Chemistry part 38, Julia Burdge,2e (2009) pdf

Unlike nylon 6,6, however, nylon 6 is not a copolymer because it is synthesi z ed from a s ingle monomer called Bringing Chemistry to life Electrically Conducting Polymers You have s

I

o

I

Figure 25.8 Structure of DNA

amino group of one amino acid and the carboxy group of another (See Figure 10.13.) One water molecule is produced as each new C-N bond is formed

The C-N bond formed between an amino group and a carboxy group is called an amide bond because RCONR'R" is the general form of an amide [ ~~ Section 10.2, Table 10.2] Poly-

mers in which the monomers are connected by amide linkages are called polyamides The amide bond between amino acids is more specifically called a peptide bond, and proteins are routinely referred to as polypeptides Although polypeptides can form from a single amino acid (e.g., poly-

valine), most proteins in organisms consist of chains of different amino acids As a result, most proteins are random copolymers (Table 25.2)

Nylon 6,6 is a synthetic condensation polymer formed from hexamethylenediamine, a ecule with two amino groups, and adipic acid, which has two carboxy groups Water is the small

mol-molecule eliminated in this condensation reaction

nH 2 N 1 CH 2 t- N H 2 + nOH- ~-f CH 2t~ -OH _

NH-fCH2~NH-~1CH2t~-+-n

Both hexamethylenediamine and adipic acid contain six carbon atoms, which is why the

resulting polymer is called nylon 6,6 Like proteins, nylon 6,6 contains amide linkages between monomers, so nylon 6,6 is a polyamide, too Also like proteins, nylon 6,6 is a copolymer, although

nylon 6,6 is an a lt e rnatin g copolymer (Table 25.2), not a random copolymer

Dacron is the trade name for the condensation copolymer formed from ethylene glycol (a diol a molecule with two alcohol groups) and para-terephthalic acid (a diacid-a molecule with two carboxy groups) The condensation reaction removes the hydrogen atom from the alcohol on one monomer and the OH group from the carboxy group on a second monomer, producing water

Deoxyribonucleic acid (DNA; see Figure 25.8), which stores the genetic information for

every known organism and has a molecular weight as large as billions of grams per mole, is a biological condensation copolymer If your body contained anything but a tiny fraction of a mole

of DNA, you would be extremely heavy! DNA is a copolymer of a five-carbon sugar called ribose, four different molecular bases (adenine, thymine, guanine, and cytosine), and phosphoric acid (H3P04) Ribonucleic acid (RNA) is a biological condensation copolymer analogous to DNA, except that RNA is made from a five-carbon sugar called ribose and the four different molecular

deoxy-bases are adenine, uracil, guanine, and cytosine RNA molecules vary greatly in size They are large molecules, but still much smaller than DNA molecules

Sample Problem 25.2 shows how to determine the monomers in a copolymer

o

I

Setup Kevlar contains an amide (linkage between C and N) We split the C-N bond, adding

an H to the exposed N and an OH to the exposed C to produce an amine and a carboxylic acid ,

Practice Problem Nylon 6, like nylon 6 , 6, is a condensation polymer (a polyamide ) Unlike nylon

6,6, however, nylon 6 is not a copolymer because it is synthesi z ed from a s ingle monomer called

Bringing Chemistry to life

Electrically Conducting Polymers

You have seen so far that it is possible to polymerize organic compounds with carbon-carbon

double bonds (alkenes) The resulting addition polymers (e.g., polyethylene) have carbon-carbon

single bonds It is also possible, though, to polymerize organic compounds with carbon-carbon

tri-

pIe bonds The simplest of these compounds (known collectively as alkynes) is acetylene (C2H2),

which is commonly used in welding torches The polymerization of acetylene is very similar to

the addition polymerization of ethylene That is, a free-radical initiator molecule attaches to one of

the carbon atoms and breaks one of the pi bonds between the two carbon atoms in acetylene The

resulting polymer, polyacetylene, thus retains a carbon-carbon double bond:

I

The structure of polyacetylene contains alternating carbon-carbon single and

carbon-carbon double bonds Recall from Chapter 9 that the carbon atoms in a C=C double bond are

