Crystalline Solids
The negative diagram above is of ordinary table salt on a penny.
Notice that each particle is very nearly a perfect little cube. When
most substances freeze, or when they separate out as a solid from a
solution that is being evaporated, they normally form crystals that have
highly regular features.
The particles in crystals are arranged in patterns that repeat
over and over again in all directions. The overall pattern that results
is called a crystal lattice. Its high degree of regularity is the
principle feature that makes solids different from liquids - a liquid
lacks this long range repetition of structure because the particles in a
liquid are jumbled and disorganized as they move about.
Because there are millions of chemical compounds, it might seem
that a enormous number of different kinds of lattices are possible. If
this were true, studying solids would be hopelessly complex.
Fortunately, however, the number of kinds of lattices that are
mathematically possible is quite limited.
To describe the structure of a crystal it is convenient to view
it as being composed of a huge number of simple, basic units called unit
cells. By repeating this simple structural unit up and down, back and
forth, in all directions, we can build the entire lattice. This is shown
below, for the simplest and most symmetrical of all unit cells, called
the simple cubic. This unit cell is a cube having atoms (or molecules or
ions) at each of its eight corners. Stacking these unit cells gives a
simple cubic lattice.
Two other cubic unit cells are also possible: face-centred cubic
and body-centred cubic. The face-centred cubic (fcc) unit cell has
identical particles at each of the corners plus another in the centre of
each face.
Many common metals - copper, silver, gold, aluminum, and lead,
for example - form crystals that have face-centred cubic lattices. Each
of these metals has the same kind of lattice, but the sizes of
their unit cells differ because the sizes of the atoms differ.
The body-centred cubic (bcc) unit cell has identical particles at
each corner plus one in the centre of the cell. The body-centred cubic
lattice is also common among a number of metals - examples are chromium,
iron, and platinum.
Again, these are substances with the same kind of lattice,
but the dimensions of the lattices reflect the size of the particular
atoms.
Not all unit cells are cubic. Some have edges of different
lengths or edges that intersect at angles other than 90o.
Although you should realize that these other unit cells (11 other types)
and the lattices they form exist, we will not go any furhter with this
topic.
Below is a cutaway view of a portion of a sodium chloride
crystal. The smaller particles represent Na+ ions. Notice
that they are located at the lattice positions that correspond to a
face-centred cubic unit cell. The Cl- ions are larger and
fill the spaces between the Na+ ions. Sodium chloride is said
to have a face-centred cubic lattice, and the cubic shape of this
lattice is the reason that NaCl crystals take on a cubic shape when they
form.
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