A Molecular Comparison of
Liquids and Solids
The fundamental difference between states of
matter is the distance between particles.
Intermolecular
Forces
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States of Matter
Because in the solid and liquid states
particles are closer together, we refer to them
as condensed phases.
Intermolecular
Forces
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The States of Matter
• The state a substance is
in at a particular
temperature and
pressure depends on
two antagonistic entities:
– the kinetic energy of the
particles;
– the strength of the
attractions between the
particles.
Intermolecular
Forces
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Intermolecular Forces
The attractions between molecules are not
nearly as strong as the intramolecular
attractions that hold compounds together.
Intermolecular
Forces
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Intermolecular Forces
They are, however, strong enough to control
physical properties such as boiling and
melting points, vapor pressures, and
viscosities.
Intermolecular
Forces
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Intermolecular Forces
These intermolecular forces as a group are
referred to as van der Waals forces.
• Dipole-dipole interactions
• Hydrogen bonding
• London dispersion forces
Intermolecular
Forces
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Ion-Dipole Interactions
• Ion-dipole interactions (a fourth type of force),
are important in solutions of ions.
• The strength of these forces are what make it
possible for ionic substances to dissolve in
polar solvents.
Intermolecular
Forces
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Dipole-Dipole Interactions
• Molecules that have
permanent dipoles are
attracted to each other.
– The positive end of one is
attracted to the negative
end of the other and viceversa.
– These forces are only
important when the
molecules are close to
each other.
Intermolecular
Forces
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Dipole-Dipole Interactions
The more polar the molecule, the higher
is its boiling point.
Intermolecular
Forces
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London Dispersion Forces
While the electrons in the 1s orbital of helium
would repel each other (and, therefore, tend
to stay far away from each other), it does
happen that they occasionally wind up on the
Intermolecular
same side of the atom.
Forces
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London Dispersion Forces
At that instant, then, the helium atom is polar,
with an excess of electrons on the left side
and a shortage on the right side.
Intermolecular
Forces
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London Dispersion Forces
Another helium nearby, then, would have a
dipole induced in it, as the electrons on the
left side of helium atom 2 repel the electrons
in the cloud on helium atom 1.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
London Dispersion Forces
London dispersion forces, or dispersion
forces, are attractions between an
instantaneous dipole and an induced dipole.
Intermolecular
Forces
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London Dispersion Forces
• These forces are present in all molecules,
whether they are polar or nonpolar.
• The tendency of an electron cloud to distort in
this way is called polarizability.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Factors Affecting London Forces
• The shape of the molecule
affects the strength of dispersion
forces: long, skinny molecules
(like n-pentane tend to have
stronger dispersion forces than
short, fat ones (like neopentane).
• This is due to the increased
surface area in n-pentane.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Factors Affecting London Forces
• The strength of dispersion forces tends to
increase with increased molecular weight.
• Larger atoms have larger electron clouds
which are easier to polarize.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Which Have a Greater Effect?
Dipole-Dipole Interactions or Dispersion Forces
• If two molecules are of comparable size
and shape, dipole-dipole interactions
will likely the dominating force.
• If one molecule is much larger than
another, dispersion forces will likely
determine its physical properties.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Practice
1. The dipole moments of acetonitrile, CH3CN, and methyl iodide,
CH3I, are 3.9 D and 1.62 D, respectively.
(a) Which of these substances has greater dipole–dipole
attractions among its molecules?
(b) Which of these substances has greater London dispersion
attractions?
(c) The boiling points of CH3CN and CH3I are 354.8 K and 315.6 K,
respectively. Which substance has the greater overall attractive
forces?
2. Of Br2, Ne, HCl, HBr, and N2, which is likely to have
(a) the largest intermolecular dispersion forces,
(b) the largest dipole–dipole attractive forces?
Intermolecular
Forces
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How Do We Explain This?
• The nonpolar series
(SnH4 to CH4) follow
the expected trend.
• The polar series
follows the trend
from H2Te through
H2S, but water is
quite an anomaly.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Hydrogen Bonding
• The dipole-dipole interactions
experienced when H is bonded to
N, O, or F are unusually strong.
• We call these interactions
hydrogen bonds.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Hydrogen Bonding
• Hydrogen bonding
arises in part from the
high electronegativity
of nitrogen, oxygen,
and fluorine.
