conditions. These conditions are dictated by the structure of the crystal, and
there is a simple rule for relating the diffraction effect to the crystal’s structure.
We will also find that the type of crystal a certain compound makes is not
necessarily arbitrary, that there is a recognizable energy of interaction between
the components of certain crystals, that crystals are not perfect, and that soci-
ety actually takes advantage of such imperfections in a big way.21.2 Types of Solids
Before we can discuss the solid phase, we need to define several different types
of solid phases. The individual particles (atoms, ions, or molecules) that make
up a solid can exist either in random arrangements in three-dimensional space
or in an ordered, repeating arrangement. Randomly arranged solids are called
amorphous(“without shape”) or glassy.As you might expect, glass is one ex-
ample of a solid that is usually amorphous; see Figure 21.1. Many polymers
have no large-scale order and so can also be considered amorphous.
Solids that are arranged in an orderly fashion are called crystalline.Most
solids form large, well-formed crystals if they are prepared carefully enough—
solidified from the liquid or gas phase very slowly, for example. (Even large
biomolecules like hormones, proteins, and DNA can form crystals. For exam-
ple, the hormone insulin was first crystallized by the American biochemist
John J. Abel in 1925.) If not prepared carefully, many solids form a host of tiny
crystals and would be described as polycrystalline.
Crystalline solids are further categorized by type of crystal. Covalently
bonded molecules can form molecular crystals,which are regular three-dimen-
sional arrangements of the individual molecules. One good example is water,
H 2 O: as a molecular compound, the individual molecules of water have some
regular arrangement in a crystal of H 2 O. Any covalent compound (like the
large biomolecules mentioned earlier) forms molecular crystals, easily or with
difficulty, given the chance. Molecular crystals are relatively soft and have low
melting points. Figure 21.2 shows an example of one molecular crystal.
Compounds that are composed of ions form ionic crystals.In this case, the
need for opposite charges—cations and anions—to neutralize each other dic-
tates a certain arrangement of ions in a crystal. Ionic crystals are typically very
hard but very brittle (that is, they break easily if subjected to sudden forces).
They also tend to have relatively high melting points. Coulombic attractions
between opposite charges is the strongest known force; it takes a lot of energy,
in the form of temperature, to break those attractions and turn an ionic solid
into a liquid.
A few solids make an almost infinite three-dimensional array of covalent
bonds to neighboring atoms. Such solids are called covalent network solids.
Diamond (a form of carbon), elemental silicon, elemental germanium, and sil-
icon dioxide are examples (see Figure 21.3). Although few solids can be de-
scribed this way, the ones that can have distinctive properties: they are very
hard with high melting points. It takes a lot of energy, either mechanical or
thermal, to break the almost infinite network of covalent bonds.
Finally, certain elements are hard but ductile and malleable, conduct elec-
tricity, are shiny, and have variable but usually high melting points. These ele-
ments are metals, and their collective characteristics are explained by an idea
called metal bonding.In this type of bonding, the electrons of the individual
metal atoms “pool” together to become electrons not of the individual atoms
but of the whole solid. This explains the electrical conductivity of metals, and732 CHAPTER 21 The Solid State: CrystalsFigure 21.1 Amorphous or glassy materials,
like the ones shown, have randomly arranged
molecules. Crystalline materials, on the other
hand, are solids that are built up of units that re-
peat in three dimensions.Figure 21.2 Molecules can form crystals, if
they are positioned regularly in three dimensions.© Nuridsany et Perennou/Photo Researchers, Inc.
© James L. Amos/CORBIS
© Peticolas/Megna Fundamental Photographs