Kelter P., Mosher M., Scott A. Chemistry. The Practical Science

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Application

HEMICAL

ENCOUNTERS:

Heart

Deﬁbrillators

FIGURE 13.28

The portable cardiac deﬁbrillator. The

deﬁbrillator stores electrical energy in

capacitors made with thin ﬁlms.

568 Chapter 13 Modern Materials

EXERCISE 13.6 Identifying Materials in the Real World

Examine and identify the following items from everyday life. Indicate whether they

are made of a composite material, a plastic, a metal, an alloy, or a ceramic: an iron

bar–reinforced concrete wall, the light switch plate, and a brass doorknocker.

Solution

The reinforced concrete wall is made from iron bars (called re-bar) buried in a mix-

ture of cement and pebbles. Therefore, it is a composite material. The light switch

plate is made from Bakelite, a plastic. Its insulating ability makes it a good choice for

an electrical plate. The doorknocker is made from brass, an alloy. The sheen and

durability of polished brass makes this item luxurious and useful.

PRACTICE 13.6

Identify the materials that make up a ﬂoppy disk, the nonstick coating in your fry-

ing pan, and the windshield of your car.

See Problems 55 and 56.

13.5 Thin Films and Surface Analysis

People who experience sudden cardiac arrest (disruption of the normal function

of the heart) are often treated with a portable cardiac deﬁbrillator like that shown

in Figure 13.28. This instrument delivers to the cardiac victim’s heart a powerful

and sudden shock, which can resynchronize the beating cycle of the heart muscle.

The deﬁbrillator contains devices called capacitors that can store large amounts

of electrical energy. At the push of a button a circuit is closed, and the capacitors

release the stored energy as one strong electrical pulse.

One of the most durable capacitors on the market is made with a thin ﬁlm of

aluminum on either side of a plastic sheet. The two sheets of thin aluminum act

as the positive and negative terminals of a battery. Because a plastic sheet separates

them, the terminals do not allow electricity to pass until a large voltage is applied.

The thin ﬁlm capacitors are particularly useful in portable devices because they

are self-healing. If a short circuit develops inside the capacitor, the heat generated

causes the aluminum to vaporize, making that portion of the capacitor ineffec-

tive. This reduces the power of the capacitor, but it is still able to function. In a

standard capacitor, a short circuit often destroys the capacitor, which would not

be desirable if you were answering a cardiac arrest emergency.

A thin ﬁlm is generally deﬁned as a ﬁlm of any material, typically only 0.1 µm

to 300 µm thick, much thinner than a layer of paint. However, like paint, a thin

ﬁlm must adhere strongly to the surface on which it is applied. Thin ﬁlms are

carefully prepared so that the chemical composition is well deﬁned (uniform

thickness, homogeneity, and so on).

How do we make a ﬁlm of a material so thin? A ﬁlm so thin is very fragile un-

less it is made directly on a surface. Many techniques have been designed to do

just that. The most common techniques are physical deposition, sputtering, and

chemical-vapor deposition.

Physical deposition relies on sublimation and deposition to place a thin ﬁlm on

a substrate. A chamber is attached to a vacuum. The material to be made into a

thin ﬁlm is placed at one end of the chamber, and the substrate is placed at the

other end. The vacuum is turned on, and heat is applied to the material to be va-

porized. When the appropriate heat and pressure have been obtained, the mater-

ial sublimes and moves through the chamber, where it deposits on the substrate.

Despite the rotation of the substrate, uniform thickness of the thin

ﬁlm is difﬁcult to obtain. Typically, thin ﬁlms of inorganic materials

such as MgF

and SiO

are made by this method.

sputtering, very high voltage is used to move a material from a

negative terminal (the cathode) to a positive terminal (the anode, see

Chapter 19), both surrounded by a low-pressure argon atmosphere.

