12 lecture outlines lo

8/3/2019 12 Lecture Outlines LO

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12 Solids


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Copyright 2007 Pearson Education, Inc., publishing as Benjamin Cummings

CHAPTER OBJECTIVES

To be able to recognize the unit cell of a crystalline solid

To be able to calculate the density of a solid given its unit cell

To understand the origin and nature of defects in crystals

To understand how X -rays are diffracted by crystalline solids

To understand the correlation between bonding and the propertiesof solids

To be able to describe the electrical properties of a solid using bandtheory

To be familiar with the properties of superconductors

To understand the differences between synthetic and biologicalpolymers

To be familiar with the properties of some contemporary materials


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Chemistry: Principles, Patterns,

and Applications, 1e

12.1 Crystalline andAmorphous Solids


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12.1 Crystalline and Amorphous Solids

The particles that make up a solid material, whether ionic, molecular,covalent, or metallic, are held in place by strong attractive forces

between them.

In discussing solids, the positions of the atoms, molecules, or ions,

which are fixed in place, are considered rather than their motion.

The constituents of a solid can be arranged in two general ways:

1. They can form a regular repeating three-dimensional structure

called a crystal lattice, thus producing a crystalline solid.

2. They can aggregate with no particular order, in which they

form an amorphous solid.


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Crystalline solids (crystals)

Have distinctive internal structures that lead to distinctive flat

surfaces, orfaces

Faces intersect at angles that are characteristic of the

substance and reflect the regular repeating arrangement of thecomponent atoms, molecules, or ions in space

Each structure produces a distinctive pattern when exposed to

X -rays that can be used to identify the material


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Crystals have relatively sharp, well-defined melting points

because all of the component atoms, molecules, or ions are the

same distance from the same number and type of neighbors:

regularity of the crystalline lattice creates local environments

that are the same

Intermolecular forces holding the solid together are uniform,

and the same amount of thermal energy is needed to break all

of the interactions simultaneously


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Amorphous solids When cleaved or broken, they produce fragments with

irregular, often curved, surfaces.

They have poorly defined patterns when exposed to X -rays

because their components are not arranged in a regular way.

An amorphous, translucent solid is called a glass.

Almost any substance can solidify in amorphous form if the

liquid phase is cooled rapidly enough.

Some solids are intrinsically amorphous, either because their

components cannot fit together well enough to form a

crystalline lattice or because they contain impurities that disrupt

the lattice.


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In an amorphous solid, the local environment, including both

the distances to the neighboring units and the numbers of

neighbors, varies throughout the material.

Different amounts of thermal energy are needed to overcome

these different interactions. Amorphous solids tend to soften slowly over a wide

temperature range rather than having a well-defined melting

point like a crystalline solid.

If an amorphous solid is maintained at a temperature just

below its melting point for long periods of time, the componentmolecules, atoms, or ions can gradually rearrange into a more

highly ordered crystalline form.


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12.2 The Arrangement of

Atoms in Crystalline Solids


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A crystalline solid consists of repeating patterns of its components inthree dimensions (a crystal lattice).

An entire crystal can be represented by drawing the structure of thesmallest identical units that, when stacked together, form the crystal.

A basic repeating unit is called a unitcell.

Unit cells are easiest to visualize in two dimensions.

The only requirement for a valid unit cell is that repeating it in spacemust produce the regular lattice.

12.2 The Arrangement of Atoms in Crystalline

Solids


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There are only seven fundamentally different kinds of unit cells, whichdiffer in the relative lengths of the edges and the angles betweenthem.

Each unit cell has six sides, and each side is a parallelogram.

Here the focus is primarily on the cubic unit cells, in which all sideshave the same length and all angles are 90.

The Unit Cell


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The three kinds of cubic unit cell1. Simple cubic unit cell consists of eight component atoms,

molecules, or ions located at the corners of the cube

2. Body-centered cubic (bcc) unit cell also contains anidentical component in the center of the cube

3. Face-centered cubic (fcc) unit cell has components in thecenter of each face in addition to those at the corners of thecube

The Unit Cell


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A solid consists of a large number of units cells arrayedin three dimensions.

Density is the mass of substance per unit volume, and

the density of the bulk material can be calculated from

the density of a single unit cell.

