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Chapter 1
Advanace Materials
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Advance Materials
Materials that are utilized in high-technology (or high-tech)applications are sometimes termed advanced materials. By hightechnology we mean a device or product that operates or functionsusing relatively intricate and sophisticated principles; examplesinclude electronic equipment (VCRs, CD players, etc.), computers,
fiber optic systems, spacecraft, aircraft, and military rocketry. Theseadvanced materials are typically either traditional materials whoseproperties have been enhanced or newly developed, high-performance materials. Furthermore, they may be of all materialtypes (e.g., metals, ceramics, polymers), and are normally relativelyexpensive.
In subsequent chapters are discussed the properties andapplications of a number of advanced materialsfor example,materials that are used for lasers, integrated circuits, magneticinformation storage, liquid crystal displays (LCDs), fiber optics, andthe thermal protection system for the Space Shuttle Orbiter.
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MODERN MATERIALS NEEDS
In spite of the tremendous progress that has been made in thediscipline of materials science and engineering within the past few years,there still remain technological challenges, including the development ofeven more sophisticated and specialized materials, as well asconsideration of the environmental impact of materials production.
Some comment is appropriate relative to these issues so as to roundout this perspective. Nuclear energy holds some promise, but thesolutions to the many problems that remain will necessarily involvematerials, from fuels to containment structures to facilities for thedisposal of radioactive waste.
Significant quantities of energy are involved in transportation.Reducing the weight of transportation vehicles (automobiles, aircraft,
trains, etc.), as well as increasing engine operating temperatures, willenhance fuel efficiency. New highstrength, low-density structuralmaterials remain to be developed, as well as materials that have higher-temperature capabilities, for use in engine components.
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Chapter 2
Atomic Structure and
Interatomic Bonding
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Why StudyAtomic Structure and
Interatomic Bonding?
An important reason to have an
understanding of interatomic bonding in solidsis that, in some instances, the type of bond
allows us to explain a materials properties.
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Why StudyAtomic Structure and
Interatomic Bonding?
This micrograph, which
represents the surface of a
gold specimen, was taken
with a sophisticated atomic
force microscope (AFM). Individualatoms for this (111)
crystallographic surface
plane are resolved. Also
note the dimensional scale
(in the nanometer range) below
the micrograph. (Imagecourtesy of Dr. Michael
Green, TopoMetrix Corporation.)
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INTRODUCTION
Some of the important properties of solid
materials depend on geometrical atomic
arrangements, and also the interactions that
exist among constituent atoms or molecules.
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FUNDAMENTAL CONCEPTS
Each atom consists of a very small nucleuscomposed ofprotons and neutrons, which isencircled by moving electrons. Both electronsand protons are electrically charged, the charge
magnitude being 1.60x10^-19 C, which isnegative in sign for electrons and positive forprotons; neutrons are electrically neutral. Massesfor these subatomic particles are infinitesimally
small; protons and neutrons have approximatelythe same mass, 1.67x10Z^-27 kg, which issignificantly larger than that of an electron,9.11x10^-31 kg.
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FUNDAMENTAL CONCEPTS
Each chemical element is characterized by
the number of protons in the nucleus, or the
atomic number(Z).1 For an electrically neutral
or complete atom, the atomic number also
equals the number of electrons. This atomic
number ranges in integral units from 1 for
hydrogen to 92 for uranium, the highest of thenaturally occurring elements.
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FUNDAMENTAL CONCEPTS
In chemistry and physics, the atomic
number(also known as the proton number) is
the number of protons found in the nucleus of
an atom and therefore identical to the charge
number of the nucleus.
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FUNDAMENTAL CONCEPTS
The atomic mass (A) of a specific atom maybe expressed as the sum of the masses of protonsand neutrons within the nucleus. Although the
number of protons is the same for all atoms of agiven element, the number of neutrons (N) maybe variable.
Atomic mass is the total mass of protons,
neutrons and electrons in a single atom is thetotal mass of protons, neutrons and electrons in asingle atom.
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FUNDAMENTAL CONCEPTS
Thus atoms of some elements have two or more different atomic
masses, which are called isotopes.
Isotopes. Atoms of the same element that have different atomic
masses.
The atomic weightof an element corresponds to the weightedaverage of the atomic masses of the atoms naturally occurring isotopes.2
The atomic mass unit(amu) may be used for computations of atomic
weight.
Atomic weight(A). The weighted average of the atomic masses of an
atoms naturally occurring isotopes. It may be expressed in terms ofatomic mass units (on an atomic basis), or the mass per mole of atoms.
