged goods); and finally, the strengths, stiffnesses, and softening temperatures of polymers,
to some degree, depend on secondary bonds that form between chain molecules.

Figure 2.23 Schematic representation of hydrogen
bonding in hydrogen fluoride (HF).
H F H F
Hydrogen
bond