For the calculation, it is necessary to know the size of

the unit cell (to obtain its volume), the molar mass of its

components, and the number of components per unit

cell.

The Unit Cell


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When counting atoms or ions in a unit cell, those lying ona face, an edge, or a corner contribute to more than oneunit cell.

An atom that lies on a face of a unit cell is shared by two

adjacent unit cells and is counted as 1/2 atom per unit cell. An atom that lies on the edge of a unit cell is shared by four

adjacent unit cells, so it contributes 1/4 atom to each.

An atom at a corner of a unit cell is shared by all eight adjacentunit cells and contributes 1/8 atom to each.

Atoms that lie entirely within a unit cell belong to only that oneunit cell.

The Unit Cell


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Substances can be viewed as consisting of identical

spheres packed together in space, and the way the

components are packed together produces the different

unit cells.

Most of the substances with structures of this type are

metals, and most metals have hcp, ccp, or bcc structures.

Particles pack together so that they can be as close

together as possible to maximize intermolecularattractions.

Packing of Spheres


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Simple cubic structure Spheres in one layer sit directly on top of those in the previous

layer.

All layers are identical, and the unit cell is the simple cubic.

Coordination number(the number of nearest neighbors) is 6. Four neighbors are in the same layer, with one neighbor above

and one neighbor below.

Simple cubic lattice is an inefficient way to pack atoms

together in space; only 52% of the total space is filled by theatoms.

The only element that crystallizes in a simple cubic unit cell is

polonium. Simple cubic unit cells are common among binary ionic

compounds where each cation is surrounded by six anions,and vice versa.

Packing of Spheres


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Body-centered cubic (bcc) structure The body-centered cubic unit cell is a more efficient way to

pack spheres together and is more common amongpure elements.

Each atom has eight nearest neighbors

(coordination number = 8), four neighbors above and fourbelow.

Atoms occupy 68% of the volume.

A body-centered cubic structure consists of a single layer ofspheres in contact with each other and aligned so that theircenters are at the corners of a square.

A second layer of spheres occupies the square-shaped holesabove the spheres in the first layer.

The third layer of spheres occupies the square holes formed bythe second layer, so that each lies directly above a sphere inthe first layer.

The alkali metals have body-centered cubic structures.

Packing of Spheres


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Hexagonal close-packed (hcp) and cubicclose-packed (ccp) structures

The most efficient way to pack spheres is the close-packedarrangement, which has two variants, the hexagonalclose-packed (hcp) and the cubic close-packed (ccp).

The hcp and ccp structures differ only in the way their layersare stacked.

Both structures have an overall packing efficiency of74% anda coordination number of12.

In both, each atom has 12 nearest neighbors (6 in the sameplane plus 3 in each of the planes immediately above and

below).

Packing of Spheres


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1. Hexagonal close-packed (hcp) structure

Noncubic unit cell

A single layer of close-packed spheres (A) has each spheresurrounded by six others in the same plane to produce ahexagonal arrangement

Above any set of seven spheres are six depressions arranged

in a hexagon Six sites can be divided into two sets, B and C

Placing an atom at a B position prohibits placing an atom at anyof the adjacent C positions and results in all the atoms in thesecond layer occupying the B positions

Packing of Spheres


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Placing the atoms in the third layer over the atoms at the A

position in the first layer gives the hexagonal close-packed

structure

Spheres in a B layer fit into the small triangular depressions

between spheres in an A layer to give two alternating layers

(A-B-A-B)

Packing of Spheres


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2. Cubic close-packed (ccp) structure

Placing the third-layer atoms over the Cpositions gives the cubic close-packed

structure Face-centered cubic unit cell with three

alternating layers (A-B-C-A-B-C)

A-B layers are identical to hexagonal close-packed, but third layer is offset from both A

and B

Packing of Spheres


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12.3 Structures of Simple

Binary Compounds


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Common Structures of Binary Compounds

The structures of most binary compounds are dictated by the packingarrangement of the largest species (the anions), with the smallerspecies (the cations) occupying appropriately sized holes in the anionlattice.

1. A simple cubic lattice of anions contains only one kind of hole,located in the center of the unit cell.

This hole is equidistant from all eight atoms at the corners of the unitcell and is called a cubic hole.