•

whether the monomer s in the final answer are reasonable , recombine them in a condensation reaction

to see if they re-form Ke v lar, the polymer in the problem

The syst e mati c n a m e of a cety lene is eth y n e,

bu t t he common n a me acetyle n e i s far m ore

•

Think About It The repeating

polymer unit in the general

structure resembles the original

monomer except that there i s a

carbon-carbon double bond in place

of a carbon-carbon triple bond If

each of the carbon-carbon s ingle

bonds at the ends of the repeating

unit in parenthese s were s plit open,

then one electron from each bond

could be combined to re-form

the second pi bond in the

carbon-carbon triple bond of the monomer

Sp2 hybridized and that it is the leftover p orbital on each carbon atom that overlaps to form the pi bond Each carbon atom in polyacetylene is identical (structurally and electronically), so there is

extended overlap of the p orbitals throughout the polymer chain The p electrons are

delocalized!-that is, they can move throughout the network of overlapping orbitals that extend the length of the polymer chain By being able to move electrons from one end of the polymer chain to the other,

poly acetylene can conduct electricity much like a wire That is, polyacetylene is a plastic that

Sample Problem 25.3 shows how to determine the structure of an electrically conducting

polymer

Sample Problem 25.3

Prop y ne ( HC=CCH3) ca n be u sed to form an e lectricall y conducting polymer Draw the structure of

polypropyne , s ho w ing at lea st three r e p eat ing unit s, and write the general formula for the polymer

Strategy Addition pol y mer s, w hether they are syn the size d from alkene s or alkynes, form via

a free-radi ca l reaction in which one pair of pi electrons in the carbon-carbon multiple bond of a monomer molecule is u se d to form carbon-carbon s ingle bonds to other monomer molecules Draw

the st ru ct ural formula of propyne suc h that the triple bond can be "opened up" to form single bonds

between co n sec uti ve monomer unit s

Setup The s tructural formula of propyne can be determined from the formula given and by analogy

to acetylene The only difference between propyne and acetylene is the CH3 group in place of one of

SECTION 25.2 Ceramics and Composite Materials 945

B-A-A-A-A-A-a) Block b) Random c) Graft

d) Alternating e) All the above

What feature is common to molecules that can undergo polymerization?

a) Fluorine b) Hydrogen bonds

c) Sulfur d) Multiple bonds

e) Lone pairs

Ceramics

The use of ceramics in the form of pottery dates back to antiquity Along with common ceramic

substances like brick and cement, modern advanced ceramics are found in electronic devices and

on the exterior of spaceships All these ceramics are polymeric inorganic compounds that share

the properties of hardness, strength, and high melting points 'Cer<iiiiics ' are ' 'u'suaiiy' 'fai-mea' by

melting and then solidifying inorganic substances (including clays)

Most ceramic materials are electrical insulators, but some conduct electricity very well at very

cold temperatures Ceramics are also good heat insulators, which is why the outer layer of the space

shuttle (shown in Figure 25.9) is made of ceramic tiles These ceramic tiles can withstand the very hot

temperatures of re-entry into the atmosphere (where temperatures can reach 1650°C), while keeping

the underlying metal structural body of the orbiter (and the astronauts within) relatively cool

Ceramics can be prepared by heating a slurry of a powder of the inorganic substance in water to a very high temperature under high pressure This process, called sintering, bonds the

The larger the lattice energy of an ionic compound, the higher the melting point

[ ~ Section 8.2, Table 8.1 ] Ceramics all have large lattice energies

-•

Multimedia

Chemical Reactions - thermite reaction

Figure 25 9 Captain Wendy Lawrence inspects the ceramic tiles on the exterior of the space shuttle

•

of bearings takes advantage of the inherent

imperfections produced by the process Porous

bearings are sometimes preferred because the

porosity allows a lubr i cant to flow throughout

the material

The "sol" in sol-gel refers to a colloidal

suspension of individual particles The "gel"

refers to the suspension of the resulting

polymer

•

particles to each other, thus producing the finished ceramic Sintering is a relatively easy process

to perform, but the resulting particle size is irregular, so the solid may contain cracks, spaces, and

make weak spots in the substance that can cause the ceramic piece to fail To avoid these lems, the sol-gel process is frequently employed for modem ceramics that are used in structural

prob- appilc · citlOns · the ' sol-gel process produces particles of nearly uniform size that are much more

likely to produce a solid ceramic without gaps or cracks

The first step in the sol-gel process is the preparation of an alkoxide of the metal or metalloid that is going to be made into the ceramic This can be illustrated with the yttrium(III) ion, which is used to prepare a yttrium-oxygen ceramic:

yttrium alkoride yttrium hydroxide ethanol

The metal hydroxide, once fOlIned, undergoes condensation polymerization to form a chain with bridging oxygen atoms between the metal atoms:

of the metal oxide-hydroxide polymer is called a gel The gel is carefully heated to remove the liquid, and what remains is a collection of tiny, remarkably uniformly sized particles Sintering of material produced by the sol-gel process produces a ceramic with relatively few imperfections

