SEVENTH E DIT ION
CHEMISTRY
& Chemical Reactivity
Enhanced Edition
John C. Kotz
SUNY Distinguished Teaching Professor
State University of New York
College of Oneonta
Paul M. Treichel
Professor of Chemistry
University of Wisconsin–Madison
John R. Townsend
Professor of Chemistry
West Chester University of Pennsylvania
Australia • Brazil • Japan • Korea • Mexico • Singapore • Spain • United Kingdom • United States
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Chemistry & Chemical Reactivity,
Enhanced Edition
John C. Kotz, Paul M. Treichel, and John R. Townsend
Publisher: Mary Finch
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T HE C HEMISTRY OF
M ODERN M ATERIALS
W
hite limestone and the minerals calcite, aragonite,
and Iceland spar are all composed of calcium carbonate. So are chalk, eggshells, and sea shells. These objects
have distinctly different physical characteristics (Figure 1),
yet they are composed primarily of the same component
particles, Ca2 and CO32 ions. What is interesting to
chemists, geologists, and biologists is that the differences
in the macroscopic characteristics result from small differences in composition (due to the presence of impurities)
and in the arrangement of the particles.
By studying the composition and structure of synthetic
and naturally occurring materials, scientists and engineers
are able to gain insight into what gives each material its
properties. They might then be able to use synthetic techniques to create new materials that are tailored to particular applications and that have predictable behaviors. They
can also extract and purify naturally occurring substances
that have properties considered desirable for specific applications. The study and synthesis of materials are the
general domain of materials science. While chemistry
serves as the foundation of materials science, understanding and working in this field may require expertise in physics, biology, and engineering.
This section explores a variety of common materials—
except organic polymers, which were described in Section
10.5—and examines the connection between composition,
atomic arrangements, and bulk properties. We will also
look at some modern materials and their applications.
Metals
Charles D. Winters
Simon Fraser/Photo Researchers, Inc.
Bonding in Metals
Figure 1 Forms of calcium carbonate. (clockwise from top) The shell of an
abalone, a limestone paving block from Europe, crystalline aragonite, common
blackboard chalk (CaCO3 and a binder), and transparent Iceland spar.
Molecular orbital (MO) theory was introduced in Chapter 9
to rationalize covalent bonding. Recall the basic concepts
of MO theory: Atomic orbitals from individual atoms in a
molecule are combined to form molecular orbitals spanning two or more atoms, with the number of MOs being
equal to the number of atomic orbitals. Electrons placed in
the lower-energy, bonding molecular orbitals make the molecule more stable energetically than the individual atoms
from which it is made.
MO theory can also be used to describe metallic bonding. A metal is a kind of “supermolecule,” and to describe
the bonding in a metal we have to look at all the atoms in
a given sample.
• Fiberoptics. Fibers made of glass are being used increasingly to
carry information.
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| 657
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658 | The Chemistry of Modern Materials
Lin
Li4
Li3
Li2
Li
Figure 2 Bands of molecular orbitals in a metal crystal. Here, the 2s
valence orbitals of Li atoms are combined to form molecular orbitals. As
more and more atoms with the same valence orbitals are added, the number
of molecular orbitals grows until the orbitals are so close in energy that they
merge into a band of molecular orbitals. If 1 mol of Li atoms, each with its
2s valence orbital, is combined, 6 1023 molecular orbitals are formed.
However, only 1 mol of electrons, or 3 1023 electron pairs, is available, so
only half of these molecular orbitals are filled.
Even a tiny piece of metal contains a very large number
of atoms and an even larger number of valence orbitals. In
1 mol of lithium atoms, for example, there are 6 1023
atoms). Considering only the 2s valence orbitals of lithium,
there are 6 1023 atomic orbitals, from which 6 1023
molecular orbitals can be created. The molecular orbitals
that we envision in lithium will span all the atoms in the
crystalline solid. A mole of lithium has 1 mol of valence
electrons, and these electrons occupy the lower-energy
bonding orbitals. The bonding is described as delocalized;
that is, the electrons are associated with all the atoms in the
crystal and not with a specific bond between two atoms.
This theory of metallic bonding is called band theory.
An energy-level diagram would show the bonding and antibonding molecular orbitals blending together into a
band of molecular orbitals (Figure 2), with the individual
MOs being so close together in energy that they are not
distinguishable. The band is composed of as many molecular orbitals as there are contributing atomic orbitals,
and each molecular orbital can accommodate two electrons of opposite spin.
In metals, there are not enough electrons to fill all of
the molecular orbitals. In 1 mol of Li atoms, for example,
6 1023 electrons, or 3 1023 electron pairs, are sufficient
to fill only half of the 6 1023 molecular orbitals. The
lowest energy for a system occurs with all electrons in orbitals with the lowest possible energy, but this is reached only
at 0 K. At 0 K, the highest filled level is called the Fermi
level (Figure 3).
In metals at temperatures above 0 K, thermal energy
will cause some electrons to occupy higher-energy orbitals.
Even a small input of energy (for example, raising the
temperature a few degrees above 0 K) will cause electrons
to move from filled orbitals to higher-energy orbitals. For
METALS
SEMICONDUCTORS AND INSULATORS
Empty levels
Filled levels
Fermi level,
0K
ENERGY
Fermi level,
0K
Conduction band
ENERGY
ENERGY
Empty levels
Electron promoted
Band gap
Positive hole below
the Fermi level
Energy Added
Filled levels
Valence band
Figure 3 Band theory applied to metals, semiconductors, and insulators. The bonding in metals and semiconductors can be described using molecular orbital
theory. Molecular orbitals are constructed from the valence orbitals on each atom and are delocalized over all the atoms. (Metals, left and center) The highest
filled level at 0 K is referred to as the Fermi level. (Semiconductors and insulators, right) In contrast to metals, the band of filled levels (the valence band) is separated from the band of empty levels (the conduction band) by a band gap. In insulators, the energy of the band gap is large.
