The Foundations of Chemistry

(Marcin) #1

AMORPHOUS SOLIDS AND CRYSTALLINE SOLIDS


We have already seen that solids have definite shapes and volumes, are not very compress-
ible, are dense, and diffuse only very slowly into other solids. They are generally
characterized by compact, ordered arrangements of particles that vibrate about fixed posi-
tions in their structures.
Some noncrystalline solids, called amorphous solids,have no well-defined, ordered
structure. Examples include rubber, some kinds of plastics, and amorphous sulfur.
Some amorphous solids are called glasses because, like liquids, they flow, although very
slowly. The irregular structures of glasses are intermediate between those of freely flowing
liquids and those of crystalline solids; there is only short-range order. Crystalline solids
such as ice and sodium chloride have well-defined, sharp melting temperatures. Particles
in amorphous solids are irregularly arranged, so intermolecular forces among their parti-
cles vary in strength within a sample. Melting occurs at different temperatures for various
portions of the same sample as the intermolecular forces are overcome. Unlike crystalline
solids, glasses and other amorphous solids do not exhibit sharp melting points, but soften
over a temperature range.
The shattering of a crystalline solid produces fragments having the same (or related)
interfacial angles and structural characteristics as the original sample. The shattering of a
cube of rock salt produces several smaller cubes of rock salt. This cleaving occurs prefer-
entially along crystal lattice planes between which the interionic or intermolecular forces
of attraction are weakest. Amorphous solids with irregular structures, such as glasses,
shatter irregularly to yield pieces with curved edges and irregular angles.

13-14


The regular external shape of a crystal is the
result of regular internal arrangements of
atoms, molecules, or ions. Crystals of the
ionic solid sodium chloride, NaCl, from a
kitchen saltshaker have the same shape as the
large crystal shown here.

510 CHAPTER 13: Liquids and Solids


E


nrichment


X-Ray Diffraction


Atoms, molecules, and ions are much too small to be seen with the eye. The arrangements
of particles in crystalline solids are determined indirectly by X-ray diffraction (scattering).
In 1912, the German physicist Max von Laue (1879–1960) showed that any crystal could
serve as a three-dimensional diffraction grating for incident electromagnetic radiation with
wavelengths approximating the internuclear separations of atoms in the crystal. Such radi-
ation is in the X-ray region of the electromagnetic spectrum. Using an apparatus such as
that shown in Figure 13-19, a monochromatic (single-wavelength) X-ray beam is defined
by a system of slits and directed onto a crystal. The crystal is rotated to vary the angle of
incidence. At various angles, strong beams of deflected X-rays hit a photographic plate.
Upon development, the plate shows a set of symmetrically arranged spots due to deflected
X-rays. Different crystals produce different arrangements of spots.

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 13.12, Solid Structures (1):
Crystalline and Amorphous Solids.


One test for the purity of a crystalline
solid is the sharpness of its melting
point. Impurities disrupt the
intermolecular forces and cause
melting to occur over a considerable
temperature range.


The lattice planes are planes within
the crystal containing ordered
arrangements of particles.


William and Lawrence Bragg are the
only father and son to receive the
Nobel Prize, which they shared in
physics in 1915.

Free download pdf