An atom or ion in a cubic hole has a coordination number of8.

Many ionic compounds that contain large cations and have a 1:1cation:anion ratio have this structure, called the cesiumchloridestructure.


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2. A face-centered cubic array of atoms or anions contains

both octahedralholes and tetrahedralholes.

Octahedral holes, one in the center of the unit cell plus a shared

one in the middle of each edge

Tetrahedral holes, located between an atom at a corner and thethree atoms at the centers of the adjacent faces

Cations of intermediate size occupy octahedral holes in an fcc

anion lattice, and small cations occupy tetrahedral holes:

larger cations have higher coordination numbers than small

cations


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The most common structure based on a face-centered cubiclattice is the sodium chloride structure, where the

octahedral holes in an fcc lattice of anions are filled with

cations: it has a 1:1 cation:anion ratio, and each ion has a

coordination number of 6

Occupation of half the tetrahedral holes by cations results in the

zinc blende structure, with a 1:1 cation:anion ratio and a

coordination number of4 for the cations with the cation beingsmaller than the anion


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3. More complex structures are possible if there are more than two kinds of atomsin a solid.

An example is the perovskite structure, in which the two metalions form an alternating body-centered array with the anions in thecenters of the square faces


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X-Ray Diffraction

X -rays are a useful tool for obtaining information about thestructures of crystalline substances because the wavelength ofX-ray radiation is comparable to the interatomic distances inmost solids.

In X -ray diffraction, a beam of X -rays is aimed at a sample of acrystalline material, and the X -rays are diffracted by layers ofatoms in the crystalline lattice.

When the beam strikes photographic film, it produces an X -raydiffraction pattern, which consists of dark spots on a lightbackground.

Bragg worked out the mathematics that allow X -ray diffraction tobe used to measure interatomic distances in crystals and to

provide information about the structures of crystalline solids.


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X-Ray Diffraction

X -rays diffracted from different planes of atoms in asolid reinforce one another if they are in phase, whichoccurs only if the extra distance they travelcorresponds to an integral number of wavelengths.

Relationship described by the Bragg equation:2dsinU = nP


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12.4 Defects in Crystals


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12.4 Defects in Crystals

Real crystals contain large numbers ofdefects (typically more than104 per milligram) ranging from variable amounts of impurities tomissing or misplaced atoms or ions

Defects occur for three main reasons:

1. It is impossible to obtain any substance in 100% pure form; someimpurities are always present.

2. Forming a crystal requires cooling the liquid phase to allow allatoms, ions, or molecules to find their proper positions, butcooling results in one or more components being trapped inthe wrong place in a lattice or in areas where two lattices thatgrow separately intersect.

3. Applying an external stress to a crystal can cause microscopic

regions of the lattice to move with respect with the rest; thisresults in imperfect alignment.


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Defects in Metals

A point defect is any defect that involves only a single

particle (a lattice point) or a very small set of points.

A line defect is restricted to a row of lattice points.

A plane defect involves an entire plane of lattice pointsin a crystal.

A vacancy occurs where an atom is missing from the

normal crystalline array; it constitutes a tiny vacuum in

the middle of a solid.


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Defects in Metals

Impurities

1. An interstitial impurity is usually a smaller atom (typicallyabout 15% smaller than the host) that can fit into the octahedralor tetrahedral holes in the metal lattice.

2. A substitutional impurity is a different atom of about thesame size that replaces one of the atoms that make up the hostlattice.

Usually chemically similar to the substance that constitutes thebulk of the sample

Generally have atomic radii that are within about 15% of theradius of the host


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Defects in Metals

Dislocations, deformations, and work hardening1. Edge dislocation

Produced by inserting an extra plane of atoms into a crystallattice

An edge dislocation in a crystal causes the planes of atoms inthe lattice to become kinked where the extra plane of atomsbegins

To shape a solid without shattering it, planes of close-packedatoms move past one another to a new position that isenergetically equivalent to the old one

Plane along which motion occurs is called a slip plane


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Defects in Metals

2. Deformation

Process begins when an edge dislocation moves through thecrystal

As the dislocation moves, only one set of contacts is broken ata time, but the net result is that the atoms in the upper half of

the lattice move to the right with respect to the lower half Intersecting dislocations in a solid prevent them from moving in

a process called pinning, which increases the mechanicalstrength of the material

Pinning also achieved by introducing selected impurities inappropriate amounts


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Defects in Metals

Most materials are polycrystalline and consist of microscopicgrains that are randomly oriented with respect to oneanother; where two grains intersect is called a grainboundary, a two-dimensional dislocation

Defect motion tends to stop at grain boundaries, so controlling

the size of the grains in a material controls its mechanicalproperties

3. Work hardening is the introduction of a dense network ofdislocations throughout the solid, which makes it very tough andhard


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The compound NiTi is known asmem

oryme

tal, and it illustrates theimportance of deformations.