Composite Materials

A composite material is made from two or more substances with different properties that remain separate in the bulk material Each contributes properties to the overall material, though, such that the composite exhibits the best properties of each of its components One of the oldest examples

of a human-made composite material is the formation of bricks from mud and straw, a process that dates back to biblical times

Modem composite materials commonly include reinforcing fibers, analogous to the straw in mud bricks, in a polymer matrix called a resin Fiberglass, Kevlar, or carbon fibers can be used to give strength to the composite, whereas polyester, polyamide, or epoxy forms the matrix that holds the fibers together These types of composites are called polymer matrix composites Composites made from a metal and a ceramic, organic polymer, or another metal are called metal matrix composites Carbide drill tips are made with a combination of softer cobalt and tougher tungsten carbide Toyota has used a metal matrix composite in the engine block of some of its cars, and some bicycles are made with aluminum metal matrix composites Composites made of a ceramic

as the primary reinforcing material accompanied by organic polymers are called ceramic matrix composites A common biological ceramic matrix composite is bone Bones consist of reinforcing fibers made of collagen (a protein) surrounded by a matrix of hydroxyapatite, [Ca5(P04)30H]

Another type of composite material that does not fit neatly into one of these categories is a

reinforced carbon-carbon composite (RCC), which consists of carbon fibers in a graphite matrix

To prevent oxidation, a silicon carbide coating is applied RCC is used on the nose cones of

mis-siles, but it is most commonly known as the coating on the nose of the space shuttle It is believed that the Space Shuttle Columbia disaster in 2003 occurred because the ceramic tiles and RCC panel material were both damaged on liftoff by the impact of falling foam insulation The extreme heat of reentry into the atmosphere damaged the orbiter's structure through the breaches in the ceramic tiles and the RCC panel, and the orbiter and all seven astronauts on board were lost

Carbon fibers can be woven into fabrics and threads to be used as the structural components

of vehicles and for sporting equipment such as bicycles, tennis rackets, and skateboards

SECTION 25.3 Liquid Crystals 947

Liquid Crystals

Liquid crystals are substances that exhibit properties of both liquids, such as the ability to flow and

to take on the shape of a container, and those of crystals, such as a regular arrangement of particles

in a lattice Some substances exhibit liquid crystal behavior when they are melted from a solid As

the temperature increases, the solid liquefies but retains some order in one or two dimensions At

a higher temperature, the ordered liquid becomes a more conventional liquid in which there is no

consistent orientation of the molecules

Liquids are isotropic, because their properties are independent of the direction of testing

Because particles in a liquid are free to rotate, there are molecules present in every possible

ori-entation, so the bulk sample gives the same measurements regardless of the direction in which

the measurements are taken or observations are made Liquid crystals are anisotropic because

the properties they display depend on the direction (orientation) of the measurement A box full

of pencils, all neatly aligned, is an analogy for anisotropy, because the pencils are long and

nar-row Looking at the pencils end on is different than looking at them from the side What you see

depends on the direction from which you view the pencils Similarly, what you see when you look

at anisotropic molecules depends on the direction from which you view them

Frederick Reinitzer discovered the first compound to exhibit liquid crystal behavior in 1888

Cholesteryl benzoate was observed to form an ordered liquid crystalline phase when melted, and

this liquid crystalline phase became an ordinary liquid at higher temperatures:

Cholesteryl benzoate

The structure of cholestery I benzoate is fairly rigid due to the presence of the fused rings and