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Metals | 659
each electron promoted, two singly occupied levels result:
a negative electron in an orbital above the Fermi level and
a positive “hole”—from the absence of an electron—below
the Fermi level.
The positive holes and negative electrons in a piece of
metal account for its electrical conductivity. Electrical conductivity arises from the movement of electrons and holes
in singly occupied states in the presence of an applied
electric field. When an electric field is applied to the metal,
negative electrons move toward the positive side, and the
positive “holes” move to the negative side. (Positive holes
“move” because an electron from an adjacent atom can
move into the hole, thereby creating a fresh “hole.”)
Because the band of unfilled energy levels in a metal is
essentially continuous—that is, because the energy gaps
between levels are extremely small—a metal can absorb
energy of nearly any wavelength. When light is absorbed,
causing an electron in a metal to move to a higher energy
state, the now-excited system can immediately emit a photon of the same energy as the electron returns to the
original energy level. This rapid and efficient absorption
and reemission of light make polished metal surfaces be
reflective and appear lustrous (shiny).
The molecular orbital picture for metallic bonding provides an interpretation for other physical characteristics of
metals. For example, most metals are malleable and ductile, meaning they can be rolled into sheets and drawn into
wires. In these processes, the metal atoms must be able to
move fairly freely with respect to their nearest neighbors.
This is possible because metallic bonding is delocalized—
that is, nondirectional. The layers of atoms can slip past
one another relatively easily, as if the delocalized electrons
were ball bearings that facilitate this motion, while at the
same time keeping the layers bonded through coulombic
attractions between the nuclei and the electrons.
In contrast to metals, rigid network solids such as diamond, silicon, and silica (SiO2) have localized bonding,
which anchors the component atoms or ions in fixed positions. Movement of atoms in these structures relative to
their neighbors requires breaking covalent bonds. As a
result, such substances are typically hard and brittle. They
will not deform under stress as metals do, but instead tend
to cleave along crystal planes (䉳 page 80).
TABLE 1
Some Common Alloys
Sterling silver
92.5% Ag, 7.5% Cu
18 K “yellow” gold
75% Au, 12.5% Ag, 12.5% Cu
Pewter
91% Sn, 7.5% Sb, 1.5% Cu
Low-alloy steel
98.6% Fe, 1.0% Mn, 0.4% C
Carbon steels
Approximately 99% Fe, 0.2–1.5% C
Stainless steel
72.8% Fe, 17.0% Cr, 7.1% Ni, and approximately 1% each of Al and Mn
Alnico magnets
10% Al, 19% Ni, 12% Co, 6% Cu,
remainder Fe
Brass
95–60% Cu, 5–40% Zn
Bronze
90% Cu, 10% Sn
Cu. Pure silver is soft and easily damaged, and the addition
of copper makes the metal more rigid. You can confirm that
an article of jewelry is sterling silver by looking for the stamp
that says “925,” which means 92.5% silver.
Gold used in jewelry is rarely pure (24 Carat) gold.
More often, you will find 18 K, 14 K, or 9 K stamped in a
gold object, referring to alloys that are 18/24, 14/24, or
9/24 gold. The 18 K “yellow” gold is 75% gold, and the
remaining 25% is copper and silver. As with sterling silver,
the added metals lead to a harder and more rigid material
(and one that is less costly).
Alloys fall in three general classes: solid solutions, which
are homogeneous mixtures of two or more elements; heterogeneous mixtures; and intermetallic compounds.
In solid solutions, one element is usually considered the
“solute” and the other the “solvent.” As with solutions in
liquids, the solute atoms are dispersed throughout the solvent such that the bulk structure is homogeneous. Unlike
liquid solutions, however, there are limitations on the size
of solvent and solute atoms. For a solid solution to form,
the solute atoms must be incorporated in such a way that
the original crystal structure of the solvent metal is preserved. Solid solutions can be achieved in two ways: with
solute atoms as interstitial atoms or as substitutional atoms
in the crystalline lattice. In interstitial alloys, the solute
atoms occupy the interstices, the small “holes” between solvent atoms (Figure 4a). The solute atoms must be substan-
Alloys: Mixtures of Metals
Pure metals often do not have the ideal properties needed
for their typical uses. It may be possible, however, to improve
their properties by adding one or more other elements to
the metal to form an alloy (Table 1). In fact, most metallic
objects we use are alloys, mixtures of a metal with one or
more other metals or even with a nonmetal such as carbon
(as in carbon steel). For example, sterling silver, commonly
used for jewelry, is an alloy composed of 92.5% Ag and 7.5%
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(a) Interstitial atoms
(b) Substitutional atoms
Figure 4 Alloys. (a) The solute atoms may be interstitial atoms, fitting into
holes in the crystal lattice. (b) The solute atoms can also substitute for one
of the lattice atoms.
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Figure 5 Photomicrograph of the surface of a heterogeneous alloy of
lead and tin.
tially smaller than the metal atoms making up the lattice to
fit into these positions. In substitutional alloys, the solute
atoms replace one of the solvent atoms in the original crystal structure (Figure 4b). For this to occur, the solute and
solvent atoms must be similar in size.
If the size constraints are not met, then the alloy will
likely form a heterogeneous mixture. When viewed under
a microscope, regions of different composition and crystal
structure can be seen in heterogeneous alloys (Figure 5).
For a solid solution to form, the electronegativities of
the alloy components must also be similar. When the two
metals have different electronegativities, the possibility exists for intermetallic compounds, substances with a definite
stoichiometry and formula. Examples of intermetallic
compounds include CuAl2, Mg2Pb, and AuCu3. In general,
intermetallic compounds are likely when one element is
relatively electronegative and the other is more electropositive. For Mg2Pb, for example, ␹ for Pb 2.3 and ␹ for
Mg 1.3 (␹ 1.0).
