The Foundations of Chemistry

BONDING IN SOLIDS

We classify crystalline solids into categories according to the types of particles in the crystal

and the bonding or interactions among them. The four categories are (1) metallic solids,

(2) ionic solids, (3) molecular solids, and (4) covalent solids. Table 13-10 summarizes these

categories of solids and their typical properties.

Metallic Solids

Metals crystallize as solids in which metal ions may be thought to occupy the lattice sites

and are embedded in a cloud of delocalized valence electrons. Nearly all metals crystal-

lize in one of three types of lattices: (1) body-centered cubic (bcc), (2) face-centered cubic

(fcc; also called cubic close-packed), and (3) hexagonal close-packed. The latter two types

are called close-packed structures because the particles (in this case metal atoms) are packed

together as closely as possible. The differences between the two close-packed structures

are illustrated in Figures 13-26 and 13-27. Let spheres of equal size represent identical

metal atoms, or any other particles, that form close-packed structures. Consider a layer

of spheres packed in a plane, A,as closely as possible (Figure 13-27a). An identical plane

of spheres, B,is placed in the depressions of plane A.If the third plane is placed with its

spheres directly above those in plane A,the ABAarrangement results. This is the hexag-

onal close-packed structure (Figure 13-27a). The extended pattern of arrangement of

planes is ABABAB.... If the third layer is placed in the alternate set of depressions in

the second layer so that spheres in the first and third layers are not directly above and

below each other, the cubic close-packed structure, ABCABCABC... , results (Figure

13-27b). In close-packed structures each sphere has a coordination numberof 12, that is, 12

nearest neighbors. In ideal close-packed structures 74% of a given volume is due to spheres

and 26% is empty space. The body-centered cubic structure is less efficient in packing;

each sphere has only eight nearest neighbors, and there is more empty space.

13-16

The term “coordination number”
is used in crystallography in a
somewhat different sense from that in
coordination chemistry (Section 25-3).
Here it refers to the number of nearest
neighbors.

516 CHAPTER 13: Liquids and Solids

TABLE 13-10 Characteristics of Types of Solids

Metallic Ionic Molecular Covalent

Particles of Metal ions in “electron Anions, cations Molecules (or atoms) Atoms

unit cell cloud”

Strongest Metallic bonds Electrostatic Dispersion, dipole–dipole, Covalent bonds

interparticle attraction between and/or hydrogen bonds

forces cations and e’s)

Properties Soft to very hard; good Hard; brittle; poor Soft; poor thermal and Very hard; poor

thermal and thermal and electrical conductors; thermal and electrical

electrical conductors; electrical conductors; low melting points conductors;* high

wide range of high melting points (272 to 400°C) melting points

melting points (400 to 3000°C) (1200 to 4000°C)

(39 to 3400°C)

Examples Li, K, Ca, Cu, Cr, Ni NaCl, CaBr 2 , K 2 SO 4 CH 4 (methane), P 4 , O 2 , C (diamond), SiO 2

(metals) (typical salts) Ar, CO 2 , H 2 O, S 8 (quartz)

*Exceptions: Diamond is a good conductor of heat; graphite is soft and conducts electricity well.