Also, when hydrogen is bonded to one of those
very electronegative elements, the hydrogen
nucleus is exposed.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Practice
1. In which of the following substances is hydrogen
bonding likely to play an important role in determining
physical properties:
methane (CH4), hydrazine (H2NNH2), methyl fluoride (CH3F), or
hydrogen sulfide (H2S)?
2. In which of the following substances is
significant hydrogen bonding possible:
methylene chloride (CH2Cl2), phosphine (PH3), hydrogen peroxide
(HOOH), or acetone (CH3COCH3)?
Intermolecular
Forces
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Summarizing Intermolecular Forces
Intermolecular
Forces
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More practice
1. List the substances BaCl2, H2, CO, HF,
and Ne in order of increasing boiling
points.
2. Given the following:
CH3CH3, CH3OH, and CH3CH2OH.
(a) Identify the intermolecular attractions
present in the following substances
(b) select the substance with the highest
boiling point:
Intermolecular
Forces
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Some Properties of Liquids
The strength of the attractions between
particles can greatly affect the properties of
a substance or solution.
With increasing IMF….
• Higher Viscosity
• Higher Surface Tension
• Higher Boiling and Melting Point
• Lower Equilibrium Vapor Pressure
Intermolecular
Forces
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Viscosity
• Resistance of a liquid
to flow is called
viscosity.
• It is related to the ease
with which molecules
can move past each
other.
• Viscosity increases
with stronger
intermolecular forces
and decreases with
higher temperature.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Properties of Liquids
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion
Intermolecular
Forces
11.3
Concave Meniscus of Water
Intermolecular
Forces
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V. Kirss, and
Geoffrey Davies, Norton Publisher, 2004, p 458
Surface Tension
Surface tension
results from the net
inward force
experienced by the
molecules on the
surface of a liquid.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Phase Changes
Intermolecular
Forces
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Energy Changes Associated
with Changes of State
The heat of fusion is the energy required to
change a solid at its melting point to a liquid.
Intermolecular
Forces
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Energy Changes Associated
with Changes of State
The heat of vaporization is defined as the
energy required to change a liquid at its
boiling point to a gas.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Energy Changes Associated
with Changes of State
• The heat added to the
system at the melting
and boiling points goes
into pulling the
molecules farther apart
from each other.
• The temperature of the
substance does not rise
during a phase change.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Practice
1. Calculate the enthalpy change upon converting
1.00 mol of ice at –25 °C to water vapor (steam)
at 125 °C under a constant pressure of 1 atm. The
specific heats of ice, water, and steam are 2.03 J/g-K, 4.18 J/g-K,
and 1.84 J/g-K, respectively. For H2O, ΔHfus = 6.01 kJ/mol and
ΔHvap = 40.67 kJ/mol.
2. What is the enthalpy change during the
process in which 100.0 g of water at 50.0 °C is
cooled to ice at –30.0 °C ?
Intermolecular
Forces
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Vapor Pressure
• At any temperature some molecules in a
liquid have enough energy to escape.
• As the temperature rises, the fraction of
molecules that have enough energy to
escape increases.
Intermolecular
Forces
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Vapor Pressure
As more molecules
escape the liquid,
the pressure they
exert increases.
Intermolecular
Forces
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Vapor Pressure
The liquid and vapor
reach a state of
dynamic equilibrium:
liquid molecules
evaporate and vapor
molecules condense
at the same rate.
Intermolecular
Forces
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Vapor Pressure
• The boiling point of a
liquid is the temperature
at which it’s vapor
pressure equals
atmospheric pressure.
• The normal boiling point
is the temperature at
which its vapor
pressure is 760 torr.
* Estimate the boiling point of diethyl ether under an
external pressure of 0.80 atm.
* At what external pressure will ethanol have a boiling
point of 60 °C?
Intermolecular
Forces
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Phase Diagrams
(prior knowledge)
Phase diagrams display the state of a
substance at various pressures and
temperatures and the places where equilibria
exist between phases.
Intermolecular
Forces
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Phase Diagrams
• The circled line is the liquid-vapor interface.
• It starts at the triple point (T), the point at
which all three states are in equilibrium.
Intermolecular
Forces
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Phase Diagrams
It ends at the critical point (C); above this
critical temperature and critical pressure the
liquid and vapor are indistinguishable from
each other.
Intermolecular
Forces
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Phase Diagrams
Each point along this line is the boiling point
of the substance at that pressure.
Intermolecular
Forces
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Phase Diagrams
• The circled line in the diagram below is the
interface between liquid and solid.