The macroscopic properties of an alloy will vary, depending on the ratio of the elements in the mixture. For
example, “stainless” steel is highly resistant to corrosion
and is roughly five times stronger than carbon and lowalloy steels. Melting point, electrical resistance, thermal
conductivity, ductility, and other properties can be similarly adjusted by changing the composition of the alloys.
Metals and their alloys are good examples of how changes
in the atomic composition and structure of a crystalline substance can have profound effects on its macroscopic chemical and physical characteristics. The same is true in semiconductors, the next class of materials we want to explore.
from semiconductors to essentially have “on” and “off”
states, which form the basis of the binary logic used in
computers. We can understand how semiconductors function by looking at their electronic structure, following the
band theory approach used for metals.
Bonding in Semiconductors: The Band Gap
The Group 4A elements carbon (in the diamond form),
silicon, and germanium have similar structures. Each atom
is surrounded by four other atoms at the corners of a tetrahedron (Figure 6). Using the band model of bonding,
the orbitals of each atom are combined to form molecular
orbitals that are delocalized over the solid. Unlike metals,
however, the result for carbon, silicon, and germanium is
two bands, a lower-energy valence band and a higherenergy conduction band. In metals, there is only a small
energy barrier for an electron to go from the filled molecular orbitals to empty molecular orbitals, and electricity
can flow easily. In electrical insulators, such as diamond,
and in semiconductors, such as silicon and germanium,
the valence and conduction bands are separated from each
other resulting in a band gap, a barrier to the promotion
of electrons to higher energy levels (see Figure 3). In the
Group 4A elements, the orbitals of the valence band are
completely filled, but the conduction band is empty.
The band gap in diamond is 580 kJ/mol—so large that
electrons are trapped in the filled valence band and cannot
make the transition to the conduction band, even at elevated temperatures. Thus, it is not possible to create positive “holes,” and diamond is an insulator, a nonconductor.
Semiconductors, in contrast, have a smaller band gap. For
common semiconducting materials, this band gap is usu-
Charles D. Winters
©Dr. James Marrow, Manchester Materials Science
Center, UMIST and University of Manchester
660 | The Chemistry of Modern Materials
Semiconductors
Semiconducting materials are at the heart of all solid-state
electronic devices, including such well-known devices as
computer chips and diode lasers. Semiconductors will not
conduct electricity easily but can be encouraged to do so
by the input of energy. This property allows devices made
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Figure 6 The structure of diamond. The structures of silicon and germanium are similar in that each atom is bound tetrahedrally to four others.
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Semiconductors | 661
ally in the range of 10 to 240 kJ/mol. (The band gap is
106 kJ/mol in silicon, whereas it is 68 kJ/mol in germanium.)
The magnitude of the band gap in semiconductors is such
that these substances are able to conduct small quantities of
current under ambient conditions, but, as their name implies, they are much poorer conductors than metals.
Semiconductors can conduct a current because thermal
energy is sufficient to promote a few electrons from the
valence band to the conduction band (Figure 7). Conduction
then occurs when the electrons in the conduction band
migrate in one direction and the positive holes in the valence band migrate in the opposite direction.
Pure silicon and germanium are called intrinsic semiconductors, with the name referring to the fact that this
is an intrinsic property of the pure material. In intrinsic
semiconductors, the number of electrons in the conduction band is determined by the temperature and the magnitude of the band gap. The smaller the band gap, the
smaller the energy required to promote a significant number of electrons. As the temperature increases, more electrons are promoted into the conduction band, and a
higher conductivity results.
In contrast to intrinsic semiconductors are materials
known as extrinsic semiconductors. The conductivity of
these materials is controlled by adding small numbers of
different atoms (typically 1 in 106 to 1 in 108) called dopants.
That is, the characteristics of semiconductors can be
changed by altering their chemical makeup, just as the properties of alloys differ from the properties of pure metals.
Suppose a few silicon atoms in the silicon lattice are replaced by aluminum atoms (or atoms of some other Group
3A element). Aluminum has only three valence electrons,
whereas silicon has four. Four Si-Al bonds are created per
aluminum atom in the lattice, but these bonds must be
deficient in electrons. According to band theory, the Si-Al
bonds form a discrete band at an energy level higher than
the valence band. This level is referred to as an acceptor
level because it can accept electrons. The gap between the
valence band and the acceptor level is usually quite small,
so electrons can be promoted readily to the acceptor level.
The positive holes created in the valence band are able to
move about under the influence of an electric potential, so
current results from the hole mobility. Because positive
holes are created in an aluminum-doped semiconductor,
this is called a p-type semiconductor (Figure 7b, left).
Now suppose phosphorus atoms (or atoms of some
other Group 5A element such as arsenic) are incorporated
into the silicon lattice instead of aluminum atoms. The
material is also a semiconductor, but it now has extra electrons because each phosphorus atom has one more valence electron than the silicon atom it replaces in the lattice. Semiconductors doped in this manner have a discrete,
partially filled donor level that resides just below the conduction band. Electrons are promoted readily to the conduction band from this donor band, and electrons in the
conduction band carry the charge. Such a material, consisting of negative charge carriers, is called an n-type semiconductor (Figure 7b, right).
One group of materials that have desirable semiconducting
properties is the III-V semiconductors, so called because they
are formed by combining elements from Group 3A (such as
Ga and In) with elements from Group 5A (such as As or Sb).
GaAs is a common semiconducting material that has electrical conductivity properties that are sometimes preferable
to those of pure silicon or germanium. The crystal structure
of GaAs is similar to that of diamond and silicon; each Ga
INTRINSIC SEMICONDUCTOR
EXTRINSIC SEMICONDUCTORS (DOPED)
p-type
Electrical Potential Applied
Group 3A
atoms added
n-type
Group 5A
atoms added
Conduction
band
Conduction
band
Conduction
band
(provides constant
supply of holes)
Acceptor level
ENERGY
Holes move toward
the negative pole
ENERGY
Positive
hole
Band gap
in pure
silicon
ENERGY
ENERGY
Electrons move toward
the positive pole
Donor level
(provides constant
supply of electrons)
Valence
band
Valence
band
(a)
Valence
band
(b)
Figure 7 Intrinsic and extrinsic semiconductors.