If a sample of this metal is warmed from room temperature to a

temperature higher than 50C, it will revert to a shape in which it has

been previously set.

NiTi exists in two different solid phases:

1. High temperature phase has cubic CsCl structure, in which a Ti

atom is embedded in the center of a cube of Ni atoms (or

vice versa)

2. Low temperature phase has a related but kinked structure, in

which one of the angles of the unit cell is no longer 90

Bending an object made of the low-temperature phase creates defects

that change the pattern of kinks within the structure. If the object is

heated, the material undergoes a transition to the cubic high-temperature

phase that causes the object to return to its original shape.

Memory Metal


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The most straightforward variant is a substitutional impurity in whicha cation or anion is replaced by another of similar charge and

size.

All of the colored gems used in jewelry are due to substitutional

impurities in simple oxide structures.

Substitutional impurities are observed in molecular crystals if the

structure of the impurity molecule is similar to the host, and they

can have major effects on the properties of the crystal.

If a cation or anion is missing, leaving a vacant site in an ionic

crystal, then the requirement that a crystal be electrically neutralimplies that there must be a corresponding vacancy of the ion with

the opposite charge somewhere in the crystal.

Defects in Ionic and Molecular Crystals


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Pairs or sets of vacancies are called Schottkydefects and are

common in simple alkali metal halides.

Sometimes one of the ions in an ionic lattice is in the wrong

position an example is the Frenkel defect where a cation

occupies a tetrahedral hole rather than an octahedral hole in the

anion lattice.

Frenkel defects common in salts that have a large anion and

a small cation and to preserve electrical neutrality, one

of the normal cation sites (octahedral) must be vacant

Frenkel defects are common in the silver halides, which

combine a small cation with large, polarizable anions

Solid electrolytes are good electrical conductors in the solid

state

Defects in Ionic and Molecular Crystals


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Nonstoichiometric Compounds

Nonstoichiometric compounds

Are solids that have intrinsically variable stoichiometries

Contain a large number of defects, usually vacancies, which

give rise to stoichiometries that can depart significantlyfrom simple integral ratios without affecting the

fundamental structure of the crystal

Consists of transition metals, lanthanides, and actinides, with

polarizable anions such as oxide and sulfide


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12.5 Correlation between Bonding

and the Properties of Solids


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12.5 Correlation between Bonding and

the Properties of Solids

Based on the nature of the forces that hold thecomponent atoms, molecules, or ions together, solidsare classified as

1. ionic;

2. molecular;

3. covalent;4. metallic.

Variation in the relative strengths of these four types ofinteractions correlates with their wide variation in

properties of these four kinds of solids.


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Ionic solids Consist of positively and negatively charged ions held together

by electrostatic forces

Strength of the attractive forces depends on the charge andsize of the ions that make up the lattice and determines manyof the physical properties of the crystal

Lattice energy, the energy required to separate 1 mol of thecrystalline ionic solid into its component ions in the gas phase,is directly proportional to the product of the ionic charges andinversely proportional to the sum of the sizes of the ions

Poor conductors of heat and electricity

High melting points

Hard but brittle; shatter under stress Dense with a dull surface

Ionic Solids


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Molecular solids Consist of atoms or molecules held together by dipole-dipole

interactions, London dispersion forces, or hydrogen bonds

Intermolecular interactions in a molecular solid are relativelyweak compared with ionic and covalent bonds

