Interatomic Bond and Atomic Structures

7/29/2019 Interatomic Bond and Atomic Structures

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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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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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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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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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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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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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MODELS

Schematic representation of the Bohr atom.


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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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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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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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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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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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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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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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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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CONFIGURATIONS


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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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PRIMARY INTERATOMIC BONDS


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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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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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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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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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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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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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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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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.



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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.



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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.



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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.



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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.



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MOLECULES

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Interatomic Bond and Atomic Structures