Page 64 - Materials Science and Engineering An Introduction

36 • Chapter 2 / Atomic Structure and Interatomic Bonding

Figure 2.12 Schematic representation of covalent
bonding in a molecule of hydrogen (H 2 ). H H H H
+

electron orbitals in the region between the two bonding atoms. In addition, the covalent
bond is directional—that is, it is between specific atoms and may exist only in the direc-
tion between one atom and another that participates in the electron sharing.
Many nonmetallic elemental molecules (e.g., Cl 2 , F 2 ), as well as molecules con-
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taining dissimilar atoms, such as CH 4 , H 2 O, HNO 3 , and HF, are covalently bonded.
Furthermore, this type of bonding is found in elemental solids such as diamond (car-
bon), silicon, and germanium and other solid compounds composed of elements that are
Tutorial Video: located on the right side of the periodic table, such as gallium arsenide (GaAs), indium
Bonding antimonide (InSb), and silicon carbide (SiC).
What is Covalent Covalent bonds may be very strong, as in diamond, which is very hard and has a
Bonding?
very high melting temperature, 3550 C (6400 F), or they may be very weak, as with
bismuth, which melts at about 270 C (518 F). Bonding energies and melting tempera-
tures for a few covalently bonded materials are presented in Table 2.3. Inasmuch as
electrons participating in covalent bonds are tightly bound to the bonding atoms, most
covalently bonded materials are electrical insulators, or, in some cases, semiconductors.
Mechanical behaviors of these materials vary widely: some are relatively strong, others
are weak; some fail in a brittle manner, whereas others experience significant amounts
of deformation before failure. It is difficult to predict the mechanical properties of cova-
lently bonded materials on the basis of their bonding characteristics.
Bond Hybridization in Carbon
Often associated with the covalent bonding of carbon (as well other nonmetallic sub-
stances) is the phenomenon of hybridization—the mixing (or combining) of two or more
atomic orbitals with the result that more orbital overlap during bonding results. For
2
2
2
example, consider the electron configuration of carbon: 1s 2s 2p . Under some circum-
stances, one of the 2s orbitals is promoted to the empty 2p orbital (Figure 2.13a), which
2
3
1
gives rise to a 1s 2s 2p configuration (Figure 2.13b). Furthermore, the 2s and 2p orbitals
3
can mix to produce four sp orbitals that are equivalent to one another, have parallel
spins, and are capable of covalently bonding with other atoms. This orbital mixing is
termed hybridization, which leads to the electron configuration shown in Figure 2.13c;
3
here, each sp orbital contains one electron, and, therefore, is half-filled.
Bonding hybrid orbitals are directional in nature—that is, each extends to and
overlaps the orbital of an adjacent bonding atom. Furthermore, for carbon, each of its
3
four sp hybrid orbitals is directed symmetrically from a carbon atom to the vertex of a
tetrahedron—a configuration represented schematically in Figure 2.14; the angle between
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3
each set of adjacent bonds is 109.5 . The bonding of sp hybrid orbitals to the 1s orbitals
of four hydrogen atoms, as in a molecule of methane (CH 4 ), is presented in Figure 2.15.
For diamond, its carbon atoms are bonded to one another with sp 3 covalent
hybrids—each atom is bonded to four other carbon atoms. The crystal structure for
diamond is shown in Figure 12.16. Diamond’s carbon–carbon bonds are extremely
strong, which accounts for its high melting temperature and ultrahigh hardness (it is
the hardest of all materials). Many polymeric materials are composed of long chains of
carbon atoms that are also bonded together using sp tetrahedral bonds; these chains
3
form a zigzag structure (Figure 14.1b) because of this 109.5 interbonding angle.
7 For these substances, the intramolecular bonds (bonds between atoms in molecule) are covalent. As noted in the
next section, other types of bonds can operate between molecules, which are termed intermolecular.
8 Bonding of this type (to four other atoms) is sometimes termed tetrahedral bonding.