Tend to be soft, low melting, and easily vaporized

((Hfus and (Hvap are low) For similar substances, the strength of the London dispersion

forces increases smoothly with increasing molecular mass


Low density

Dull surface

Molecular Solids


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Covalent solids Formed by networks or chains of atoms or molecules held

together by covalent bonds

A perfect single crystal of a covalent solid is a single giantmolecule

Tend to be very hard and have high melting points

Not easily deformed Brittle, tend to shatter when subjected to large stresses


Low density

Dull surface

Covalent Solids


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Metallic solids Packing efficiency in metallic crystals tends to be high, sometallic solids are dense, with each atom having as many as12 nearest neighbors

Every lattice point in a pure metallic element is occupied by anatom of the same metal

Reflect light, called luster Have high electrical and thermal conductivity

Have high heat capacity

Are malleable and ductile

Easily deformed under stress

High density

Melting points depend strongly on electron configuration

Metallic Solids


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Bonding in metallic solids is quite different from the bonding inthe other kinds of solids; the valence electrons are delocalizedthroughout the crystal, providing a strong cohesive force thatholds the metal atoms together

Strength of metallic bonds varies dramatically

Metallic bonds tend to be weakest for elements that havenearly empty or nearly full valence shells, and are strongest forelements with half-filled valence shells

Melting points of the metals increase to a maximum aroundGroup 6 and then decrease again from left to right across the sand dblocks

Other properties related to the strength of metallic bonds, suchas enthalpies of fusion, boiling points, and hardness, havesimilar periodic trends

Metallic Solids


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Substitutional alloys An alloy is a mixture of metals with metallic properties that

differ from those of its constituent elements.

Substitutional alloys are metallic solids that contain largenumbers of substitutional impurities and can be formed by

substituting one metal atom for another of similar size in thelattice.

Interstitial alloys are metallic solids that contain smallnumbers of interstitial impurities and can be formed by insertingsmaller atoms into holes in the metal lattice.

Metallic Solids


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Most metallic substances are alloys, and thecomposition of most alloys varies over wide ranges.

Certain metals combine in only fixed proportions to

form intermetallic compounds.1. Their compositions are determined by the relative sizes of

their component atoms and the ratio of the totalnumber of valence electrons to the number of atomspresent

2. Have structures and physical properties quite different

from those of their constituent elements

Metallic Solids


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12.6 Bonding in Metals and

Semiconductors


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Band Theory

An approach to metallic bonding

Assumes that the valence orbitals of the atoms in a solid

interact, generating a set of molecular orbitals that

extend throughout the solid


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Band Theory

One-dimensional systems

A linear arrangement ofn metal atoms, each of which contains

a single electron in an sorbital

If the distance between the metal atoms is short enough for the

orbitals to interact, they produce bonding, antibonding, and

nonbonding molecular orbitals

The lowest-energy orbital is the completely bonding molecular

orbital, the highest-energy orbital is the completely

antibonding molecular orbital, and those in the middle are

nonbonding molecular orbitals

Energy separation between adjacent orbitals decreases as the

number of interacting orbitals increases


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Band Theory

The result is a continuum of energy levels, each of which

corresponds to a particular molecular orbital extending

throughout the linear array of metal atoms

Continuous set of allowed energy levels is called an energy

band Difference in energy between the highest and lowest energy

levels is the bandwidth, which is proportional to the strength of

the interaction between orbitals on adjacent atoms; the

stronger the interaction, the larger the bandwidth


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Band Theory

Band has as many energy levels as molecular orbitals

Band for a one-dimensional system contains n energy levels

corresponding to the combining ofsorbitals from nmetal

atoms

Each of the original s orbitals could contain a maximum of twoelectrons, so the band can accommodate a total of2n

electrons, but each of the metal atoms contain only a single

electron in each sorbital so there are only n electrons to place

in the band

Electrons occupy the lowest energy levels, so only the lower

half of the band is filled (the bonding molecular orbitals) and

results in the strongest possible bonding


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Band Theory

Multidimensional systems In two- or three-dimensional systems with atomsthat contain electrons in p and dorbitals, theresulting energy-level diagrams are essentially thesame as the diagram of the one-dimensional systemexcept they contain as many bands as there aredifferent types of interacting orbitals.

Each band will have a different bandwidth and willbe centered at a different energy, corresponding to

the energy of the parent atomic orbital of an isolatedatom.


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Band Theory

Band gap The energy difference between the highest level of

one band and the lowest level of the next is the

band gap and represents a set of energies that do

not correspond to any allowed combinations ofatomic orbitals.