Inside the device, the high voltage forms argon cations that race to the

cathode. Their impact dislodges metal atoms from the cathode with

extremely high kinetic energy. These ejected atoms ﬂy in all direc-

tions. Some strike the anode and form a thin ﬁlm. Typically, thin ﬁlms of metals

(such as silicon, titanium, aluminum, gold, and silver) are made via sputtering

techniques.

chemical-vapor deposition, a thin ﬁlm is prepared when a compound placed

on a surface undergoes a reaction. For example, scratch-resistant sunglasses can

be made by coating the lenses with a carbon-based thin ﬁlm. In this process, a

mixture of methane (CH

) and hydrogen (H

) is passed over the lenses. Applica-

tion of intense microwave radiation to this mixture causes the molecules to break

apart into their individual atoms. The carbon atoms re-form into a diamond-like

thin ﬁlm. The hydrogen atoms react with any carbon that starts to form graphite-

like ﬁlms. Other methods include the reaction of a metal(IV) halide with hydro-

gen gas to prepare metal ﬁlms and the reaction of silane (SiH

) and ammonia

13.5 Thin Films and Surface Analysis 569

Vacuum

pump

Physical Deposition

Substrate Source

material

Heated

platform

Argon gas

–

Sputtering

Target

Ground

Power

supply

Magnetic

field

Substrate

Sputtered

target atom

Source

gases

Chemical-Va

or De

osition

Substrate

Product

gases

Product

gases

Reaction zone

Chemical Vapor Deposition

Capacitors can be made from thin ﬁlms.

(NH

) to make silicon nitride ceramic thin ﬁlms. This method makes thin ﬁlms

of metals, alloys, ceramics, and polymers.

Applications of thin ﬁlms range from the fabrication of microelectronic de-

vices to use as novel catalysts and as protective coatings. For instance, thin ﬁlms

of gold that absorb harmful UV radiation from the sun are used to create protec-

tive eyewear for astronauts in spaceﬂight (see Figure 13.29).

Thin ﬁlms are analyzed using a variety of techniques, including scanning tun-

neling microscopy (STM) and atomic force microscopy (AFM). Atomic force

microscopy is similar to STM (Chapter 9) in that an atomic-sized needle (with a

diameter less than 10 nm) is used. However, in AFM, the needle rests on the sur-

face to be explored. A laser is directed at the top of the needle and is used to calcu-

late the distance of the needle from the surface. As the surface is scanned, the nee-

dle moves up and down through the surface’s peaks and crevices. The laser beam

records the changing distance to the surface, and a picture of the surface is pro-

duced (see Figure 13.30). This method is very useful for exploring surface defects.

Modiﬁcations of AFM can provide speciﬁc information about the surface, such as

the type of atom and the thickness of the ﬁlm.

EXERCISE 13.7 That’s Pretty Thin

How many atoms of tin (atomic radius = 141 pm) are needed to make a one-atom-

thick thin ﬁlm that covers an area equal to that of a sheet of paper measuring 8.5 by

11 in? Assume the atoms are aligned in rows and columns as they cover the piece of

paper.

First Thoughts

To answer the question, we need to determine the area of the paper. Then we’ll con-

sider the area occupied by one atom (assuming it is a square that is 282 pm on each

side—the diameter of the tin atom).

570 Chapter 13 Modern Materials

FIGURE 13.29

Soichi Noguchi, a Japanese astronaut,

took a walk in space during the August,

2005, mission of the space shuttle

Discovery. The face shield on his suit is

lowered, showing the gold thin ﬁlm.

Tip

Cantilever

Computer

Movable

platform

Laser

Photodiode array for

lateral and vertical

deflection measurement

Sample

FIGURE 13.30

A block diagram illustrating atomic force microscopy (AFM). The atomic force microscope

measures the deﬂection of an atomic-sized needle from the surface of a substrate using

a laser. The data obtained can be used to draw a picture of the hills and valleys on a

substrate.

Solution

8.5 in = 21.59 cm; 11 in = 27.94 cm

Area of paper = 21.59 cm × 27.94 cm = 603.22 cm

Area of atom = 282 pm × 282 pm = 79524 pm



1 ×10

−12

1pm

100 cm



= 7.952 × 10

–16

Number of atoms

603.22 cm

7.952 ×10

−16

= 7.6 × 10

atoms

Further Insight

That is quite a few atoms, though it is much less than a mole. In fact, this number of

atoms amounts to 1.3 ×10

–6

mol, or 1.5 × 10

–4

g, of tin. The change in mass of the

paper would be less than a milligram!