• The melting point at each pressure can be
found along this line.
Intermolecular
Forces
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Phase Diagrams
• Below the triple point the substance cannot
exist in the liquid state.
• Along the circled line the solid and gas
phases are in equilibrium; the sublimation
point at each pressure is along this line.
Intermolecular
Forces
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Phase Diagram of Water
• Note the high critical
temperature and critical
pressure.
– These are due to the
strong van der Waals
forces between water
molecules.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Phase Diagram of Water
• The slope of the solidliquid line is negative.
– This means that as the
pressure is increased at a
temperature just below the
melting point, water goes
from a solid to a liquid.
Intermolecular
Forces
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Phase Diagram of Carbon Dioxide
Carbon dioxide
cannot exist in the
liquid state at
pressures below
5.11 atm; CO2
sublimes at normal
pressures.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Structures of Solids
• We can think of
solids as falling into
two groups:
– crystalline, in which
particles are in highly
ordered arrangement.
Intermolecular
Forces
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Solids
• We can think of
solids as falling into
two groups:
– amorphous, in which
there is no particular
order in the
arrangement of
particles.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Attractions in Ionic Crystals
In ionic crystals, ions pack themselves so as to
maximize the attractions and minimize repulsions
between the ions.
Intermolecular
Forces
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Bonding in Solids
Intermolecular
Forces
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Covalent-Network and
Molecular Solids
• Diamonds are an example of a covalentnetwork solid, in which atoms are covalently
bonded to each other.
– They tend to be hard and have high melting
points.
Intermolecular
Forces
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Covalent-Network and
Molecular Solids
• Graphite is an example of a molecular solid,
in which atoms are held together with van der
Waals forces.
– They tend to be softer and have lower melting
points.
Intermolecular
Forces
© 2009, Prentice-Hall, Inc.
Metallic Bonding, Alloys &
Semiconductors
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Metallic Solids
• Metals are not covalently
bonded, but the
attractions between
atoms are too strong to
be van der Waals forces.
• In metals valence
electrons are delocalized
throughout the solid.
Intermolecular
Forces
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Alloys
• Alloys contain more than one element and have
the characteristic properties of metals.
• Solid Solution alloys are homogeneous
mixtures.
• Heterogeneous alloys: The components are not
dispersed uniformly (e.g., pearlite steel has two
phases: almost pure Fe and cementite, Fe3C).
• Pure metals and alloys have different physical
properties.
• An alloy of gold and copper is used in jewelry (the
alloy is harder than the relatively soft pure 24
karat gold).
• 14 karat gold is an alloy containing 58% gold.
Intermolecular
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Langella, APSI
Metal Alloys-Solid Solutions
Substance has mixture of
element and metallic properties.
1.Substitutional Alloy: some metal atoms
replaced by others of similar size.
Electronegativities usually are similar. The atoms
must have similar atomic radii.
The elements must have similar bonding
characteristics.
• brass = Cu/Zn
Intermolecular
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Metal Alloys
(continued)
2.Interstitial Alloy: Interstices (holes) in closest packed
metal structure are occupied by small atoms. Solute atoms
occupy interstices “small holes” between solvent atoms.
One element (usually a nonmetal) must have a significantly
smaller radius than the other (in order to fit into the
interstitial site).
steel = iron + carbon
3.Both types: Alloy steels contain a mix of substitutional
(Cr, Mo) and interstitial (Carbon) alloys.
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Substitutional
Alloy
Interstitial Alloy
Intermolecular
Forces
Presented By, Mark
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Alloys vs. Pure Metal
• The alloy is much harder, stronger, and less ductile than the pure metal
(increased bonding between nonmetal and metal).
An example is steel (contains up to 3% carbon).
mild steels (<0.2% carbon) - useful for chains, nails, etc.
medium steels (0.2-0.6% carbon) - useful for girders, rails, etc.
high-carbon steels (0.6-1.5% carbon) - used in cutlery, tools,
springs.
Other elements may also be added to make alloy steels.
Addition of V and Cr increases the strength of the steel and improves its
resistance to stress and corrosion.
The most important iron alloy is stainless steel. It contains C, Cr (from
ferrochrome, FeCr2), and Ni.
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Which two substances are most
likely to form an interstitial alloy?
•
•
•
•
•
Nickel and titanium
Silver and tin
Tin and lead
Copper and zinc
Tungsten and carbon
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Which two substances are most
likely to form an interstitial alloy?