If the width of adjacent bands is larger than the

energy gap between them, overlapping bands

result, in which molecular orbitals derived from two

or more kinds of valence orbitals have similar

energies.


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Band Theory

Band theory can explain the properties ofmetals

Metals conduct electricity because only a very small amount of

energy is required to excite an electron from a filled level to an

empty one where it is free to migrate rapidly throughout the

crystal in response to an applied electric field. Metals have high heat capacities because the electrons in the

valence band can absorb thermal energy by being excited to

the low-lying empty energy levels.

Metals are lustrous because electrons can be excited from

many different filled levels in a metallic solid and can thendecay back to any of the many empty levels, causing light of all

wavelengths to be absorbed and reemitted.


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Requirements for Metallic Behavior

For a solid to exhibit metallic behavior, it must have a set ofdelocalized orbitals forming a band of allowed energy levels, and the

resulting band must be only partially filled with electrons.

Band theory explains the correlation between the valence-electron

configuration of a metal and the strength of metallic bonding.

Metals with six to nine valence electrons (Groups 6 to 9) fill

valence bands halfway, and the elements of these groups

exhibit physical properties consistent with the presence of the

strongest metallic bonding.


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Insulators

Electrical insulators are materials that conduct electricity poorlybecause their valence bands are full.

Energy gap between the highest filled levels and the lowest empty

levels is so large that the empty levels are inaccessible; thermal

energy cannot excite an electron from a filled level to an empty one.


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Semiconductors

If a solid has a filled valence band with a relatively

low-lying empty band above it (a conduction band),

then electrons can be excited by thermal energy from the

filled band into the vacant band where they can then

migrate throughout the crystal, resulting in electrical

conductivity.

Semiconductors are substances that have

conductivities between those of metals and insulators.

Many binary compounds of the main group

elements exhibit semiconducting behavior.


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Semiconductors

Temperature and conductivity

Thermal energy can excite electrons across the

band gap in a semiconductor, so increasing the

temperature increases the number of electrons that

have sufficient kinetic energy to be promoted into

the conduction band.

Electrical conductivity of a semiconductor increases

rapidly with increasing temperature, whereas the

electrical conductivity of metals decreases slowly

with increasing temperature.


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Semiconductors

n-andp-type semiconductors Doping is a process that is used to tune the electrical

properties of commercial semiconductors by deliberately

introducing small amounts of impurities.

1. Adding an element with more valence electrons than the atoms

of the host populates the conduction band, resulting in an

n-type semiconductorwith increased electrical conductivityn indicates that the added charge carrier are negative

(electrons).

2. Adding an element with fewer valence electrons than the atoms

of the host generates holes in the valence band, resulting in

a p-type semiconductorthat also exhibits increased

electrical conductivity p standing for positive charge carrier(a hole).

Chemistry: Principles Patterns


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12.7 Superconductors


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12.7 Superconductors

Onnes measured the resistivity of samples (resisitivity and

conductivity of a material are inversely proportional).

Onnes discovered that the resistivity of many metallic elements

decreased suddenly to zero, rather than continuing to decrease

slowly with decreasing temperature a phenomenon called

superconductivitybecause a resistivity of zero means that an

electrical current can flow forever. Superconductors are solids that at a characteristic

superconducting transition temperature (Tc) exhibit zero

resistance to the flow of electrical current.

At temperatures lower than theirTc, superconductors completely

expel a magnetic field from their interior, a phenomenon called theMeissnereffect.


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BCS Theory

Bardeen, Cooper, and Schrieffer formulated a theory forsuperconductivity called the BCS theory.

According to the BCS theory, electrons are able to travel through a

solid with zero resistance because of an attractive interaction

between two electrons that are at some distance from each other.

As one electron moves through the lattice, the surrounding nuclei are

attracted to it and the motion of the nuclei can create a short-lived

hole that pulls the second electron in the same direction as the first;

to avoid colliding with the approaching second electron, the nuclei

return to their original positions.


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BCS Theory

Pairs of electrons are called Cooperpairs, and they

migrate through a crystal as a unit.

Electrons in Cooper pairs change partners frequently.