PRACTICE 13.7

How many moles of nickel (atomic radius = 125 pm) would be needed to coat a

photograph measuring 4.0 in × 6.0 in? Assume that the nickel atoms are arranged

neatly in rows and columns as they cover the photo. How many layers of nickel

atoms would be needed to increase the mass of the photograph by 1.00 g?...by

20.0 g? Is it possible to place a thin ﬁlm of gold on a photograph measuring 4.0 in ×

6.0 in with only 0.050 mg of Au (atomic radius = 144 pm)?

See Problems 61–66 and 68.

13.6 On the Horizon—

What Does the Future Hold?

Health science professions utilize cutting-edge technology wherever possible to

provide the best care for their patients. Visit the doctor’s ofﬁce, have your annual

dental exam, or stop by the emergency room, and you’ll notice the use of modern

materials that improve the quality of your care. These materials are so ingrained

in our society that it really would be hard to live our current lifestyle without

them. In fact, ﬁfty years ago we would never have been able to believe that plastics

would stop bullets, that a ceramic bone would be used to repair a fracture, or that

a plastic heart could keep someone alive. With recent advances in the technology

of modern materials, it appears that anything is possible. Just read your newspa-

per to see what’s coming.

“Green Chemistry”

One of the main thrusts in modern materials science is the development of envi-

ronmentally benign technologies, or

Green Chemistry. Replacing toxic catalysts

in manufacturing processes and using aqueous solutions instead of ﬂammable

organic solvents are just two of many things that can be done to make the chem-

ical manufacturing industry more environmentally friendly. Replacing haz-

ardous pesticides with the application of plant proteins to trigger the production

of substances that defend against insects is another approach to helping clean up

the environment. Twelve principles to help guide chemists in crafting a more

environmentally friendly science were articulated by Paul Anastas of the White

House Ofﬁce of Security and Technology Policy and John Warner of the Univer-

sity of Massachusetts–Boston. Six of those principles are waste prevention, atom

economy (using as much of each reagent as possible in a reaction), reducing the

13.6 On the Horizon—What Does the Future Hold? 571

Application

HEMICAL

ENCOUNTERS:

“Green Chemistry”

FIGURE 13.31

Hot ﬂame and cool ﬂower. The aerogel

is an excellent insulator for this ﬂower.

572 Chapter 13 Modern Materials

Zeolites show promise as useful

materials. They contain pockets that can

be engineered to hold small molecules.

use of hazardous chemicals, energy efﬁciency, use of catalytic reactions, and pol-

lution prevention.

The use of specially engineered zeolites (see Chapter 8) as catalyst compo-

nents is promising. One area where a zeolite catalyst could have great impact is in

the manufacture of phenol (primarily used as to make the resin in plywood

sheets). Research on the use of a zeolite to directly oxidize benzene (C

) to

phenol (C

OH) is currently under way. Typically, the manufacture of phenol

from benzene is a two-step process. The preparation of phenol via this method

generates a lot of by-products. Zeolites may be the perfect solution to this prob-

lem, reducing the large amount of waste generated.

Biopolymers

Research into the modiﬁcation of natural polymers (biopolymers) has the poten-

tial to have tremendous impact in the health professions. Biopolymers include

nucleic acids (DNA and RNA), proteins (such as enzymes), and some carbohy-

drates (such as cellulose, chitin, and starch). These compounds can be chemi-

cally modiﬁed in the laboratory to make biological polymers with interesting

properties.

Biopolymers are environmentally friendly because they degrade (break down

to chemically more benign compounds) in the environment and are often reab-

sorbed by living creatures. For example, dead trees (composed of cellulose and

other natural polymers) decay and become food for other organisms. Cellulose is

biodegradable. The recent use of cellulose instead of Styrofoam “peanuts” as

packaging materials has been an excellent application of Green Chemistry. The

two materials do the same job, acting as cushions for our breakables when

shipped. However, the cellulose peanut can be biodegraded (which greatly re-

duces its volume when added to water), and large quantities of nonrenewable

resources are not needed to manufacture it. Replacing synthetic polymers with

biopolymers helps promote Green Chemistry.