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662 | The Chemistry of Modern Materials
Charles D. Winters
atom is tetrahedrally coordinated to
four As atoms, and vice versa. This
Leads
structure is often referred to as the Semiconductor
zinc blende structure (Figure 13.10).
It is also possible for Group 2B
and 6A elements to form semiconLens
ducting compounds, such as CdS.
The farther apart the elements are Figure 8 Light-emitting diodes (LEDs). (left) A schematic
found in the periodic table, how- drawing of a typical LED. (right) Traffic signs with LED light
ever, the more ionic the bonding require much less energy input than incandescent lights and
becomes. As the ionic character of are now being widely used.
the bonding increases, the band gap
will increase, and the material will become an insulator rather
by hooking the positive terminal of a battery to the p-type
than a semiconductor. For example, the band gap in GaAs is
semiconductor and the negative terminal to the n-type semi140 kJ/mol, whereas it is 232 kJ/mol in CdS.
conductor. Negative electrons move from the n-type to the
These materials can be modified further by substituting
p-type, and positive holes move from the p-type to the n-type.
other atoms into the structure. For example, in one widely
When electrons move across the p–n junction, they can drop
used semiconductor, aluminum atoms are substituted for galfrom the conduction band into a hole in the valence band
lium atoms in GaAs, giving materials with a range of compositions (Ga1–xAlxAs). The importance of this modification is
LIGHT-EMITTING DIODE (LED)
that the band gap depends on the relative proportions of the
elements, so it is possible to control the size of the band gap
by adjusting the stoichiometry. As Al atoms are substituted for
Ga atoms, for example, the band gap energy increases. This
consideration is important for the specific uses of these map-type
n-type
terials in devices such as LEDs.
Applications of Semiconductors:
Diodes, LEDs, and Transistors
14a_ICh_0656-0669.indd 662
ENERGY
The combination of p- and n-type semiconducting materials in a single electronic device launched the microelectronics and computer industries. When a semiconductor
is created such that it is p-type on one half and n-type on
the other, a marvelous device known as the p–n rectifying
junction, or diode, results. Diodes, which allow current to
flow easily in only one direction when a voltage is applied,
are the fundamental building blocks of solid-state electronic devices. They are used for many circuitry applications, such as switching and converting between electromagnetic radiation and electric current.
LEDs, or light-emitting diodes (Figure 8), are now used
in the lights in the dashboards of cars and in their rear
warning lights, in traffic lights, and in toys. These semiconducting devices are made by combining elements such
as gallium, phosphorus, arsenic, and aluminum. When attached to a low-voltage (say 6–12 V) source, they emit light
with a wavelength that depends on their composition.
Furthermore, they emit light with a brightness that rivals
standard incandescent lights, and the light can be focused
using a tiny plastic lens.
An LED has a simple construction. It consists of a
p-type semiconductor joined to an n-type semiconductor
(Figure 9). A voltage is applied to the material, perhaps
Conduction band,
Ec
Electrons
Fermi level, Ef
Holes
When electrons and holes
meet at the p–n junction,
energy is evolved as light.
Valence band, Ev
p–n junction
Figure 9 Mechanism for the emission of light from an LED constructed
from n- and p-type semiconductors. When p- and n-type semiconductors are
joined, the energy levels adjust so that the Fermi levels (Ef) are equal. This
causes the energy levels of the conduction (Ec) and valence (Ev) bands to
“bend.” Also, holes flow from the p side to the n side, and electrons flow from
n to p until equilibrium is reached. No more charge will flow until a voltage is
applied. When an electric field is applied, occasionally electrons in the conduction band will move across the band gap and combine with holes in the valence
band. Energy is then evolved as light. The energy of the emitted light is approximately equal to the band gap. Therefore, by adjusting the band gap, the color
of the emitted light can be altered. (See S. M. Condren, et al.: Journal of
Chemical Education, Vol. 78, pp. 1033–1040, 2001.)
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Ceramics | 663
Figure 10 Gallium arsenide
n–p–n composition. This arrangement forms a device
known as a transistor. A transistor amplifies an electrical
signal, making it ideal for powering loudspeakers, for example. Transistors can also be used for processing and
storing information, a critical function for computer chips.
By combining thousands of these transistors and diodes,
an integrated circuit can be made that is the basis of what
we commonly refer to as computer chips, devices for controlling and storing information (Figure 11).
NASA JPL
(GaAs) solar panel. This panel
was built for NASA’s Deep Space
1 probe. The array uses 3600
solar cells, which convert light
to electricity to power an ion
propulsion system. (DS1 was
launched on October 24, 1998,
and sent back images of Comet
Borrelly in deep space. The
spacecraft was retired on
December 18, 2001.)
Ceramics
Let’s go back to our original examples of various forms of
CaCO3, including sea shells and chalk. Chalk is so soft that
it will rub off on the rough surface of a blackboard. In
contrast, sea shells are inherently tough. They are designed
to protect their soft and vulnerable inhabitants from the
powerful jaws of sea-borne predators or rough conditions
underwater. Chalk, sea shells, and the spines of sea urchins
(Figure 12) are all ceramics, but they are obviously different from one another. Clearly, there is a great deal of
variability in this class of materials.
You may be accustomed to thinking of “ceramics” as the
objects that result from high-temperature firing, such as
pottery. From a materials chemistry perspective, however,
other materials such as clay, which largely consists of hydrated silicates of various compositions, are also considered ceramics. Ceramics are solid inorganic compounds.
Their composition includes metal and nonmetal atoms,
and the bonding between atoms ranges from very ionic to
covalent (䉳 Section 9.2). In general, ceramics are hard,
relatively brittle, and inflexible, and they are usually good
thermal insulators. Some ceramics can be electrically conductive, but most are electrical insulators. Some, like glass,
another type of ceramic, can be optically
transparent, whereas other ceramics are completely opaque.
