In the previous pages, some of the mechanisms that bond together
the multitude of individual atoms or molecules of a solid material
were discussed. These forces may be primary chemical bonds, as
in metals and ionic solids, or they may be secondary van der Waals’
forces of solids, such as in ice, paraffin wax and most polymers.
In solids, the way the atoms or molecules arrange themselves contributes
to the appearance and the properties of the materials.
Atoms can be gathered together as an aggregate through a number
of different processes, including condensation, pressurization,
chemical reaction, electrodeposition, and melting. The process
usually determines, at least initially, whether the collection
of atoms will take to form of a gas, liquid or solid. The state
usually changes as its temperature or pressure is changed. Melting
is the process most often used to form an aggregate of atoms.
When the temperature of a melt is lowered to a certain point,
the liquid will form either a crystalline solid or and amorphous
A solid substance with its atoms held apart at equilibrium spacing,
but with no long-range periodicity in atom location in its structure
is an amorphous solid. Examples of amorphous solids are glass
and some types of plastic. They are sometimes described as supercooled
liquids because their molecules are arranged in a random manner
some what as in the liquid state. For example, glass is commonly
made from silicon dioxide or quartz sand, which has a crystalline
structure. When the sand is melted and the liquid is cooled rapidly
enough to avoid crystallization, an amorphous solid called a glass
is formed. Amorphous solids do not show a sharp phase change from
solid to liquid at a definite melting point, but rather soften
gradually when they are heated. The physical properties of amorphous
solids are identical in all directions along any axis so they
are said to have isotropic properties, which will be discussed
in more detail later
More than 90% of naturally occurring and artificially prepared
solids are crystalline. Minerals, sand, clay, limestone, metals,
carbon (diamond and graphite), salts ( NaCl, KCl etc.), all have
crystalline structures. A crystal is a regular, repeating arrangement
of atoms or molecules. The majority of solids, including all metals,
adopt a crystalline arrangement because the amount of stabilization
achieved by anchoring interactions between neighboring particles
is at its greatest when the particles adopt regular (rather than
random) arrangements. In the crystalline arrangement, the particles
pack efficiently together to minimize the total intermolecular
The regular repeating pattern that the atoms arrange in is called
the crystalline lattice. The scanning tunneling microscope (STM)
makes it possible to image the electron cloud associated individual
atoms at the surface of a material. Below is an STM image of a
platinum surface showing the regular alignment of atoms.
Courtesy: IBM Research, Almaden Research Center.
Crystal structures may be conveniently specified by describing
the arrangement within the solid of a small representative group
of atoms or molecules, called the ‘unit cell.’ By
multiplying identical unit cells in three directions, the location
of all the particles in the crystal is determined. In nature,
14 different types of crystal structures or lattices are found.
The simplest crystalline unit cell to picture is the cubic, where
the atoms are lined up in a square, 3D grid. The unit cell is
simply a box with an atom at each corner. Simple cubic crystals
are relatively rare, mostly because they tend to easily distort.
However, many crystals form body-centered-cubic (bcc) or face-centered-cubic
(fcc) structures, which are cubic with either an extra atom centered
in the cube or centered in each face of the cube. Most metals
form bcc, fcc or Hexagonal Close Packed (hpc) structures; however,
the structure can change depending on temperature. These three
structures will be discussed in more detail on the following page.
Crystalline structure is important because it contributes to
the properties of a material. For example, it is easier for planes
of atoms to slide by each other if those planes are closely packed.
Therefore, lattice structures with closely packed planes allow
more plastic deformation than those that are not closely packed.
Additionally, cubic lattice structures allow slippage to occur
more easily than non-cubic lattices. This is because their symmetry
provides closely packed planes in several directions. A face-centered
cubic crystal structure will exhibit more ductility (deform more
readily under load before breaking) than a body-centered cubic
structure. The bcc lattice, although cubic, is not closely packed
and forms strong metals. Alpha-iron and tungsten have the bcc
form. The fcc lattice is both cubic and closely packed and forms
more ductile materials. Gamma-iron, silver, gold, and lead have
fcc structures. Finally, HCP lattices are closely packed, but
not cubic. HCP metals like cobalt and zinc are not as ductile
as the fcc metals.