atomic structure and bonding in solids
TRANSCRIPT
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POLYMERS CERAMICS METALS
DUCTILITY Varies Poor Good
HighCONDUCTIVITY(ELECTRICAL & THERMAL)
Low Low
HARDNESS/STRENGTHLow
medium Very highMedium
high
CORROSION RESISTANCE Fair good Good Fair poor
STIFFNESS Low High Fair
FRACTURE TOUGHNESSLow
mediumLow High
MACHINABILITY Good Poor Good
Classes of materials
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Electronic structure (distribution of electrons in atomic orbitals) Number of electrons and (tendency for an
atom to attract an electron)
Why study bonding?
Because the properties of materials (strength, hardness,
conductivity, etc..) are determined by the manner in which
atoms are connected.
Also by how the atoms are arranged in space.
What determines the nature of the chemical
bond between atoms?
Crystal Structure
electronegativity
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Nucleus: Z = # protons
orbital electrons:n = principalquantum number
n=3 2 1
= 1 for hydrogen to 94 for plutonium
N = # neutrons
Atomic mass A Z + N
Adapted from Fig. 2.1,
Callister 6e.
BOHR ATOM
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have discrete energy states
tend to occupy lowest available energy state.
I
ncreasingenergy
n=1
n=2
n=3
n=4
1s
2s
3s
2p
3p
4s
4p3d
Electrons...ELECTRON ENERGY STATES
Remember that n is the principal quantum number
s, p,d and f signify thesubshells which the
electrons occupy.
Different types ofsubshells have different
numbers of energy states
Within each energystate there are two
possible spin orientations
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have complete s and p subshells
are unreactive.
Stable electron configurations...
Z Element Configuration
2 He 1s2
10 Ne 1s22s22p6
18 Ar 1s22s22p63s23p6
36 Kr 1s22s22p63s23p63d104s24p6
Adapted from Table 2.2,Callister 6e.
Valence electrons are the electrons that occupy the
outermost filled shell.
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Why? Valence (outer) shell usually not filled completely.
Most elements: Electron configuration not stable.ElementHydrogenHelium
LithiumBerylliumBoronCarbon...
NeonSodiumMagnesiumAluminum...
Argon...Krypton
Atomic #12
3456
10111213
18...36
Electron configuration1s1
1s2 (stable)1s22s11s22s21s22s22p11s22s22p2...
1s22s22p6 (stable)1s22s22p63s11s22s22p63s21s22s22p63s23p1...
1s22s22p63s23p6 (stable)...
1s22s22p63s23p63d104s246 (stable)
Adapted from Table 2.2,
Callister 6e.
SURVEY OF ELEMENTS
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Increasing Electronegativity
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He
Ne
Ar
Kr
Xe
Rn
in
ertgases
accept1e
accept2e
giveup1e
give
up2e
g
iveup3
e
FLi Be
Metal
Nonmetal
Intermediate
H
Na Cl
Br
I
At
O
SMg
Ca
Sr
Ba
Ra
K
Rb
Cs
Fr
Sc
Y
Se
Te
Po
Columns: Similar Valence Structure
Electropositive elements:
Readily give up electronsto become + ions.
Electronegative elements:
Readily acquire electrons
to become - ions.
THE PERIODIC TABLE
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Ranges from 0.7 to 4.0,
Smaller electronegativity Larger electronegativity
Large values: tendency to acquire electrons.
ELECTRONEGATIVITY
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Na (metal)
unstable
Cl (nonmetal)
unstableelectron
+ -CoulombicAttraction
Na (cation)stable Cl (anion)stable
Occurs between + and - ions.
Requires electron transfer.
Large difference in electronegativity required. Example: NaCl
IONIC BONDING
Stable because the s and
p subshells are filled!
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Predominant bonding in Ceramics
Give up electrons Acquire electrons
He
-
Ne
-
Ar
-
Kr-
Xe-
Rn-
F
4.0
Cl3.0
Br2.8
I2.5
At2.2
Li
1.0
Na
0.9
K0.8
Rb0.8
Cs0.7
Fr
0.7
H
2.1
Be1.5
Mg1.2
Ca1.0
Sr1.0
Ba0.9
Ra
0.9
Ti1.5
Cr1.6
Fe1.8
Ni1.8
Zn1.8
As2.0
CsCl
MgO
CaF2
NaCl
O3.5
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 is adapted from Linus Pauling, The Nature of the
Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell
University.
EXAMPLES: IONIC BONDING
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Requires shared electrons
Example: CH4
C: has 4 valence e,needs 4 more
H: has 1 valence e,
needs 1 more
Electronegativitiesare comparable.
shared electrons
from carbon atom
shared electronsfrom hydrogenatoms
H
H
H
H
C
CH4
Adapted from Fig. 2.10, Callister 6e.