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FUNDAMENTAL CONCEPTS
1 amu/atom (or molecule) = 1 g/mol
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ELECTRONS IN ATOMS - ATOMIC
MODELSDuring the latter part of the nineteenth century it was
realized that many phenomena involving electrons in solids
could not be explained in terms of classical mechanics. What
followed was the establishment of a set of principles and laws
that govern systems of atomic and subatomic entities thatcame to be known as quantum mechanics.
Quantum mechanics. A branch of physics that deals
with atomic and subatomic systems; it allows only discrete
values of energy that are separated from one another. Bycontrast, for classical mechanics, continuous energy values are
permissible.
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ELECTRONS IN ATOMS -ATOMIC
MODELSOne early outgrowth of quantum mechanics was the
simplified Bohr atomic model, in which electrons are assumed
to revolve around the atomic nucleus in discrete orbitals, and
the position of any particular electron is more or less well
defined in terms of its orbital.
Bohr atomic model. An early atomic model, in which
electrons are assumed to revolve around the nucleus in
discrete orbitals.
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ELECTRONS IN ATOMS - ATOMIC
MODELS
Schematic representation of the Bohr atom.
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ELECTRONS IN ATOMS - ATOMIC
MODELSAnother important quantum-mechanical
principle stipulates that the energies of electrons are
quantized; that is, electrons are permitted to have
only specific values of energy. An electron maychange energy, but in doing so it must make a
quantum jump either to an allowed higher energy
(with absorption of energy) or to a lower energy
(with emission of energy). Often, it is convenient tothink of these allowed electron energies as being
associated with energylevels or states.
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ELECTRONS IN ATOMS - ATOMIC
MODELS
(a) Thefirst three electron
energy states for the
Bohr hydrogen
atom.
(b) Electron energy
states for the first
three
shells of the
wavemechanical
hydrogenatom.
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ELECTRONS IN ATOMS - ATOMIC
MODELSBohr model was eventually found to have some
significant limitations because of its inability to
explain several phenomena involving electrons. A
resolution was reached with a wave-mechanicalmodel, in which the electron is considered to exhibit
both wavelike and particle-like characteristics.
Wave-mechanical model. Atomic model in
which electrons are treated as being wavelike.
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ELECTRONS IN ATOMS -
QUANTUM NUMBERS
Quantum numbers. A set of four
numbers, the values of which are used to label
possible electron states. Three of the quantum
numbers are integers, which also specify thesize, shape, and spatial orientation of an
electrons probability density; the fourth
number designates spin orientation.
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ELECTRONS IN ATOMS -
QUANTUM NUMBERSUsing wave mechanics, every electron in an atom is
characterized by four parameters called quantum numbers.
The size, shape, and spatial orientation of an electrons
probability density are specified by three of these quantum
numbers. Furthermore, Bohr energy levels separate intoelectron subshells, and quantum numbers dictate the number
of states within each subshell. Shells are specified by a
principal quantum number n, which may take on integral
values beginning with unity; sometimes these shells aredesignated by the letters K, L, M, N, O, and so on, which
correspond, respectively, to n 1, 2, 3, 4, 5, . . . .
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ELECTRONS IN ATOMS -
QUANTUM NUMBERS
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONS
Electron configuration is the distribution
of electrons of an atom or molecule (or other
physical structure) in atomic or molecular
orbitals.
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONSThe preceding discussion has dealt primarily with
electron statesvalues of energy that are permitted
for electrons. To determine the manner in which
these states are filled with electrons, we use thePauli exclusion principle, another quantum-
mechanical concept. This principle stipulates that
each electron state can hold no more than two
electrons, which must have opposite spins.
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONS
When all the electrons occupy the lowest
possible energies in accord with the foregoing
restrictions, an atom is said to be in its ground state.Theground state of a quantum
mechanical system is its lowest-energy state the
energy of the ground state is known as the zero-
point energy of the system
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONSThe valence electrons are those that occupy the
outermost filled shell. These electrons are extremely
important; as will be seen, they participate in the bonding
between atoms to form atomic and molecular aggregates.
Valence electron is an electron that is associated with
an atom, and that can participate in the formation of a
chemical bond; in a single covalent bond, both atoms in the
bond contribute one valence electron in order to form ashared pair.