Biopolymers can assist in the delivery of drugs to patients by helping to make

the drugs more readily absorbable by the body. Another recently issued patent

illustrates the use of a biopolymer to enhance the ﬂavor of chewing gum. Because

compounds made by biological systems are often chiral (see Section 12.14), the

use of biopolymers as catalysts in a chemical reaction can provide an environ-

mentally benign method to make medicines.

Aerogels

A special type of glass known as an aerogel has attracted attention lately. The

aerogel shown in Figure 13.31 is, as its name implies, a silica glass whose structure

is mostly air. We can think of the aerogel as a ceramic foam. It is produced by

crystallizing the porous silica structure in a solvent and then removing the solvent

to give a nearly transparent silica structure that has the appearance of soapsuds.

What makes aerogels so interesting is that they have tremendous surface

area. A 1-g sample of an aerogel can have as much as 1000 m

of surface area—

equivalent to about 1.4 acre of farmland or one-seventh of the street area taken

up by the Empire State Building in New York City. Combine the high surface area

and extremely low density (0.003 to 0.35 g/mL) with the high resistance to tem-

perature common among the ceramics (some aerogels are completely stable up to

3000°C), and you have a perfect insulator for a spacecraft. In fact, these materials

have signiﬁcantly greater insulating power than ﬁberglass. Research is currently

being conducted on the manufacture of ﬂexible aerogels that could be used to

make insulation for homes. Just imagine a ﬁreﬁghter’s coat as thin and light as a

sheet but insulating enough to protect its wearer against the hottest ﬁres.

Key Words 573

The Bottom Line

■

In a crystalline solid, the atoms, ions, or molecules

are highly ordered in repeating units that make up

the lattice of the crystal. Amorphous solids, al-

though their atoms, ions, or molecules inhabit rigid

and ﬁxed locations, lack the long-range order in a

crystal. (Section 13.1)

■

The unit cell is the basic repeating unit that, by

simple translation in three dimensions, can be

used to represent the entire crystalline lattice. (Sec-

tion 13.1)

■

The simple cubic unit cell, the body-centered cubic

unit cell, and the face-centered cubic unit cell make

up most of the crystalline lattices of the metallic el-

ements. (Section 13.1)

■

Metals often adopt a closest packed structure. These

structures include the hexagonal closest packed

structure and the cubic closest packed structure.

(Section 13.1)

■

We can calculate the volume and density of a

unit cell by means of simple geometry. In doing

so, we assume the atoms are hard spheres.

(Section 13.1)

■

Band theory describes why metals are electrical and

thermal conductors, shiny, malleable, and ductile.

(Section 13.2)

■

The valence band and the conduction band result

from the nearly inﬁnite number of atomic orbitals

that overlap to form the molecular orbitals in a

metal. The band gap can be used to assess whether

a compound is a conductor, a semiconductor, or an

insulator. (Section 13.2)

■

Alloys are mixtures of two or more metals. They in-

clude the interstitial and substitutional alloys. Amal-

gams are special alloys of mercury. (Section 13.2)

■

Ceramics, which include glasses and superconduc-

tors, are compounds that contain both ionic and

covalent bonds. (Section 13.3)

■

Plastics, named after their ability to be molded into

shape, are polymeric materials. (Section 13.4)

■

Composite materials are made of an intimate com-

bination of two or more materials. (Section 13.5)

■

“Green Chemistry” is the practice of chemistry via

environmentally benign methods. (Section 13.6)

aerogel A silica glass whose structure is mostly air;

also known as a ceramic foam. (p. 572)

alloy A solution of two or more metals. (p. 556)

amalgam A solution made from a metal dissolved in

mercury. (p. 557)

amorphous solid A solid whose atoms, ions, or mole-

cules occupy fairly rigid and ﬁxed locations but

that lacks a high degree of order over the long

term. (p. 541)

Key Words

band gap A small energy gap that exists between the

valence band and the conduction band (p. 553)

band theory A metal is a lattice of metal cations spaced

throughout a sea of delocalized electrons. (p. 551)

biopolymer A polymer of biological materials, such as

DNA or cellulose. (p. 572)

body-centered cubic (bcc) unit cell A unit cell built via

the addition of an atom, ion, or molecule at the

center of the simple cubic unit cell. (p. 543)

ceramic A nonmetallic material made from inorganic

compounds. (p. 557)

chemical-vapor deposition A thin ﬁlm prepared when a

compound placed on a surface undergoes a reaction.

