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8/3/2019 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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Solids: Conductors, Insulators
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
Molecular orbital theory gives a more
detailed picture of the 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
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 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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Solids: Conductors, Insulators
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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Solids: Conductors, Insulators
and Semiconductors
Conductor Insulator
Band gap = 5.5 eV
530 kJ/molNo gap
GraphiteDiamond
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Solids: Conductors, Insulators
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
Semiconducting elements form the basis
of solid state electronic devices.
When silicon is doped with phosphorus, itbecomes an n-type semiconductor, in
which electric current is carried by
electrons.
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Semiconductors
Semiconducting elements form the basis
of solid state electronic devices.
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
Semiconducting elements form the basis
of solid state electronic devices.
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.