http://www.ck12.org Chapter 13. States of Matter
FIGURE 13.16
Three unit cells of the cubic crystal sys-
tem. Each sphere represents an atom or
an ion. In the simple cubic system, the
atoms or ions are at the corners of the unit
cell only. In the face-centered unit cell,
there are also atoms or ions in the center
of each of the six faces of the unit cell. In
the body-centered unit cell, there is one
atom or ion in the center of the unit cell in
addition to the corner atoms or ions.1.Ionic crystals—The ionic crystal structure, discussed in the chapterIonic and Metallic Bonding, consists of
alternating positively charged cations and negatively charged anions. The ions may either be monatomic or
polyatomic. Generally, ionic crystals form from a combination of metal cations and Group 16 or 17 nonmetal
anions, although nonmetallic polyatomic ions are also common components of ionic crystals. Ionic crystals
are hard and brittle and have high melting points. Ionic compounds do not conduct electricity as solids, but
they do conduct when molten or dissolved in water.
2.Metallic crystals—Metallic crystals, also discussed in the chapterIonic and Metallic Bonding, consist of
metal cations surrounded by a “sea” of mobile valence electrons. These electrons, also referred to as delo-
calized electrons, do not belong to any one atom and are capable of moving through the entire crystal. As a
result, metals are good conductors of electricity. As seen in the table above (Table13.3), metallic crystals
can have a wide range of melting points.
3.Covalent network crystals—A covalent network crystal consists of atoms at the lattice points of the crystal,
with each atom being covalently bonded to its nearest neighbor atoms. The covalently bonded network is
three-dimensional and contains a very large number of atoms. Network solids include diamond (Figure
13.17), quartz, many metalloids, and oxides of transition metals and metalloids. Network solids are hard and
brittle, with extremely high melting and boiling points. Being composed of atoms rather than ions, they do
not conduct electricity well in any state.
4.Molecular crystals—Molecular crystals typically consist of molecules at the lattice points of the crystal, held
together by relatively weak intermolecular forces. The intermolecular forces may be dispersion forces in the
case of nonpolar substances or dipole-dipole forces in the case of polar substances. Some molecular crystals,
such as ice, have molecules held together by hydrogen bonds. When one of the noble gases is cooled and
solidified, the lattice points are individual atoms rather than molecules. However, because the atoms are held
together by dispersion forces and not by covalent or metallic bonds, the properties of such a crystal are most
similar to the crystals of molecular substances. In all cases, the intermolecular forces holding the particles
together are far weaker than either ionic or covalent bonds. As a result, the melting and boiling points of
molecular crystals are much lower. Lacking ions or free electrons, molecular crystals are poor electrical
conductors.