5 Steps to a 5 AP Chemistry 2019

Solids, Liquids, and Intermolecular Forces ❮ 171

Figure 12.2 The three types of unit cell of the cubic lattice.

Simple cubic Body-centered cubic Face-centered cubic

Five types of crystalline solid are known:

1. In atomic solids, individual atoms are held in place by London forces. The noble gases
   are the only atomic solids known to form.


2. In molecular solids, lattices composed of molecules are held in place by London forces,
   dipole–dipole forces, and hydrogen bonding. Solid methane and water are examples of
   molecular solids.


3. In ionic solids, lattices composed of ions are held together by the attraction of the
   opposite charges of the ions. These crystalline solids tend to be strong, with high melt-
   ing points because of the strength of the intermolecular forces. NaCl and other salts are
   examples of ionic solids. Figure 12.3 shows the lattice structure of NaCl. Each sodium
   cation is surrounded by six chloride anions, and each chloride anion is surrounded by
   six sodium cations.


4. In metallic solids, metal atoms occupying the crystal lattice are held together by metallic
   bonding. In metallic bonding, the electrons of the atoms are delocalized and are free to
   move throughout the entire solid. This explains electrical and thermal conductivity, as
   well as many other properties of metals.


5. In covalent network solids, covalent bonds join atoms together in the crystal lattice,
   which is quite large. Graphite, diamond, and silicon dioxide (SiO 2 ) are examples of
   network solids. The crystal is one giant molecule.

ENRICHMENT Several types of unit cells are found in solids. The cubic system is the most common type.
Three types of unit cells are found in the cubic system:

1. The simple cubic unit cell has particles located at the corners of a simple cube.


2. The body-centered unit cell has particles located at the corners of the cube and in
   the center of the cube.


3. The face-centered unit cell has particles at the corners and one in the center of
   each face of the cube, but not in the center of the cube itself.
   Figure 12.2 shows three types of cubic unit cells.