Page 72 - Materials Science and Engineering An Introduction

44 • Chapter 2 / Atomic Structure and Interatomic Bonding

Mixed metallic–ionic bonds are observed for compounds composed of two metals
when there is a significant difference between their electronegativities. This means that
some electron transfer is associated with the bond inasmuch as it has an ionic compo-
nent. Furthermore, the larger this electronegativity difference, the greater the degree of
ionicity. For example, there is little ionic character to the titanium–aluminum bond for
the intermetallic compound TiAl 3 because electronegativities of both Al and Ti are the
same (1.5; see Figure 2.9). However, a much greater degree of ionic character is present
for AuCu 3 ; the electronegativity difference for copper and gold is 0.5.

EXAMPLE PROBLEM 2.3

Calculation of the Percent Ionic Character for the C-H Bond
Compute the percent ionic character (%IC) of the interatomic bond that forms between carbon
and hydrogen.

Solution
The %IC of a bond between two atoms/ions, A and B (A being the more electronegative) is a
function of their electronegativities X A and X B , according to Equation 2.16. The electronega-
tivities for C and H (see Figure 2.9) are X C 2.5 and X H 2.1. Therefore, the %IC is
2
%IC = 51 - exp[ -(0.25)(X C - X H ) ]6 * 100
2
= 51 - exp[ -(0.25)(2.5 - 2.1) ]6 * 100
= 3.9%
Thus the COH atomic bond is primarily covalent (96.1%).

2.9 MOLECULES
Many common molecules are composed of groups of atoms bound together by strong
covalent bonds, including elemental diatomic molecules (F 2 , O 2 , H 2 , etc.), as well as a
host of compounds (H 2 O, CO 2 , HNO 3 , C 6 H 6 , CH 4 , etc.). In the condensed liquid and
solid states, bonds between molecules are weak secondary ones. Consequently, mo-
lecular materials have relatively low melting and boiling temperatures. Most materials
that have small molecules composed of a few atoms are gases at ordinary, or ambient,
temperatures and pressures. However, many modern polymers, being molecular materi-
als composed of extremely large molecules, exist as solids; some of their properties are
strongly dependent on the presence of van der Waals and hydrogen secondary bonds.

2.10 BONDING TYPE-MATERIAL CLASSIFICATION
CORRELATIONS
In previous discussions of this chapter, some correlations have been drawn between
bonding type and material classification—namely, ionic bonding (ceramics), covalent
bonding (polymers), metallic bonding (metals), and van der Waals bonding (molecular
solids). We summarized these correlations in the material-type tetrahedron shown in
Figure 2.25b—the bonding tetrahedron of Figure 2.25a, on which is superimposed the
10
bonding location/region typified by each of the four material classes. Also included
10 Although most atoms in polymer molecules are covalently bonded, some van der Waals bonding is normally
present. We chose not to include van der Waals bonds for polymers because they (van der Waals) are intermolecular
(i.e., between molecules) as opposed to intramolecular (within molecules) and not the principal bonding type.