Metal Bonding 2

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Copyright by Houghton Mifflin Company. All rights reserved. 1

Solids: Conductors, Insulators

and Semiconductors Conductors: mostly metals

Insulators: mostly nonmetal materials

well 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 mobile

electrons 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 metal

crowd together into bands, this picture

of metal bonding is called band theory.


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Bonding in Metals



According to band theory, the electrons in a

crystal become free to move when they are

excited to the unoccupied orbitals of a

band.


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Bonding in Metals



In a metal, this requires little energy

since the unoccupied orbitals lie just above

the occupied orbitals of highest energy.


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Bonding In

Metals:

Lithium

according toMolecular

OrbitalTheory


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Sodium According to Band Theory

Conduction band:

empty 3s antibonding

Valence band:

full 3s bonding

No gap


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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, amazingpossibilities


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Diamond

Diamond has a three-dimensional networkstructure 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 3550C.

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 withsp3 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.

Graphites 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 resonantelectrons in graphites 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 is

buckminsterfullerene, C60.

Buckminsterfullerenes show potential forapplications in superconductivity and catalytic

activity.


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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/molNo 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 electrical

conductivity increases as temperature increases.


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Semiconductors



When silicon is doped with phosphorus, itbecomes an n-type semiconductor, in

which electric current is carried by

electrons.


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Semiconductors



When silicon is doped with boron, itbecomes a p-type semiconductor, in

which an electrical current is carried by

positively 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 rectifieris 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:

Ap-njunction as a

rectifier.

Metal Bonding 2

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