It is also possible to have ceramics in which
impurity atoms are included in the composition. As we saw with metals and semiconductors, impurity atoms can have dramatic effects
on the characteristics of a material.
Jan Hinsch/Photo Researchers, Inc.
© Will & Deni McIntyre/Photo Researchers, Inc.
of the p-type semiconductor, and energy is released as light.
(The mechanism of light emission by an LED is similar to
that described for excited atoms in Section 6.3.) If the band
gap energy is equivalent to the energy of light in the visible
region, light can be observed. Because the band gap energy
can be adjusted by changing the composition of the doped
semiconductor, the wavelength of the light can also be altered, giving light of different colors.
The same device that forms the LED can be run in
reverse to convert light that falls on it into an electrical
signal. Solar panel cells work in this manner (Figure 10).
They are generally GaAs-based p–n junction materials that
have a band gap corresponding to the energy of visible
light. When sunlight falls on these devices, a current is
induced. That current can be used either immediately or
stored in batteries for later use. A similar technology is
used in simpler devices referred to as photodiode detectors. They have an abundance of applications, ranging
from the light-sensitive switches on elevator doors to sensitive detection equipment for scientific instruments.
The p- and n-type semiconductor materials can also be
constructed into a sandwich structure of either p–n–p or
Figure 11 Integrated circuits. (left) A wafer on which a large number of integrated circuits has
been printed. (right) A close-up of a semiconductor chip showing the complex layering of circuits
that is now possible.
14a_ICh_0656-0669.indd 663
Glass: A Disordered Ceramic
An amorphous, or noncrystalline, solid material is generally referred to as a glass (see
Section 13.4). Glasses are formed by melting
the raw material and then cooling it from the
liquid state rapidly so that the component
atoms do not have time to crystallize into a
regular lattice structure. A wide range of materials, including metals and organic poly-
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664 | The Chemistry of Modern Materials
© Jeffrey L. Rotman/Corbis
13c). Because the network is changed by such an addition,
these network modifiers can dramatically alter the physical
characteristics of the material, such as melting point, color,
opacity, and strength. Soda-lime glass—made from SiO2,
Na2O (soda), and CaO (lime)—is a common glass used in
windows and for containers. The metal oxides lower the
melting temperature by about a thousand degrees from
that of pure silica. Pyrex glass, also called borosilicate glass,
incorporates an additional component, boric oxide. The
boric oxide raises the softening temperature and minimizes the coefficient of thermal expansion, enabling the
glass to better withstand temperature changes. Because of
its excellent thermal properties, this type of glass is used
for beakers and flasks in chemistry laboratories and for
ovenware for the kitchen.
An important characteristic of some glasses is their optical transparency, which allows them to be used as windows and lenses. Glasses can also be reflective. The combination of transparency and reflectivity is controlled by
the material’s index of refraction. All materials have an
index of refraction that determines how much a beam of
light will change its velocity when entering the material.
The index of refraction is defined relative to the speed of
light in a vacuum, which is defined as exactly 1. (The index
of refraction velocity of light in a vacuum/velocity in
material.) On this basis, dry air has an index of refraction
of 1.0003, and typical values for silicate glasses range from
1.5 to 1.9.
The change in the velocity of the electromagnetic wave
once it enters the material causes the beam to bend, or
change direction within the material. If light hits a surface
at some incident angle relative to the line perpendicular
to the surface, some of the light will be reflected at the
same angle, and some will be transmitted into the material
at a refracted angle (Figure 14). Both the incident angle
Figure 12 A sea urchin. The spines of the urchin are composed primarily
of CaCO3, but a significant amount of MgCO3 is present as well.
mers, can be coaxed into a glassy form. However, the bestknown glasses are silicate glasses. These are derived from
SiO2, which is plentiful, inexpensive, and chemically unreactive. Each silicon atom is linked to four oxygen atoms
in the solid structure, with a tetrahedral arrangement
around each silicon atom. The SiO2 units are linked together to form a large network of atoms (Figure 13). Over
a longer distance, however, the network has no discernible
order or pattern.
Glasses can be modified by the presence of alkali metal
oxides (such as Na2O and K2O) or other metal or nonmetal oxides (such as CaO, B2O3, and Al2O3). The added
impurities change the silicate network and alter the properties of the material. The oxide ions are incorporated into
the silicate network structure, and the resultant negative
charge is balanced by the interstitial metal cations (Figure
O
Na
Si
(a)
(b)
(c)
Figure 13 Representation of glass structure. (a) Silica glass (SiO2) may have some order over a short distance but much less
order over a larger portion of the solid (b). (c) The SiO2 structure can be modified by adding metal oxides, which leads to a lower
melting temperature and other desirable properties. (In this simple representation, the gray Si atoms are shown at the center of a
planar triangle of red O atoms; in reality, each Si atom is surrounded tetrahedrally by O atoms. The structure is not planar but is
three dimensional.)
14a_ICh_0656-0669.indd 664
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Ceramics | 665
Incident
light
Reflected
light
i
r
Refracted
light
(a)
Charles D. Winters
i angle of incidence
r angle of refraction
ir
i
(b)
Figure 14 Refraction of light. (a) When light enters a different medium,
its velocity changes. This causes the path of a photon to change direction in
the material. (b) Observing an object in a glass of water illustrates the effect
of light refraction.
Courtesy Dr. Joanna Aizenberg
Simon Fraser/Photo Researchers, Inc.
and the index of refraction will affect how much of the
light is reflected and the angle at which it bends in the
second material. You can observe this effect by putting an
object in a glass of water and looking at the apparent bend
that results in the object (Figure 14b).