2 ~

about carbon-carbon single bonds (i.e., bonds between Sp3 -hybridized carbon atoms), but not about carbon-carbon double bonds (i.e., bonds between Sp2 -hybridized carbon atoms)

ity and this particular shape makes it possible for the cholesteryl benzoate molecules to arrange

themselves in an orderly manner, in much the way that pencils, chopsticks, or tongue depressors

can be arranged

There are three different types of ordering (called nematic, smectic, and cholesteric) that a liquid crystal can adopt (see Figure 25.10) A nematic liquid crystal contains molecules ordered

in one dimension only Nematic molecules are all aligned parallel to one another, but there is no

organization into layers or rows A smectic liquid crystal is ordered in two dimensions Smectic

molecules are aligned parallel to one other and are further arranged in layers that are parallel to

one another Cholesteric liquid crystal molecules are parallel to each other within each layer, but

each layer is rotated with respect to the layers above and below it The layers are rotated because

there are repulsive intermolecular forces between layers

Liquid crystalline substances were curiosities for many years but have found applications in many fields Liquid crystal displays (LCDs) in calculators, watches, and laptop computers are pos-

sible, for example, because polarized light can be transmitted through liquid crystals in one phase

but not transmitted through another (Figure 25.11) Polarized light is produced by passing

ordi-nary light (which oscillates in all directions perpendicular to the beam) through a filter that allows

(b) cholesteric, and (c) smectic liquid

•

crystals

948 CHAPTER 25 Modern Materials

Figure 25.11 Liquid crystal

display When the current is on, the

liquid crystal molecules are aligned and

polarized light cannot pass through

When the current is off, the molecules

are not aligned and light can pass

through

•

Figure 25.12 LCD thermometer

A material that changes color as the

temperature changes is called thermochromic

-,

molecules not aligned,

light pas s es, bright region

Polarizer

Top plate

LC layer Bottom plate

_ _ -I-Polarizer

~-t-Mirror

Current on:

molecules aligned, light blocked, dark region

only the light waves oscillating in one particular plane to pass through This plane-polarized light can be rotated by a twisted liquid crystal such that it is allowed to pass through another filter when

a voltage is applied to the liquid If the light passes through the liquid when the voltage is off, though, the light will not pass through Thjs principle can be used in calculator and watch displays because each digit is made up of no more than the seven segments shown in Figure 25.11, and the

specific digit depends on which segments are bright and which are dark LCDs in laptop ers and televisions are more sophisticated, but still operate on the same basic principle

comput-Liquid crystals can also be used in thermometers (Figure 25.12) The spacing between tal layers depends on temperature, and the wavelength of light reflected by the crystal depends on tills spacing Thus, the color reflected by the liquid crystal will change with temperature, and this color can be used to indicate the temperature to which the liquid crystal is exposed

crys-Sample Problem 25.4 lets you practice determining whether or not a molecule might exhibit liquid crystal properties

Would the following molecule make a good liquid crystal? Why or why not?

~C <

Strategy Cho le steryl benzoate exhibits liquid crysta l properties because it has structurally rigid region s ( fused rings and si-hybridized carbon atoms) and because itis relatively long compared to

SECTION 25.4 Biomedical Materials 949

its width Examine the structure in que s tion to see if it has rigid region s and to see if it is longer in

one dimension than in another

Setup Carbon atoms that are sp2-hybridized contribute to the rigidity of a molecule's shape

Solution The left-hand portion of the molecule contains an Sp2_ hybridized carbon atom, a benzene

ring, another sp2-hybridized carbon atom, and another benzene ring These feature s are relatively

rigid and should allow the molecule to maintain its shape when heated The chain of CH2 groups

ending in a CH3 group is not rigid due to free rotation about the carbon-carbon single bonds (i.e.,

sp3-hybridized carbon atoms) The overall s hape of the molecule is longer than it i s wide, though,

which combined with a large portion that is rigid should make the molecule a good candidate for

liquid crystal behavior

Practice Problem Which compound would you expect to exhibit liquid crystal behavior , and why?

25 3 1 Which of the following is a good

analogy for anisotropy? ( Select all that apply )

25 3 2 What characteristics make a molecule

likely to exhibit liquid crystal

propertie s? (Se lect all that apply ) a) Long narrow s hape

b ) Flexibility c) Rigidity d) Low molar mass

e) High molar mas s

Many modern materials are finding uses in medical application s Replacement joints, dental

implants, and artificial organs all contain modern polymers, composites, and ceramics To

func-tion successfully in a biomedical application, though, a material mu s t first be compatible with the

living system The human body very easily recognizes foreign substances and attacks them in an

effort to rid them from the body Thus, a biomaterial must be designed with enough similarity to

the body's own systems that the body will accept the material as its own Additionally, if the

poly-mer, ceramic, or composite contains leftover chemicals from it s manufacture, these contaminants

may lead to adverse reactions with the body over the lifetime of the medical implant It is

impor-tant, therefore, that the substance be as pure as possible

The physical properties of a biomedical material are important, too , because the longer the material lasts, the less often the medical device has to be replaced during the lifetime of the patient ·