(p. 569)

closest packed structure A structure wherein the atoms,

molecules, or ions are arranged in the most efﬁcient

manner possible. (p. 544)

composite material A material made from two or more

different substances. (p. 565)

conduction band The collection of empty molecular or-

bitals on a metal. (p. 552)

conductor A substance that contains a very small band

gap. (p. 554)

crosslinking Reactions that covalently bond two or

more individual strands of a polymer together.

(p. 563)

crystal lattice A highly ordered framework of atoms,

molecules, or ions. (p. 542)

crystalline solid A solid made from atoms, ions, or

molecules in a highly ordered long-range repeating

pattern. (p. 541)

cubic closest packed (ccp) structure A closest packed

structure made by staggering a second and third row

of particles so that none of the rows line up. (p. 544)

diffraction pattern A pattern of constructive and de-

structive interference after EMR passes through a

solid material. (p. 546)

dope To add an impurity to a pure semiconductor to

alter its conductive properties. (p. 554)

ductile Able to be pulled or drawn into a wire.

(p. 552)

face-centered cubic unit cell A unit cell built via the ad-

dition of an atom, ion, or molecule at the center of

each side of the simple cubic unit cell. (p. 543)

ﬁber A polymer whose chains are aligned in one di-

rection. (p. 563)

glass An amorphous solid ceramic material. (p. 559)

Green Chemistry The practice of chemistry using envi-

ronmentally benign methods. (p. 571)

hexagonal closest packed (hcp) structure A closest packed

structure made by staggering a second and third row

of particles in such a way that the ﬁrst and third

rows line up. (p. 544)

insulator A substance that contains a very large band

gap. (p. 554)

interstitial alloy An alloy made by the addition of

solute metals placed in the existing spaces within a

solvent metal. (p. 556)

ionic solids Solids made up of ionic compounds held

together by strong electrostatic forces of attraction

between adjacent cations and anions. (p. 542)

liquid crystal A transitional phase that exists between

the solid and liquid phases for some molecules. Typ-

ically, these molecules have long, rigid structures

with strong dipole moments. (p. 550)

malleable Capable of being shaped. (p. 552)

molecular solids Solids made up of molecules that are

held together by intermolecular forces. (p. 542)

nonbonding molecular orbital A molecular orbital that is

not involved in bonding or antibonding. (p. 551)

n-type semiconductor A semiconductor that contains

electrons (negative charges) in the conduction band.

(p. 554)

phonons The induced vibrations produced in a metal

lattice. (p. 561)

photovoltaic device A device in which light energy can

be used to generate an electric current. (p. 555)

physical deposition The process of sublimation and de-

position used to place a thin ﬁlm on a substrate.

(p. 568)

plastics A thermally moldable polymer. (p. 563)

p-type semiconductor A semiconductor that is electri-

cally conductive if we apply a small electric ﬁeld.

The semiconductor contains positive holes in the

valence band. (p. 554)

semiconductor A metal that contains a small band gap.

(p. 553)

simple cubic unit cell An arrangement of particles

within a crystalline solid comprised of six particles

occupying the corners of a cubic box. (p. 542)

sputtering The process of producing a thin ﬁlm on a

substrate by using high voltage to move a material

from a negative terminal (the cathode) to a positive

terminal (the anode). (p. 569)

substitutional alloy An alloy made by directly replacing

solvent atoms with solute atoms. (p. 556)

superconductor A ceramic whose resistance to the ﬂow

of electrons drops to zero as the temperature is re-

duced. (p. 561)

transition temperature The temperature of a super-

conductor at which the resistance to the ﬂow of

electrons becomes negligible. (p. 562)

tensile strength Resistance to breaking when a sub-

stance is stretched. (p. 563)

unit cell The repeating pattern that makes up a crys-

talline solid. (p. 542)

valence band The collection of ﬁlled molecular orbitals

on a metal. (p. 552)

574 Chapter 13 Modern Materials

Focus Your Learning 575

Focus Your Learning

The answers to the odd-numbered problems and some selected

problems appear at the back of the book, as represented by the

blue numbering.

