Lo Ppt13 Miller Short2

8/3/2019 Lo Ppt13 Miller Short2

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Solids: Conductors, Insulators

and Semiconductors• Conductors: mostly metals

• Insulators: mostly nonmetal materials

– we’ll study allotropes of carbon and study

their properties

• Semiconductors: metalloids



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and Semiconductors

Conductor Insulator Semiconductor

Valence Band

in red

Conduction Band: white

Band gapNo gap



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Bonding in Metals

• The electron-sea model is a simple

depiction of a metal as an array of positive

ions surrounded by delocalized valenceelectrons. – Metals are good conductors of electricity because

of the mobility of these delocalized valence

electrons.

– A metal also conducts heat well because the mobileelectrons can carry additional kinetic energy.



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Bonding in Metals



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Bonding in Metals

• Molecular orbital theory gives a more

detailed picture of the bonding in metals.

– Because the energy levels in a metalcrowd together into bands, this pictureof metal bonding is called band theory.



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Bonding in Metals

• Molecular orbital theory gives a more

detailed picture of the bonding in metals.

– In a metal, this requires little energy since the unoccupied orbitals lie just abovethe occupied orbitals of highest energy.



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Bonding In

Metals:

Lithium

according toMolecular

Orbital

Theory



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Magnesium

3s bonding and antibonding should be full



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Magnesium

Conduction band:

empty

Valence band:

full

No gap: conductor

Conductor



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and Semiconductors

Conductor Insulator Semiconductor

Valence Band

in red

Conduction Band: white

Band gapNo gap



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Allotropes of Carbon

• Diamond: high thermal conductivity,extremely strong, insulator

• Graphite: high thermal conductivity,conductor

– electrodes for electrolysis and batteries;essentially pencil “lead”

• Fullerenes: discovered in 1986, amazing possibilities



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Diamond

• Diamond has a three-dimensional network structure in which each carbon is singly-bondedto four others with sp3 hybridization.

• Diamond is a covalent network solid

– each carbon covalently bonded to 4 others.

• Diamonds are the hardest substance known.

– must break carbon-carbon bonds • Diamonds have a melting point of 3550°C.

– must break carbon-carbon bonds



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Structure of Diamond



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Diamond

• Diamond has a three-dimensional network structure inwhich each carbon is singly-bonded to four others with sp3 hybridization.

• Why do diamonds conduct heat? – Metals conduct heat because the the mobile electrons can

carry additional kinetic energy.

– Diamonds are insulators and have no mobile electrons.

– Diamonds conduct heat through high frequency (= high

energy) vibrations that transmit over long distances• Diamonds conduct heat 4 times better than copper!



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Graphite

• Graphite has a layered structure, in which

the carbon atoms in each layer bond to three

other carbons with sp2 orbitals.• Graphite’s primary use is in the

manufacture of electrodes for electrolysis

and batteries. – Of the covalent network solids, only graphiteconducts electricity.

– This is due to the delocalization of the resonant p electrons in graphite’s sp

2

hybridization.



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Structure of Graphite



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Fullerenes

• The fullerenes are a family of molecules with a

closed cage of carbon atoms arranged in

pentagons and hexagons. Each carbon is sp2

hybridized.

– The most symmetrical member isbuckminsterfullerene, C60.

– Buckminsterfullerenes show potential forapplications in superconductivity and catalyticactivity.



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Figure 13.25:A frame

model

of C60.By permission of Dr.

Richard Smalley, Rice

University

Buckminsterfullerene



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and Semiconductors

Conductor Insulator

Band gap = 5.5 eV

≈ 530 kJ/mol No gap

GraphiteDiamond



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and Semiconductors

Semiconductor

Band gap

Band Gap for Semiconductors

Diamond 5.5 eV

Si 1.1 eV

Ge 0.67 eV



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Semiconductors

• Metalloids: semiconducting elements – low electrical conductivity at room temperature

– Electrical conductivity increases with temp.

• Gap between valence and conduction band

is intermediate in size



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Semiconductors

• Semiconducting elements form the basis of

solid state electronic devices.

– A striking property of these elements is that their

conductivities increase markedly when they aredoped with small quantities of other elements.

– Metalloids (such as silicon or germanium) aresemiconducting elements whose electricalconductivity increases as temperature increases.



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Semiconductors



– When silicon is doped with phosphorus, itbecomes an n-type semiconductor, inwhich electric current is carried byelectrons.



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Semiconductors



– When silicon is doped with boron, itbecomes a p-type semiconductor, inwhich an electrical current is carried bypositively charged holes



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Semiconductors



– Joining a p-type semiconductor to an n-type semiconductor produces a p-n

junction, which can function as a rectifier.

– A rectifier is a device that allows current to

flow in one direction, but not the other.



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Figure 13.29: Effect of doping silicon.



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Figure 13.30:

A p-n junction as a

rectifier.

Lo Ppt13 Miller Short2

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