Most biomedical materials must possess great strength and flexibility to perform in the body For

example, the materials in artificial joints and heart valves must be able to flex many times without

breaking Materials used in dental implants must show great hardness, moreover , so as not to crack

during biting and chewing It takes years of research to develop successful biomedical materials

that meet these needs

Think About It Double bonds

in a molecular structure indicate

the presence of pi bonds Recall that it is the pi bonds that restrict

rotation about bonds in a molecule

[ ~ Sect i on 9 5] and lend rigidity

to the s tructure

•

•

Dental Implants

Dental implant materials have been used for many years The oldest dental fillings were made of

various materials including lead (which fell out of favor due to its softness the danger of lead poisoning was as yet unknown), tin, platinum, gold, and aluminum The remains of Confeder-ate soldiers have been shown to include fillings made of a lead-tungsten mixture, probably from shotgun pellets; tin-iron; a mercury amalgam; gold; and even radioactive thorium (The dentist probably thought he was using tin, which is similar in appearance.) Many modern fillings are made of dental amalgam [ ~~ Chapter 19] , a solution of several metals in mercury Modern dental amalgam consists of 50 percent mercury and 50 percent of an alloy powder that usually contains

(in order of abundance) silver, tin, copper, and zinc These metals tend to expand slightly with age, causing fissures and cracks in the tooth that may require further intervention (e.g., crowns, root canals, or tooth replacement) Some people consider amalgam fillings to be unsightly, too

Dental composite materials are now used that have several advantages over traditional

amal-gams The composites can be made in a wide range of colors, for example, to match the color of the other teeth The existing healthy portion of the tooth can be etched with acid, moreover, to cre-ate pores into which the composite material can bond With traditional dental amalgam, the dentist must create indentations in the healthy tooth to hold the amalgam in place Destroying a portion of

the healthy tooth is undesirable and can be avoided with the use of the composite

The dental composite consists of a matrix (made from a methacrylate resin) and a silica

filler The composite material is applied to the cavity in a putty-like consistency and then is dried and polymerized by a photochemical reaction initiated by light of a particular wavelength Because the light does not penetrate very far into the composite, the thickness of the applied composite is

critical If the layer of composite is too thick, some of the composite will remain soft A properly constructed dental composite filling will last more than 10 years Over time, though, composite fillings tend to shrink, leaving breaches that can cause leakage, a situation that must be addressed

to prevent further tooth decay

Porcelain ceramic fillings and crowns are very common, but the ceramic has two

disadvan-tages: it is very hard and can wear on the opposing teeth, and it is brittle and may crack if subjected

to great force Most dentists, therefore, do not recommend porcelain ceramic crowns and fillings for the molar teeth, which do the bulk of the grinding work during chewing

A material used in dentistry must have properties that maximize both patient comfort and the lifetime of the implant Dental fillings and tooth replacements must be resistant to acids, for example, because many foods (such as citrus fruits and soft drinks) contain acids directly Any food containing carbohydrates can produce acids, though, if traces are left in the mouth because the bacteria that reside there consume carbohydrates, producing acids in the process These acids eat away at the natural hydroxyapatite in teeth, creating caries (cavities) Materials like dental amalgam, gold, and dental composite resist attack by acids in the mouth

A dental material should also have low thermal conductivity; that is, it should conduct heat poorly This is especially important in applications where the implant material is in contact with the nerve inside the tooth If the implant material conducts heat well, then whenever hot or cold foods come in contact with the implant, the hot or cold will be transmitted through the material to the nerve, causing pain Metals are good thermal conductors, so dental amalgams must not touch nerves

Finally, dental materials should resist wear (so they last a long time) and they should resist expansion and contraction as the temperature fluctuates The close spacing and precision fits involved in dental fillings and implants would result in discomfort and possibly failure of the implant if the material expanded or contracted appreciably with changes in temperature

Soft Tissue Materials

Burn victims who have lost large amounts of skin do not have enough cells to grow new skin,

so a synthetic substitute must be used The most promising artificial skin material is based on

a polymer of lactic acid and glycolic acid Both of these compounds contain an alcohol group

(-OH) and a carboxy group (-COOH), so they can form a polyester copolymer in a condensation reaction that mirrors the formation of Dacron polyester (Section 25.1):

