Boron atom
Oxygen atom(a) (b)Figure 10.1Two-dimensional representation of B 2 O 3. (a) Amorphous B 2 O 3 exhibits only short-range
order. (b) Crystalline B 2 O 3 exhibits long-range order as well.336 Chapter Ten
A
solid consists of atoms, ions, or molecules packed closely together, and the
forces that hold them in place give rise to the distinctive properties of the var-
ious kinds of solid. The covalent bonds that can link a fixed number of atoms
to form a certain molecule can also link an unlimited number of them to form a solid.
In addition, ionic, van der Waals, and metallic bonds provide the cohesive forces in
solids whose structural elements are respectively ions, molecules, and metal atoms. All
these bonds involve electric forces, with the chief differences among them being in the
ways in which the outer electrons of the structural elements are distributed. Although
very little of the matter in the universe is in the solid state, solids constitute much of
the physical world around us, and a large part of modern technology is based on the
special characteristics of various solid materials.10.1 CRYSTALLINE AND AMORPHOUS SOLIDS
Long-range and short-range orderMost solids are crystalline,with the atoms, ions, or molecules of which they are
composed falling into regular, repeated three-dimensional patterns. The presence of
long-range orderis thus the defining property of a crystal, although relatively few
samples of crystalline solids consist of single crystals. Most are polycrystalline and are
composed of a great many small crystals (sometimes called crystallites).
Other solids lack the definite arrangements of their member particles so conspicu-
ous in crystals. They may be regarded as supercooled liquids whose stiffness is due to
an exceptionally high viscosity. Glass, pitch, and many plastics are examples of such
amorphous(“without form”) solids.
Amorphous solids do exhibit short-range orderin their structures, however. The dis-
tinction between the two kinds of order is nicely exhibited in boron trioxide (B 2 O 3 ),
which can occur in both crystalline and amorphous forms. In each case every boron atom
is surrounded by three oxygen atoms, which represents a short-range order. In a B 2 O 3
crystal a long-range order is also present, as shown in a two-dimensional representation
in Fig. 10.1. Amorphous B 2 O 3 , a vitreous or “glassy” substance, lacks this additional reg-
ularity. Crystallization from the vitreous state is so sluggish that it ordinarily does not oc-
cur, but it is not unknown. Glass may devitrify when heated until it has not quite be-
gun to soften, and extremely old glass specimens are sometimes found to have crystallized.bei48482_ch10.qxd 1/22/02 10:03 PM Page 336