have to be subject to the same interaction forces (magnitude and direction),
i.e., be in the same environment. As a result the entities are arranged in a
regularly repeating pattern or lattice. This is illustrated in Figure 15.1a.
In principle, the building entities have afixed, perfectly ordered, and
permanent position. In practice, this is not fully true, and the following
points can be mentioned:
The molecules (or atoms, or ions) are subject toheat motion. Hence
only the average positions will be fixed.
Occasionally, two molecules may interchange positions. In other
words,diffusioncan occur in a crystalline material, but the time
scales involved are centuries rather than seconds.
Crystals are virtually never perfect, but contain defects, often due to
incorporation of a foreign molecule. A defect generally leads to a
dislocationin the crystal lattice, as is illustrated in Figure 15.1b.
Some solid materials are ‘‘polycrystalline’’, i.e., they are composites of
many small crystalline domains of various orientations; see Figure
15.1c.Noncrystalline solid states are discussed in Section 16.1.1.
Bonds Involved. Crystals can be classified according to the nature
of the main bonds keeping the building entities together. The following list is
very roughly in the order of decreasing bond strength, heat of fusion, and
melting temperature.
- Covalent. The prime example is carbon in diamond or graphite,
where each carbon atom is covalently bonded to four others. Another
example is quartz, (SiO 2 )n; in a large crystal—in fact one giant molecule—n
can be as large as 10^24. - Metallic. Metal crystals consist of an array of cations, through
which electrons can freely move. - Ionic. Salts typically form ionic crystals, i.e., a regular packing of
cations and anions. In several of these, e.g., NaCl, the ions simply alternate
in a cubic array. - Molecular. This concerns, for instance, triglycerides and sugars.
The bonds involved are due to van der Waals attraction and often also
hydrogen bonds.
The crystal bonds in foods are almost exclusively of Types 3 and 4.
Often, a combination of ionic and hydrogen bonds and van der Waals forces
is involved. Consequently, we will consider the building entities of a crystal
to be molecules, unless stated otherwise. Some crystals contain a fixed molar
proportion of solvent molecules, e.g., potassium oxalateðK 2 C 2 O 4 ?2H 2 OÞ;