Page 67 - Materials Science and Engineering An Introduction

2.7 Secondary Bonding or van der Waals Bonding • 39

Figure 2.19 Schematic illustra-
+ + + + +
Metal atom tion of metallic bonding.
- -
- - Ion core
- - -
-
+ + + + +
-
+ - - -
-
- - Sea of valence
+ - + + + + electrons
- -
- - - -
+ + + - - + - +
per atom. Figure 2.19 illustrates metallic bonding. The free electrons shield the positively
charged ion cores from the mutually repulsive electrostatic forces that they would other-
wise exert upon one another; consequently, the metallic bond is nondirectional in charac-
ter. In addition, these free electrons act as a “glue” to hold the ion cores together. Bonding
energies and melting temperatures for several metals are listed in Table 2.3. Bonding may
be weak or strong; energies range from 62 kJ/mol for mercury to 850 kJ/mol for tungsten.
Tutorial Video: Their respective melting temperatures are 39 C and 3414 C ( 39 F and 6177 F).
Bonding Metallic bonding is found in the periodic table for Group IA and IIA elements and,
What is Metallic
Bonding? in fact, for all elemental metals.
Metals are good conductors of both electricity and heat as a consequence of their free
electrons (see Sections 18.5, 18.6, and 19.4). Furthermore, in Section 7.4, we note that at
room temperature, most metals and their alloys fail in a ductile manner—that is, fracture
occurs after the materials have experienced significant degrees of permanent deforma-
tion. This behavior is explained in terms of deformation mechanism (Section 7.2), which
is implicitly related to the characteristics of the metallic bond.

Concept Check 2.3 Explain why covalently bonded materials are generally less dense
than ionically or metallically bonded ones.
[The answer may be found at www.wiley.com/college/callister (Student Companion Site).]

2.7 SECONDARY BONDING OR
VAN DER WAALS BONDING

secondary bond Secondary bonds, or van der Waals (physical) bonds, are weak in comparison to the
primary or chemical bonds; bonding energies range between about 4 and 30 kJ/mol.
van der Waals bond
Secondary bonding exists between virtually all atoms or molecules, but its presence may
be obscured if any of the three primary bonding types is present. Secondary bonding is
evidenced for the inert gases, which have stable electron structures. In addition, second-
ary (or intermolecular) bonds are possible between atoms or groups of atoms, which
themselves are joined together by primary (or intramolecular) ionic or covalent bonds.
dipole Secondary bonding forces arise from atomic or molecular dipoles. In essence,
an electric dipole exists whenever there is some separation of positive and negative
portions of an atom or molecule. The bonding results from the coulombic attraction
between the positive end of one dipole and the negative region of an adjacent one, as
indicated in Figure 2.20. Dipole interactions occur between induced dipoles, between
induced dipoles and polar molecules (which have permanent dipoles), and between po-
hydrogen bonding lar molecules. Hydrogen bonding, a special type of secondary bonding, is found to exist