13.1 The Structure of Crystals

Skill Review

1. What key feature distinguishes a crystalline solid from an

amorphous solid?

2. Cite an example of a crystalline solid and of an amorphous

solid.

3. Use the descriptive term parallelepiped to deﬁne both a crys-

tal lattice and a unit cell.

4. Cite an example of an object made from parallelepipeds that

are visible to the naked eye.

5. Which of these diagrams represents the body-centered cubic

structure of iron?

6. Which of these diagrams represents the face-centered cubic

structure of strontium?

7. How many complete atoms would you expect to ﬁnd in a

unit cell made from the simple cubic unit cell, but with addi-

tional atoms placed along every edge of the unit cell?

8. How many complete atoms would you expect to ﬁnd in a

unit cell made from the simple cubic unit cell, but with addi-

tional atoms placed only at the face of two of the sides of the

unit cell?

9. Without any further information, indicate whether each of

the following three solids is most likely to be molecular, ionic,

or metallic: Solid A is the only one of the three that conducts

electricity in the solid state. Solid B melts when placed in

boiling water. Solid C dissolves in water, and the resulting

solution conducts electricity.

10. As part of a display of fruit, a grocer places two types of ap-

ples (red and yellow) in a pattern. The apples are to be placed

in a slanted display with a wooden frame. The ﬁrst layer is

made of red apples, and the second layer is placed above the

ﬁrst in the “dimples” or gaps. Show how the third layer of red

apples would be placed if the grocers, remembering their

chemistry, wanted to make the display resemble hexagonal

closest packing.

11. Calculate the volume of a fcc unit cell with each of the fol-

lowing atomic radii:

a. 125 pm b. 210 pm c. 302 pm

12. Calculate the volume of a bcc unit cell if it is composed of

atoms with each of the following radii:

a. 200 pm b. 183 pm c. 164 pm

Chemical Applications and Practices

13. Suppose the label on a can of soup reports 220 mg of sodium

per serving of soup. How many grams of NaCl will be present

in one serving of the soup?

14. When passing through the small openings within crystal

structures, X-rays diffract into a pattern. Say a researcher is

deriving the structure of an interesting protein by using

X-rays with a frequency of 1.94 × 10

−1

. What is the

wavelength, in nanometers, of the X-rays?

15. The metal nickel has several remarkable uses. When alloyed

with other metals, it can form a type of “memory metal”

called nitinol, which is a nearly 1:1 mixture of nickel and

titanium. Nickel crystallizes in a face-centered cubic arrange-

ment. The density of nickel is 8.90 g/cm

. From this informa-

tion, calculate the radius of an atom of this coinage metal?

(Currently the percentage of nickel in a U.S. “nickel” coin is

approximately 25%.)

(

)(

)

16. The element iridium is currently an important part of an

extinction theory concerning dinosaurs. Unusual deposits

of iridium indicate that a large, iridium-containing meteor

may have hit our planet, resulting in a dust cloud from the

impact blocking the light from the sun. Iridium crystallizes

in a face-centered cubic structure. What is the density of this

corrosion-resistant metal whose radius is 136 pm?

17. Titanium metal and its oxide have a wide variety of uses. Its

relatively light weight and strength make it ideal in some

alloys. Titanium dioxide is found in paints and some lip-

sticks. Pure metallic titanium crystallizes in a body-centered

cubic structure. Titanium has a density of 4.5 g/cm

. What is

the radius of an atom of titanium? What percent of the unit

cell is empty space?

18. Metallic chromium has an atomic radius of 125 pm. Judging

on the basis of the density of chromium (7.2 g/cm

), which

of three cubic unit cells (simple, face-centered, or body-

centered) would you predict for chromium?

19. One way to verify the numerical value of Avogadro’s number

is to use X-ray data and unit cell calculations. Although the

reasoning is the same for any selection, use the following

speciﬁc information about copper to make the veriﬁcation.

Copper crystallizes with a face-centered unit cell with an

edge length of 3.6 × 10

−10

m. The density of copper is ap-

proximately 8.9 g/cm

20. A hypothetical metal was determined to have a fcc unit cell

and a density of 0.185 mol/cm

. What is the calculated radius

of the metal?