•
•
•
•
•
Nickel and titanium
Silver and tin
Tin and lead
Copper and zinc
Tungsten and carbon
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Bonding Models for Metals
Intermolecular
Forces
APSI 2014
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Bonding Models for Metals
•Electron Sea Model: A regular
array of metals in a “sea” of
electrons. The electron-sea model
is a qualitative interpretation of
band theory (molecular-orbital
model for metals).
•Band (Molecular Orbital) Model:
Electrons assumed to travel around
metal crystal in MOs formed from
valence atomic orbitals of metal
atoms.
•Conduction Bands: closely spaced
empty molecular orbitals allow
conductivity of heat and electricity.
Intermolecular
Forces
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Molecular Orbital Theory
Recall that atomic
orbitals mix to give
rise to molecular
orbitals.
Intermolecular
Forces
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Molecular-Orbital Model for
Metals
• Delocalized bonding requires the atomic orbitals on one
atom to interact with atomic orbitals on neighboring atoms.
• Example: Graphite electrons are delocalized over a whole
plane, while benzene molecules have electrons
delocalized over a ring.
• Recall that the number of molecular orbitals is equal to the
number of atomic orbitals. Each orbital can hold two
electrons.
• In metals there are a very large number of orbitals.
• As the number of orbitals increases, their energy spacing
decreases and they band together.
• The available electrons do not completely fill the band of
orbitals.
Intermolecular
Forces
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Intermolecular
Forces
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Molecular-Orbital Model for
Metals
• Therefore, electrons can be promoted to unoccupied
energy bands.
• Because the energy differences between orbitals are
small the promotion of electrons requires little energy.
• As we move across the transition metal series, the
antibonding band starts becoming filled.
• Therefore, the first half of the transition metal series has
only bonding-bonding interactions and the second half has
bonding–antibonding interactions.
• We expect the metals in the middle of the transition metal
series (group 6B) to have the highest melting points.
• The energy gap between bands is called the band gap.
Intermolecular
Forces
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Molecular Orbital Theory
In such elements,
the energy gap
between molecular
orbitals essentially
disappears, and
continuous bands
of energy states
result.
Intermolecular
Forces
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Formation of Bands
When atoms come together to form a compound, their atom orbital energies mix to
form molecular orbital energies. As more atoms begin to mix and more molecular
orbitals are formed, it is expected that many of these energy levels will start to be very
close to, or even completely degenerate, in energy. These energy levels are then said
to form bands of energy remember each orbital only holds two electrons. Intermolecular
Forces
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The electronic band structure of nickel.
The left side of the figure shows the electron configuration of a
single Ni atom, while the right-hand side of the figure shows how
these orbital energy levels broaden into energy bands in bulk
Intermolecular
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APSI 2014
nickel. The horizontal dashed gray line denotes the position
of
the
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Fermi Level, which separates the occupied molecular orbitals
Types of Materials
Rather than having molecular orbitals separated by an
energy gap, these substances have energy bands.
The gap between bands determines whether a substance
is a metal, a semiconductor, or an insulator.
Intermolecular
Forces
Presented By, Mark
Langella, APSI Chemistry
2014 , PWISTA .com
Energy bands in metals, semiconductors, and insulators.
Metals are characterized by the highest-energy electrons
occupying a partially filled band. Semiconductors and insulators
have an energy gap that separates the completely filled band
(shaded in blue) and the empty band (unshaded), knownIntermolecular
as the
Forces
band gap and represented by the symbol Eg. ThePresented
filled By,
band
Mark is
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called the valence band (VB), and the empty band is
called the
Metals
• Valence electrons are
in a partially-filled
band.
• There is virtually no
energy needed for an
electron to go from
the lower, occupied
part of the band to the
higher, unoccupied
part.
• This is how a metal
conducts electricity.
Intermolecular
Forces
Presented By, Mark
Langella, APSI Chemistry
2014 , PWISTA .com
Insulators
• The energy band
gap in insulating
materials is
generally greater
than ~350 kJ/mol.
• They are not
conductive.
Intermolecular
Forces
Presented By, Mark
Langella, APSI Chemistry
2014 , PWISTA .com
Semiconductors
Semiconductors have a
gap between the
valence band and
conduction band of
~50-300 kJ/mol.
Intermolecular
Forces
Presented By, Mark
Langella, APSI Chemistry
2014 , PWISTA .com
An intrinsic semiconductor is a semiconductor in its pure state.