As the temperature of the solid increases, the vibrations

of the atoms in the lattice increase continuously, until the

electrons cannot avoid colliding with them collisions

result in the loss of superconductivity at higher

temperatures.

S


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Bednorz and Muller showed that certain mixed-metal oxides

containing La, Ba, and Cu exhibited superconductivity above 30 K

and were called high-temperature superconductors.

The 1-2-3 superconductor was discovered by Chu and Wu and

was found to be superconducting at temperatures higherthan 95 K.

High-Temperature Superconductors



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12.8 Polymeric Solids


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12.8 Polymers

Polymers are giant molecules formed from a carefully controlled series of

condensation or addition reactions and consist of basic structural unitscalled monomers, which are repeated many times in each molecule.

Polymerization is the process by which monomers are connected into

chains or networks by covalent bonds.

Polymers can form via a condensation reaction, in which two monomer

molecules are joined by a new covalent bond and a small molecule is

eliminated, or by an addition reaction, where the components of a species

A-B are added to adjacent atoms of a carbon-carbon multiple bond.

Plastic is the property of a material that allows it to be molded into almost

any shape.


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Synthetic Polymers

Synthetic polymers include plastics, fibers, and rubbers.

Fibers are particles that are more than a hundred times longer than

they are wide.

Synthetic polymers have commercial advantages over biological

polymers because they can be produced inexpensively, andscientists can select monomer units to tailor the physical properties

of the resulting polymer for particular purposes.

The best-known example of a synthetic polymer is nylon,whose

monomers are linked by amide bonds, so its physical properties are

similar to those of some proteins.

Replacing the flexible CH2units in nylon by aromatic rings produces

a stiffer and stronger polymer called Kevlar.


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Synthetic Polymers

Polyesters are synthetic polymers that are linked byester bonds.

Polymers based on skeletons that contain only carbonare all synthetic.

Substances such as glass can also be formed intofibers, producing fiberglass.

Most synthetic fibers are neither soluble nor low melting,and multi tep processes are required to manufacturethem and to form them into objects.

Pyrolysis is a high-temperature decomposition reactionthat can form fibers.



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Chemistry: Principles, Patterns


12.9 Contemporary

Materials


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12.9 Contemporary Materials

In addition to polymers, other materials such as

ceramics, high-strength alloys, and composites play a

major role in almost every aspect of our lives.


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Ceramics

A ceramic is any nonmetallic, inorganic solid that is strong enoughfor use in structural application.

Traditional ceramics are based on metal silicates or aluminosilicates

and are the materials used to make pottery, china, bricks, and

concrete.

Modern ceramics contain a wider range of components and are

classified as

1. ceramicoxides, which are based on metal oxides;

2. nonoxide ceramics, which are based on metal carbides.

All modern ceramics are hard, lightweight, and stable at very high

temperatures they are also brittle, tending to crack or break under

stresses.


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Ceramics

Producing a high-strength ceramic involves a process calledsintering, which fuses the grains into a dense and strong material.

One of the most widely used raw materials for making ceramics is

clay, which consists of hydrated alumina and silica that contain a

broad range of impurities.

When hydrated, clays can be molded, but duringhigh-temperature heat treatment, called firing, a dense and

strong ceramic is produced.

The necessary fine powders of ceramic oxides with uniformly sized

particles can be produced by the sol-gel process.


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Superalloys

Superalloys are high-strength alloys, often with acomplex composition, that are used in systems requiring

mechanical strength, high surface stability, and

resistance to high temperatures.

Most superalloys are based on nickel, cobalt, or iron.

Pure nickel or cobalt is easily oxidized, but adding small

amounts of other metals results in an alloy that has

superior properties.

Superalloys exhibit unusually high temperature stability

and resistance to oxidation.


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Composite Materials

Composite materials have two distinct components:

1. The matrix (which constitutes the bulk of the material)

2. Fibers or granules that are embedded within the matrix and

limit the growth of cracks by pinning defects in the bulk material

There are three distinct types of composite material, distinguished by

the nature of the matrix:

1. Polymer-matrix composites have reinforcing fibers

embedded in a polymer matrix.

2. Metal-matrix composites have a metal matrix and fibers of

boron, graphite, or ceramic.

3. Ceramic-matrix composites use reinforcing fibers, usuallyceramic, to make the matrix phase less brittle.

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