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONS
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ELECTRONS IN ATOMS - ELECTRON
CONFIGURATIONS
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ATOMIC BONDING IN SOLIDS
BONDING FORCES AND ENERGIES
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDSPerhaps ionic bonding is the easiest to describe
and visualize. It is always found in compounds that
are composed of both metallic and nonmetallic
elements, elements that are situated at thehorizontal extremities of the periodic table. Atoms of
a metallic element easily give up their valence
electrons to the nonmetallic atoms.
Ionic bondis a type of chemical bond formedthrough an electrostatic attraction between two
oppositely charged ions.
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
The attractive bonding forces are
coulombic; that is, positive and negative ions,
by virtue of their net electrical charge, attract
one another.
Coulombic force. A force between
charged particles such as ions; the force is
attractive when the particles are of oppositecharge.
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
In covalent bonding stable electron
configurations are assumed by the sharing of
electrons between adjacent atoms. Two atoms that
are covalently bonded will each contribute at leastone electron to the bond, and the shared electrons
may be considered to belong to both atoms.
Covalent bond. A primary interatomic bond that
is formed by the sharing of electrons between
neighboring atoms.
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
Metallic bonding, the final primary bonding type, is
found in metals and their alloys. A relatively simple model has
been proposed that very nearly approximates the bonding
scheme. Metallic materials have one, two, or at most, three
valence electrons.
Metallic bond. A primary interatomic bond involving the
nondirectional sharing of nonlocalized valence electrons (sea
of electrons) that are mutually shared by all the atoms in the
metallic solid.
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ATOMIC BONDING IN SOLIDS -
PRIMARY INTERATOMIC BONDS
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIESSECONDARY BONDING OR VAN DER WAALS BONDING
Secondary, van der Waals, orphysical bonds are weak in
comparison to the primary or chemical ones; bonding
energies are typically on the order of only 10 kJ/mol (0.1
eV/atom). Secondary bonding exists between virtually allatoms or molecules, but its presence may be obscured if any
of the three primary bonding types is present. Secondary
bonding is evidenced for the inert gases, which have stable
electron structures, and, in addition, between molecules inmolecular structures that are covalently bonded.
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
Secondary bonds. Interatomic
anintermolecular bonds that are relatively
weak and for which bonding energies are
relatively small. Normally atomic or moleculardipoles are involved. Secondary bonding types
are van der Waals and hydrogen.
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
Secondary bonding forces arise from
atomic or molecular dipoles.
Dipole (electric). A pair of equal yet
opposite electrical charges that are separated
by a small distance.
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
Hydrogen bonding, a special type of secondary
bonding, is found to exist between some molecules
that have hydrogen as one of the constituents. These
bonding mechanisms are now discussed briefly.Hydrogen bond. A strong secondary interatomic
bond that exists between a bound hydrogen atom
(its unscreened proton) and the electrons of adjacent
atoms.
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
FLUCTUATING INDUCED DIPOLE BONDS
a.) A dipole may be created or induced in
an atom or molecule that is normally
electrically symmetric; that is, the overall
spatial distribution of the electrons is
symmetric with respect to the positively
charged nucleus.
ATOMIC BONDING IN SOLIDS
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
FLUCTUATING INDUCED DIPOLE BONDS
b.) All atoms are experiencing constant
vibrational motion that can cause
instantaneous and short-lived distortions of
this electrical symmetry for some of the atoms
or molecules, and the creation of small
electric dipoles.
ATOMIC BONDING IN SOLIDS
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIESPOLAR MOLECULE-INDUCED DIPOLE BONDS
Permanent dipole moments exist in some
molecules by virtue of an asymmetrical arrangement
of positively and negatively charged regions; suchmolecules are termedpolar molecules.
Polar molecule. A molecule in which there
exists a permanent electric dipole moment by virtue
of the asymmetrical distribution of positively and
negatively charged regions.
ATOMIC BONDING IN SOLIDS
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
PERMANENT DIPOLE BONDS
Van der Waals forces will also exist
between adjacent polar molecules. The
associated bonding energies are significantly
greater than for bonds involving induced
dipoles.
ATOMIC BONDING IN SOLIDS
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ATOMIC BONDING IN SOLIDS -
BONDING FORCES AND ENERGIES
MOLECULES
At the conclusion of this chapter, let us
take a moment to discuss the concept of a
molecule in terms of solid materials.
Molecule. A group of atoms that are
bound together by primary interatomic
bonds.
ATOMIC BONDING IN SOLIDS
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ATOMIC BONDING IN SOLIDS -
MOLECULES
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