For every electron that jumps into the conduction band, the
missing electron will generate a hole that can move freely in the
valence band. The number of holes will equal the number of
electrons that have jumped. The higher the temp more electrons
into conduction band.
Intermolecular
Forces
Presented By, Mark
Langella, APSI
The following pictures show the electron populations of the bands of MO
energy levels for four different materials:
(a) Classify each material as an insulator, a semiconductor, or a metal.
(b) Arrange the four materials in order of increasing electrical conductivity.
Explain.
Intermolecular
(c) Tell whether the conductivity of each material increases or decreases
Forces
Presented By, Mark
when the temperature increases.
Langella, APSI
Semiconductors
• Among elements, only silicon,
germanium and graphite
(carbon), all of which have 4
valence electrons, are
semiconductors.
• Inorganic semiconductors (like
GaAs) tend to have an
average of 4 valence electrons
(3 for Ga, 5 for As).
Intermolecular
Forces
Presented By, Mark
Langella, APSI Chemistry
2014 , PWISTA .com
Doping
By introducing very
small amounts of
impurities that have
more valence electrons
(n-Type) or fewer (pType) valence
electrons, one can
increase or decrease
the conductivity of a
semiconductor.
Presented By, Intermolecular
Mark
Langella, APSI Chemistry
Forces
2014 , PWISTA .com
The addition of controlled small amounts of impurities (doping) to a semiconductor changes the
electronic properties of the material.
•Left: A pure, intrinsic semiconductor has a filled valence
band and an empty conduction band (ideally).
•Middle: The addition of a dopant atom that has more
valence electrons than the host atom adds electrons to the
conduction band (i.e., phosphorus doped into silicon). The
resulting material is an n-type semiconductor.
Intermolecular
•Right: The addition of a dopant atom that has fewer valence
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Presented By, Mark
electrons than the host atom leads to fewer electrons
in the
Langella, APSI
Intermolecular
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Langella, APSI
Intermolecular
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Langella, APSI
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Which of the following is a p-type semiconductor?
•
•
•
•
•
Sulfur-doped carbon
Boron-doped germanium
Phosphorus-doped silicon
Ultra-pure silicon
Carbon-doped copper
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Which of the following is a p-type semiconductor?
•
•
•
•
•
Sulfur-doped carbon
Boron-doped germanium
Phosphorus-doped silicon
Ultra-pure silicon
Carbon-doped copper
Intermolecular
Forces
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Langella, APSI
Practice Exercises
Which of the following elements, if doped into silicon,
would yield an n-type semiconductor? Ga; As; C.
Suggest an element that could be used to dope silicon to
yield a p-type material.
Intermolecular
Forces
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Langella, APSI
Diode- Used to switch and convert
between electromagnetic radiation and
electric current
• Semiconductor created that has p-type
on one half and n-type on the other half
• Known as “p-n junction”
Intermolecular
Forces
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Langella, APSI
Intermolecular
Forces
Presented By, Mark
Langella, APSI
Light emitting diodes.
The heart of a light emitting diode is a p-n junction where an
applied voltage drives electrons and holes to meet. Bottom:
The color of light emitted depends upon the band gap ofIntermolecular
the
Forces
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By,
Mark
semiconductor used to form the p-n junction. For display
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Color
Red
λ
Voltage Drop
610 < λ < 760 1.63 < ΔV < 2.03
Orange
590 < λ < 610 2.03 < ΔV < 2.10
Yellow
570 < λ < 590 2.10 < ΔV < 2.18
Green
500 < λ < 570 1.9[63] < ΔV < 4.0
Blue
450 < λ < 500 2.48 < ΔV < 3.7
Violet
400 < λ < 450 2.76 < ΔV < 4.0
Composition
Aluminium gallium arsenide (AlGaAs)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Gallium arsenide phosphide (GaAsP)
Aluminium gallium indium phosphide (AlGaInP)
Gallium(III) phosphide (GaP)
Traditional green:
Gallium(III) phosphide (GaP)
Aluminium gallium indium phosphide (AlGaInP)
Aluminium gallium phosphide (AlGaP)
Pure green:
Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN)
Zinc selenide (ZnSe)
Indium gallium nitride (InGaN)
Silicon carbide (SiC) as substrate
Silicon (Si) as substrate—under development
Indium gallium nitride (InGaN)
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Chemistry 2014 , PWISTA .com
Solar Cells
Intermolecular
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Langella, APSI