This combination of the transmission and reflection
characteristics of glass has allowed scientists and engineers
to develop optical fibers (Figure 15). Optical fibers are
designed to have a property called total internal reflection,
whereby all the light that enters at one end of the fiber
stays within the fiber through reflections with the interior
surface as the light travels from one end of the fiber to
Figure 15 Optical fibers. (left) Glass fibers transmit light along the axis of
the fiber. (right) Bell Laboratory scientist Joanna Aizenberg recently discovered that a deep-sea sponge, made chiefly of silica (SiO2), has a framework
that has the characteristics of optical fibers. (For more about the structure
of the sponge, see Figure 2 on page 27.)
14a_ICh_0656-0669.indd 665
the other. Total internal reflection in these fibers is
achieved by controlling the ratio of the indices of refraction between the fiber’s core and its outside surface.
Chemically, the index of refraction is controlled by adjusting the quantity and type of cationic network modifiers
that are added to the glass. The index of refraction of a
glass fiber can be controlled so that it has one value at the
core of the fiber but changes smoothly across the radius
of the fiber to a different value at the surface. This is accomplished by an ion-exchange process during fiber production in which, for example, K ions are replaced by
Tl ions.
Optical fibers are transforming the communications industry in an amazing fashion. Instead of transmitting information using electrons traveling through metallic wires,
optical fibers allow communication to occur by transmitting
photons through glass fiber bundles. Signal transmission by
optical fibers, known as photonics, is much faster and more
economical than transmission using copper wires and cables. For example, the quantity of copper required to carry
the equivalent amount of information transmitted by optical
fiber would weigh 300,000 times more than the optical fiber
material!
Fired Ceramics for Special Purposes:
Cements, Clays, and Refractories
Other classes of ceramics include cements, clays, and refractories. Unlike glasses, these ceramics are processed by
shaping, drying, and then firing, without ever melting the
solid.
Cements are extremely strong and are commonly used
as structural materials. They can be formed into almost any
shape. When mixed with water, they produce a paste that
can be poured into molds and allowed to dry and harden.
Clays are generally mixtures of hydrated alumina
(Al2O3) and silica (SiO2), but may also contain other ingredients, such as tricalcium silicate, (3 CaO · SiO2), dicalcium silicate, (2 CaO · SiO2), and MgO. Their composition is irregular, and, because they are powders, their
crystallinity extends for only short distances.
Clays have the useful property of becoming very plastic
when water is added, a characteristic referred to as hydroplasticity. This plasticity, and clay’s ability to hold its shape
during firing, are very important for the forming processes
used to create various objects.
The layered molecular structure of clays results in microscopic platelets that can slide over each other easily
when wet. The layers consist of SiO4 tetrahedra joined with
AlO6 octahedra (see Section 21.7). In addition to these
basic silicon- and aluminum-based structures, different
cations can be substituted into the framework to change
the properties of the clay. Common substituents include
Ca2, Fe2, and Mg2. Different clay materials can then be
created by varying the combinations of layers and the substituent cations.
12/28/07 10:30:55 AM
now called piezoelectricity, is the property that allows a mechanical distortion
(such as a slight bending) to induce an
electrical current and, conversely, an
electrical current to cause a distortion in
the material.
Not all crystalline ceramics exhibit
piezoelectricity. Those that do have a specific unit cell structure (Section 13.2) that
can loosely trap an impurity cation. The
ion’s position shifts when the unit cell is
deformed by mechanical stress. This shift
causes an induced dipole (see Section
12.3) and, therefore, a potential difference
across the material that can be converted
Figure 16 Aerogel, a networked matrix of SiO2. (left) NASA’s Peter Tsou holds a piece of aerogel. It is to an electrical signal.
In addition to the minerals originally
99.8% air, is 39 times more insulating than the best fiberglass insulation, and is 1000 times less dense
tested
by the Curie brothers, materials
than glass. (right) Aerogel was used on a NASA mission to collect the particles in comet dust. The particles entered the gel at a very high velocity, but were slowed gradually. Scientists studied the tracks made
known to exhibit the piezoelectric effect
by the particles and later retrieved the particles and studied their composition.
include titanium compounds of barium
and lead, lead zirconate (PbZrO3), and
Refractories constitute a class of ceramics that are caammonium dihydrogen phosphate (NH4H2PO4).
pable of withstanding very high temperatures without
Materials that exhibit piezoelectricity have a great
deforming, in some cases up to 1650 °C (3000 °F), and
many applications, ranging from home gadgets to sophisthat are thermally insulating. Because of these properties,
ticated medical and scientific applications. One use with
refractory bricks are used in applications such as furnace
which you may be familiar is the automatic ignition syslinings and in metallurgical operations. These materials
tems on some barbecue grills and lighters (Figure 17).
are thermally insulating largely because of the porosity
All digital watch beepers are based on piezoceramics, as
of their structure; that is, holes (or pores) are dispersed
are smoke detector alarms. A less familiar application
evenly within the solid. However, while porosity will make
is found in the sensing lever of some atomic force mia material more thermally insulating, it will also weaken
croscopes (AFMs) and scanning-tunneling microscopes
it. As a consequence, refractories are not as strong as
(STMs), instruments that convert mechanical vibrations
cements.
to electrical signals.
An amazing example of the use of porosity to increase
Scientists and engineers are always searching for materials
the insulating capacities of a ceramic is found in a material
with new and useful properties. Perhaps the most dramatic
developed at NASA called aerogel (Figure 16; see Case
property that has been observed in newly developed ceramStudy: The World’s Lightest Solid, page 607). Aerogel is
ics is superconductivity at relatively high temperatures.
more than 99% air, with the remainder consisting of a
networked matrix of SiO2. This makes aerogel about 1000
times less dense than glass but gives the material extraordinary thermal insulating abilities. NASA used aerogel on
a mission in which a spacecraft flew through the tail of the
comet Wild 2 and returned to Earth with space particles
embedded in the aerogel.