SECTION 25.4 Biomedical Materials 95 1

This copolymer forms the structural mesh that supports the growth of skin tissue cells from

a source other than the patient Once the skin cells grow on the structural mesh, the artificial skin

is applied to the patient, and the copolymer mesh eventually disappears as the ester linkages are

hydrolyzed During this treatment, the patients must take drugs that will suppress their immune

rejecting the new skin

Sutures, commonly known as stitches, are now made of the same lactic acid-glycolic acid

did not degrade so they eventually had to be removed again This type of suture is still used in

owner

Artificial hearts have not yet been perfected to the point that they can work on a permanent basis,

and hold it in place

of the nanofiber wound dressing

Artificial Joints

of the PMMA and polyethylene parts

no more than 20 years in a patient before needing to be replaced again

H ydroly s i s is e ssentially th e oppos i t e of a con d ensation re a ct ion

,

Recall that allotrop es are different forms of the

sa me element [ ~~ Section 2 6]

(a)

Despite the progress made so far on developing safe, durable, and useful artificial joints,

Nanotechnology

A sharp-pointed stylus is moved over the surface of the substance under study, and the

mounted on a probe that reflects a laser beam into a detector From the differences in deflection

The scanning tunneling microscope (STM) works on a similar principle but only for samples

that conduct electricity The STM measures the peak and valley heights of the sample from the

and the up and down movements are translated into an image of the atomic and molecular

struc-ture of the substance [Figure 2S.13(c)]

Graphite, Buckyballs, and Nanotubes

Carbon 'exists' as 'seve'ral allotropes, one of which is graphite Graphite consists of sheets of carbon

Intermolecular forces [ ~~ Chapter 11] hold the sheets together Because of the delocalization of

Figure 25 13 ( a ) Atomic force microscope ( AFM ) ( b ) AFM image of a yeast cell (c) STM image of iron atom s arranged to displa y the Chinese

characters for atom on a copper su rface

SECTION 25.5 Nanotechnolog y 9:>3

is an electrical conductor within its sheets, but not between them Electrical conductivity is much

though, a compound consisting of 60 carbon atoms was isolated from graphite rods that had been

of each face:

/ / ,

in such exotic places as stars and interstellar media and have been observed in such mundane

ure 25.14 The differences in arrangement of hexagons give the two kinds of nanotubes different

electrical conductivities

Carbon nanotubes of different diameters can nest inside each other, producing walled nanotubes Most nanotubes are single-walled, though, and they can be as narrow as 1 nm

piece of steel! Because nanotubes consist only of carbon atoms, they can come into close

con-tact, thereby allowing intermolecular forces to hold large numbers of tubes together This, in turn,

makes it possible to align groups of tubes, potentially forming long strands and fibers Lightweight

Besides carbon, other elements have been fashioned into nanotubes, too Molybdenum

Many universities have developed large-scale facilities for nanotechnology research in

pro-duction of silicon chip-based electronic circuits, nanotechnology may provide the pathway to

•

Figure 25.15 Energ y b an d s in

metals (conductors), se miconductor s,

and insulators

•

The group num ber for a main group element

corresponds to the number of valence electrons

thatthe atom has [ ~~ Section 8.5]

-i11 ~ k Valence band

Semiconductors

energies of the bonding and antibonding bands depend on the energies of the atomic orbitals that combined to form them in the first place As a result, the band structure of a bulk sample depends

on the original atoms' energy levels The band energies and the gaps (or lack of gaps) between the

behavior of the material

Metals have no band gap, so they are good conductors of electricity The valence band and

energy balTier to the movement of electrons from one atom to another in a metal

valence band to the conduction band Nonconductors (electrical insulators) have large band gaps,

so it is nearly impossible to promote electrons from the valence band to the conduction band

Silicon, germanium, and carbon in the form of graphite are the only elemental

semicon- ~

semiconductors consist of combinations of elements whose valence electron count totals 8 For

contributes three valence electrons and phosphorus contributes five, giving a total of eight valence

and Cd) and Group 6A elements

Sample Problem 25.5 lets you practice identifying combinations of elements that can exhibit semiconductor properties

Sample

State whether each of the following combinations of elements could form a semiconductor: (a)

Ga-Se , ( b ) In-P , ( c ) Cd-Teo

Strategy Count the v alence electrons in each t y pe of atom If they total eight for the two elements,

then the combination w ill probabl y form a se miconductor