21. Assuming that the following metals crystallize in the fcc unit

cell, which would you predict to have the greater density,

Co (radius = 125 pm) or Rh (radius =135 pm)?

22. Assuming that the following metals crystallize into a bcc unit

cell, which would you predict to have the greater density,

V (radius = 131 pm) or Cr (radius = 125 pm)?

13.2 Metals

Skill Review

23. Identify the metals in the following list of elements.

B,Na,Al,Ca,Cr,Se,Sb,Bi

24. List two physical characteristics that would enable you to dif-

ferentiate between calcium and carbon (graphite).

25. How many valence electrons are found in an atom of potas-

sium? How many molecular orbitals are found when three

atoms of potassium combine?

26. How many valence electrons are found in an atom of cal-

cium? How many molecular orbitals are found when three

calcium atoms combine?

27. The following three energies represent the band gap values

for three samples. Which of the values in this list is most

likely to be that of a metal?...that of a semiconductor? . . .

that of an insulator? 85 kJ/mol, 450 kJ/mol, 2.5 kJ/mol.

28. Which of these energies would you expect copper metal to

possess as its band gap: 0.06 kJ/mol, 73 kJ/mol, or 510 kJ/mol?

29. The particular types of alloys known as amalgams use mer-

cury as a component. Explain, using the delocalized electron

576 Chapter 13 Modern Materials

bonding model, how the metal components in any alloy

allow the atoms of different metals to bond.

30. How does the amalgam picture you created in Problem 29

allow for varying ratios of components, unlike the set ratio of

typical compounds?

Chemical Applications and Practices

31. A grocery stocker wishes to illustrate the two types of alloys

to his friends by using red and green apples to repressent

atoms. Explain how this would be accomplished.

32. A grocery stocker creates a stack of soup cans to display two

different kinds of soups. Judging on the basis of the pattern

in which they are stacked, which type of alloy do they appear

to represent?

33. Sodium metal is very reactive, yet under controlled condi-

tions its ability to absorb heat on a per-gram basis can be very

useful. Some nuclear reactor designs make use of sodium in-

stead of water as a heat transfer agent.

a. How many valence electrons does sodium have?

b. Using band theory, explain why thermal energy causes

the electrons in sodium, like other metals, to populate

antibonding orbitals.

34. Show the construction of a molecular orbital diagram from

the combination of two sodium atoms, of three sodium

atoms, and of four sodium atoms.

35. Semiconductors can also be used in lasers. If a laser were

constructed and it produced a wavelength of 820 nm, what

would be the band gap energy (in joules per photon)?

36. What would be the band gap energy (in kilojoules per mole)

for a laser whose wavelength was 720 nm?

37. Doping silicon allows for the production of n-type and

p-type semiconductors. Would doping silicon with indium

make an n- or a p-type semiconductor? Explain the basis for

your answer.

38. Silicon and selenium both have semiconductor properties.Ex-

plain why doping both with arsenic would have opposite re-

sults in terms of producing n-type or p-type semiconductors.

39. The blue color of the Hope diamond is due to small amounts

of boron in the carbon lattice. Predict what electronic prop-

erties this diamond must have.

40. What properties would you expect to ﬁnd in a diamond

doped with small quantities of arsenic in the carbon lattice?

13.3 Ceramics

Skill Review

41. Explain why the bonding and resulting “band gap”in ceramic

materials make them good insulators.

42. Describe the difference between the bonding in the glass in a

window and the plaster in a wall.

43. Most compounds melt over a very small temperature range

that can even be used to help identify the compound. How-

ever, glass materials do not have such a small temperature

range for melting. Explain why the melting temperature for

glasses is replaced with a transition temperature.

44. Color, brittleness, and other properties of glass can be inﬂu-

enced by the presence of impurities. However, the glass used

in ﬁber optics must be based on very pure SiO

. Explain how

this material, in the form of glass ﬁbers, can be used to send

messages using light.

45. The frequency used for a typical MRI scan is in the range of

250 MHz. What are the wavelength and energy (in joules per

photon) for this technique?