Modern Ceramics with Exceptional Properties
In 1880, Pierre Curie and his brother Jacques worked in
a small laboratory in Paris to examine the electrical properties of certain crystalline substances. Using nothing
more than tin foil, glue, wire, and magnets, they were able
to confirm the presence of surface charges on samples of
materials such as tourmaline, quartz, and topaz when they
were subjected to mechanical stresses. This phenomenon,
14a_ICh_0656-0669.indd 666
Charles D. Winters
NASA JPL
666 | The Chemistry of Modern Materials
Figure 17 Devices that depend on the piezoelectric effect. These devices
work by using a mechanical stress to produce an electric current. Piezoelectric
devices are widely used in ignitors and in devices that convert electric impulses
to vibrations, such as in the timing circuit of a wristwatch.
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Biomaterials: Learning from Nature | 667
Courtesy of University of Kentucky Public Relations
Once again, we see that combining atoms into sometimes complex chemical compositions allows scientists to
develop materials with particular properties. In ceramics,
which are normally electrically insulating, this includes
even the ability to conduct electricity.
Biomaterials: Learning
from Nature
Most of the materials described so far in this chapter come
from nonliving sources and, in many cases, are the result
of laboratory syntheses. However, an important branch of
materials research deals with examining, understanding,
and even copying materials produced by living systems.
The study of naturally occurring materials has led to
the development of synthetic materials that possess important properties. A good example is rubber (Chapter 10,
page 483). The polymer we know as rubber was initially
obtained from certain trees and chemically modified to
convert it to a useful material. Natural rubber was found
to be so useful that chemists eventually achieved the synthesis of a structurally identical material. Research on rubber, which spanned more than 200 years, has had important consequences for humans as evidenced by the myriad
applications of rubber today.
Today, scientists continue to look to nature to provide
new materials and to provide clues to improve the materials we already use. The sea urchin and its ceramic spines
(Figure 12) and the sponge whose skeleton has the characteristics of optical fibers (Figure 15) are just two examples where biomaterials research has focused on sea
life in a search for new materials. Scientists have also examined conch shells to understand their incredible fracture strength. They used scanning electron microscopy
(SEM) to scrutinize the structure of the shell when it was
fractured. What they discovered was a criss-crossed, layered structure that is the equivalent of a “ceramic plywood” (Figure 20). This microarchitecture prevents fractures that occur on the outside surface of the shell from
Figure 18 Superconductivity. When a superconducting material is cooled
to a low temperature, say in liquid nitrogen (boiling point is 77 K), it generates a very strong magnetic field. In this photo, a 1-pound magnet is levitated in the field created by the cooled superconductor.
Copyright © 2000 Nature Publishing Inc.
Used with permission.
Superconductivity is a phenomenon in which the electrical
resistivity of a material drops to nearly zero at a particular
temperature referred to as the critical temperature, Tc
(Figure 18). Most metals naturally have resistivities that decrease with temperature in a constant manner but still have
significant resistivity even at temperatures near 0 K.
A few metals and metal alloys have been found to exhibit
superconductivity. For metals, however, the critical temperatures are extremely low, between 0 and 20 K. These temperatures are costly to achieve and difficult to maintain.
Recent scientific attention has, therefore, focused on a class
of ceramics with superconductive critical temperatures near
100 K. These materials include YBa2Cu3O7, with Tc 92 K
(Figure 19), and HgBa2Ca2Cu2O3, with Tc 153 K.
Figure 19 The lattice of YBa2Cu3O7, a superconductor. Yttrium ions are
yellow; barium ions are red; copper ions are green; and oxygen ions are
blue. (Reprinted with permission of Dr. Klaus Hermann of the Fritz Haber
Institution.)
14a_ICh_0656-0669.indd 667
Figure 20 A scanning electron microscope picture of the shell of the
conch. Photos from S. Kamat, X. Su, R. Ballarini, and A. H. Heuer. Structural
basis for the fracture toughness of the shell of the conch Strombus gigas.
Nature. Vol. 405, pp. 1036–1040, 2000.
12/28/07 10:30:59 AM
668 | The Chemistry of Modern Materials
Jonathan Wilker, Purdue University
being transferred into the inner layers. Figure 21 Strong mussels. (left) A common blue mussel can cling to
A
The discovery has inspired materials engi- almost any surface, including this Teflon sheet, even underwater. (right) The HNHECH3
A
neers to create materials that are signifi- adhesive precursor is a protein interlinked with iron(III) ions. Side chains
KHNH
O
A
cantly strengthened by incorporating a on the protein are dihydroxyphenylalanine (DOPA), and an iron(III)
KO OH
A
fibrous ceramic matrix, such as SiC (silicon ion binds to the hydroxyl groups (–OH) in three side chains.
A
HNH H
O
carbide) whiskers.
CH3
CH3 N
B
A
A
In another area of research focusing on
KHN
N O
E
O
O
HO
A
A
sea creatures, the connective tissues of sea
O O
ON
O
NH
A
E
ONE
¨ {
cucumbers and other echinoderms (marine
A
Fe
A
O
E
HO ; A ' O
invertebrates with tube feet and calciteG
HN
O A
A
covered, radially symmetrical bodies) have
H3CH NO
A
been studied in an attempt to discover how
HNE
NH
O
these animals can reversibly control the stiffE
M D
D G
O
O
ON
ness of their outer skin. The connective tisCH3
B
B
O
N
H
M
E
E D
sues of these animals include the protein
H O
O
N
D
collagen in a cross-linked fiber structure,
EH
H3C
OH
similar to the dermis, an inner layer of the
skin consisting of sensitive connective tissue
of many mammals. At the same time, other proteins and
Scientists who have researched mussel adhesives have
soluble molecules in the echinoderm system allow the anibeen able to determine that the amino acid 3,4-dihydroxymals to change the characteristics of the connective tissue
phenylalanine (DOPA) is the agent primarily responsible
in response to their nervous system. As a result, creatures
for the strength of the adhesion. But DOPA alone cannot
such as sea cucumbers can move about and, in some cases,
explain the incredible strength of the mussel glues. The
can defend themselves by hardening their skin to an almost
secret lies in the combination of an Fe3 ion with DOPA
to form a cross-linked matrix of the mussel’s protein
shell-like consistency. The ensuing laboratory research has
(Figure 21). The curing process, or hardening of the natfocused on the formulation of a synthetic collagen-based
ural proteinaceous liquid produced by the mussel, is a
polymer composite material in which the stiffness can be
result of the iron–protein interaction that occurs to form
changed repeatedly through a series of oxidation and reducFe(DOPA)3 cross-links.
tion reactions. Scientists are now developing models for
synthetic skin and muscle based on their findings.