46. a. Metals are known for their ability to conduct electricity.

However, the structure and bonding in metals also cause

resistance to conductance. Explain how phonons con-

tribute to the resistance.

b. What causes resistance to change at the transition tem-

perature in superconductors?

Chemical Applications and Practices

47. Silicates are formed from the connections between tetrahe-

dral structures made of a silicon atom bonded to four oxygen

atoms. Diagram this tetrahedral anion with a −4 charge.

48. Joining of the tetrahedral silicates typically occurs through

sharing of one of the oxygen atoms from one tetrahedron to

another. Draw a diagram of how the SiO

−4

ion could be used

to produce two different substances.

49. Glass containers are known to be relatively inert. In fact, cor-

rosive acids are typically stored in glass containers. However,

hydroﬂuoric acid (HF) will react with glass and must be

stored in polyethylene containers. This property of HF has

been used as a method of glass etching. (However, the fumes

produced are extremely harmful.) In the reaction shown

here, how many grams of glass would react with 10.0 g of HF?

4HF(aq) + SiO

(s) → 2H

O(l) + SiF

(g)

50. Safety glass is found in automobiles manufactured in the

United States. It consists of a “polymer sandwich”of a layer of

plastic between two glass layers. Using the known properties

of plastics and glass, explain why this arrangement may help

reduce some impact injuries in automobile accidents.

51. During a typical day, you are likely to come in contact with

the materials discussed in this chapter. Classify each of the

following as a metal, an alloy, a glass, a plastic, or a ceramic

(more than one answer may be appropriate).

a. the body of the pen you use;

b. a contact lens;

c. the frames on a pair of glasses.

52. Classify each of the following as a metal, an alloy, a glass, a

plastic, or a ceramic:

a. a milk jug;

b. the body of the spark plug in an automobile engine;

Focus Your Learning

577

c. the frame of an automobile;

d. an etched plaque.

Section 13.4 Plastics

Skill Review

53. What distinguishing property of plastics forms the basis of

this quote? “All plastics are polymers, but not all polymers are

plastics.”

54. Deﬁne the term plastic. Provide an example of a plastic and

an example of a polymer that isn’t a plastic.

55. Determine whether each of the following everyday items

contains a plastic, a metal,an alloy, a glass, or a ceramic (more

than one answer may be appropriate).

a. the frame of a compact mirror;

b. a mirror;

c. the body of the computer you use;

d. a drinking cup used at the dentist’s ofﬁce.

56. Determine whether each of the following everyday items

contains a plastic, a metal, an alloy, a glass, or a ceramic:

a. a CD case;

b. the outer body of a cell phone;

c. a leisure suit;

d. a burner on a stove.

Chemical Applications and Practices

57. Some manufacturers are using composites to obtain desir-

able properties for the exterior of new automobiles. Describe

what advantages a composite material might provide for

automobile exteriors. What disadvantages might exist?

58. Why would a manufacturer consider constructing an air-

plane wing using composite materials?

59. Most hydrocarbon polymers make poor electrical conduc-

tors, although there are some that do conduct electricity.

Explain (noting the types of bonds in the polymers) why this

property arises in polymers.

60. The construction of a polymer made from ethyne (C

)

could conceivably be quite advantageous. Considering the

fact that a resulting polymer could contain a nearly inﬁnite

string of alternating single and double bonds, what advan-

tage could you foresee to such a polymer?

Section 13.5 Thin Films and Surface Analysis

Skill Review

61. Approximate the number of gold atoms that would make up

the thickness of a gold ﬁlm on an astronaut’s visor. Assume

that the ﬁlm is 215 nm thick and that the radius of a gold

atom is 144 pm.

62. How many moles of gold would be needed to prepare a thin

ﬁlm that is 576 pm thick covering an area similar in size to

a postage stamp (1 in by 1 in)? The radius of a gold atom is

144 pm.

63. How many atoms of silver (atomic radius =145 pm) must be

deposited to completely cover an area that measures 2.54 cm

by 2.54 cm? Assume the layer is only one atom thick.

64. A notecard that measures 3 in by 5 in is coated on one side

with a thin ﬁlm of copper (atomic radius = 128). How much

will this coating add to the mass of the notecard?