Research on adhesive materials represents another area
in which sea creatures can provide some clues. Getting
things to stick together is important in a multitude of apThe Future of Materials
plications. The loss of the space shuttle Columbia in early
The modern tools and techniques of chemistry are making
2003, caused by loss of some ceramic tiles when a piece of
it possible for scientists not only to develop novel materials,
insulating foam that fell off during launch hit them, ofbut also to proceed in new and unforeseen directions. The
fered a sobering lesson in adhesive failure under extreme
field of nanotechnology is an example. In nanotechnology,
conditions of temperature and humidity. If you look
structures with dimensions on the order of nanometers
around, you will probably find something with an adhesive
are used to carry out specific functions. For example, scilabel, something with an attached plastic part, something
entists can now create a tube of carbon atoms embedded
with a rubber seal, or perhaps something taped together.
in a slightly larger carbon tube to act as a ball bearing at
Adhesives have also proven useful for medical applications,
the molecular level (Figure 22).
where specialized glues help doctors seal tissues within the
Nanoscience has provided profoundly important applihuman body. For every type of sticking application, differcations for medicine, computing, and energy consumpent properties are needed for the adhesive material.
tion. For example, scientists have developed quantum dots
Nature provides numerous examples of adhesion.
(Figure 23), nanometer-scale crystals of different materials
Geckos and flies that can walk on glass while completely
that can emit light and can even be made to function as
inverted hold clues to the kind of biologically based adhelasers. Quantum dots have been used as biological markers
sion that could be the basis of synthetic analogs. Marine
by attaching them to various cells. By shining light on
mussels, which can stick quite well to wood, metal, and
them, the quantum dots will fluoresce in different colors,
rock, also hold great interest for scientists studying adheallowing the cells to be imaged.
sion (Figure 21).
14a_ICh_0656-0669.indd 668
12/28/07 10:31:01 AM
The Future of Materials | 669
Jian-Min You, University of Illinois, Urbana-Champaign, originally published
in Science 300, 1419–1421 (2003)
materials to achieve different properties for special functions. In many cases, we can look to nature to provide
answers and suggestions on how to proceed.
SUGGESTED READ INGS
1. S. M. Sze: Semiconductor Devices: Physics and Technology.
New York: John Wiley & Sons, 1985.
2. X. Su, S. Kamat, and A. H. Heuer: “The structure of sea
urchin spines, large biogenic single crystals of calcite.”
Journal of Materials Science, Vol. 35, pp. 5545–5551, 2000.
3. M. J. Sever, J. T. Weisser, J. Monahan, S. Srinivasan, and
J. J. Wilker: “Metal-mediated cross-linking in the generation of marine mussel adhesive.” Angewandte Chemie
International Edition, Vol. 43, pp. 447–450, 2004.
Figure 22 A molecular bearing: double-walled carbon nanotubes. (left)
An electron diffraction image of a double-walled carbon nanotube. (right) A
model of the material.
Other ongoing research is being carried out on nanoscale
drug delivery, a technology that allows one to deliver medicinal agents directly into the cells that need them.
One way that scientists have been able to achieve such
breakthroughs is by studying the structures of materials
that are already known. They have been learning to manipulate atoms and molecules so that they will arrange
themselves in specific ways to achieve desired shapes and
functions. In a process referred to as self-assembly, molecules or atoms will arrange themselves based on their
shapes, the intermolecular forces between them, and their
interactions with their environment.
Chemistry is the key to understanding and developing
materials. The atomic compositions and long-term atomic
arrangements of different materials fundamentally determine their properties and characteristics. Chemists can use
analytical instruments to determine these structures. They
can then exploit this knowledge to manipulate or develop
© Robert Rathe
Figure 23 Quantum
14a_ICh_0656-0669.indd 669
dots. Quantum dots are a
special kind of semiconductor. They have diameters in the 2–10 nm range
and have unique properties. Among these is the
ability to tune the band
gap so different colors
result on excitation.
STUDY QUESTIONS
Blue-numbered questions have answers in Appendix P and
fully-worked solutions in the Student Solutions Manual.
1. What is the maximum wavelength of light that can excite an electron transition across the band gap of
GaAs? To which region of the electromagnetic spectrum does this correspond?
2. Which of the following would be good substitutional
impurities for an aluminum alloy?
(a) Sn
(b) P
(c) K
(d) Pb
3. The amount of sunlight striking the surface of the earth
(when the sun is directly overhead on a clear day) is approximately 925 watts per square meter (W/m2). The
area of a typical solar cell is approximately 1.0 cm2. If
the cell is running at 25% efficiency, what is its energy
output per minute?
4. Using the result of the calculation in Question 3, estimate the number of solar cells that would be needed
to power a 700-W microwave oven. If the solar cells
were assembled into a panel, what would be the approximate area of the panel?
5. Describe how you could calculate the density of pewter
from the densities of the component elements, assuming that pewter is a substitutional alloy. Look up the
densities of the constituent elements and carry out this
calculation. Densities can be found on a website such
as www.webelements.com, or go to ChemistryNOW and
click on the periodic table tool. Click on the symbol of
each of the elements in pewter. A table of atomic properties includes the element’s density.
6. Calculate an approximate value for the density of aerogel using the fact that it is 99% air, by volume, and the
remainder is SiO2 (Density 2.3 g/cm3). What is the
mass of a 1.0-cm3 piece of aerogel?
12/28/07 10:31:01 AM