COVALENT BONDING
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He-
Ne-
Ar-
Kr-
Xe-
Rn-
F4.0
Cl3.0
Br2.8
I
2.5
At2.2
Li1.0
Na0.9
K0.8
Rb
0.8
Cs0.7
Fr
0.7
H2.1
Be1.5
Mg1.2
Ca1.0
Sr1.0
Ba0.9
Ra0.9
Ti1.5
Cr1.6
Fe1.8
Ni1.8
Zn1.8
As2.0
SiC
C(diamond)
H2O
C2.5
H2
Cl2
F2
Si1.8
Ga1.6
GaAs
Ge1.8
O2.0
columnIVA
Sn1.8
Pb1.8
Molecules ofnonmetals
Molecules ofmetals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (aboutcolumn IVA)
EXAMPLES: COVALENT BONDING
It is possible for bonds to be partially covalent and partially ionic in
nature. Look in Chapter 2 to see how to evaluate this aspect of bonds
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Arises from a sea ofdonated valence electrons(1, 2, or 3 from each atom).
Primary bond for metals and their alloys
+ + +
+ + +
+ + + Adapted from Fig. 2.11, Callister 6e.
METALLIC BONDING
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Arises from interaction between dipoles
Permanentdipoles-molecule induced
Fluctuating dipoles
+ - secondarybonding
+ -
H Cl H Clsecondarybonding
secondarybonding
HH HH
H2 H2
secondarybonding
ex: liquid H2asymmetric electron
clouds
+ - + -secondary
bonding
-general case:
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.13, Callister 6e.
SECONDARY BONDING
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Type
Ionic
Covalent
Metallic
Secondary
Bond Energy
Large!
Variablelarge-Diamondsmall-Bismuth
Variablelarge-Tungstensmall-Mercury
smallest
Comments
Nondirectional (ceramics)
Directional
semiconductors, ceramicspolymer chains)
Nondirectional (metals)
Directionalinter-chain (polymer)
inter-molecular
SUMMARY: BONDING
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Net force is given by the sum of an
attractive force and a repulsive force
Potential is given by the integral of the net
force curve with respect to distance:
Note: equilibrium separation occurs
where the net force = 0 and the energy is
at a minimum.
=
dFE
Bonding Forces and Energies
repulsive, attractive, and net
forces
repulsive, attractive, and netenergies
Bond length, r
FF
r
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Indicates how much energy must be supplied to completely
disassociate the two atoms
Depth of the potential well indicates bonding strength
Deep well
Shallow well
Bonding energy: Minimum of the potential vs. distance curve.
strongly bonded
weakly bonded
Bonding Forces and Energies
Eo=
bond energy
Energy (r)
ro
r
unstretched length
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Bond length, r
Bond energy, Eo
FF
r
Melting Temperature, Tm
Eo=
bond energy
Energy (r)
ror
unstretched length
r
larger Tm
smaller Tm
Energy (r)
ro
Tm is larger if Eo is larger.
PROPERTIES FROM BONDING: TM
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Example: Bonding energy and TM
Use the data below to estimate the bonding energy of
copper which has a melting temperature of 1084C.
34108.8W
15384.2Fe
6603.4Al
-390.7Hg
E0, eV/atom TM, C
tungsten
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Solution: Plot the data
-500
0
500
1000
1500
20002500
3000
3500
4000
0 2 4 6 8 10
Bonding Energy (eV/atom)
Meltin
gTemperat
ure(C)
E0=3.6 eV/atom
With this analysis we estimate E0 of copper = 3.6 eV/atom.
The measured value is 3.5 eV/atom.
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Elastic modulus, E
E ~ curvature at ro
crosssectionalarea Ao
L
length, Lo
F
undeformed
deformed
LFAo
= E Lo
Elastic modulus
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
rounstretched length
E is larger if Eo is larger.
PROPERTIES FROM BONDING: E
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Coefficient of thermal expansion,
~ symmetry at ro
is larger if Eo is smaller.
L
length, Lo
unheated, T1
heated, T2
= (T2-T1)LLo
coeff. thermal expansion
r
smaller
larger
Energy
ro
PROPERTIES FROM BONDING:
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A comparison of the type of bonding found in different materials:
For brass, the bonding is .. since it is a metal alloy.
For rubber, the bonding is with some .Rubber is composed primarily of carbon and hydrogen atoms
For BaS, the bonding is predominantly .. (but with some covalentcharacter)
on the basis of the relative positions of Ba and S in the periodic table.
For solid xenon, the bonding is . since xenon is an inert gas.
For nylon, the bonding is .. with perhaps some ..
Nylon is composed primarily of carbon and hydrogen
For AlP the bonding is predominantly (but with some ionic character)
on the basis of the relative positions of Al and P in the periodic table.
metallic
covalent Van der Waals
ionic
Van der Waals
covalent Van der Waals
covalent
secondarybonding
Bonding Types: Summary
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Ceramics
(Ionic & covalent bonding):
Metals
(Metallic bonding):
Polymers(Covalent & Secondary):
secondarybonding
Large bond energylarge Tm
large E
small
Variable bond energymoderate Tm
moderate E
moderate
Directional PropertiesSecondary bonding dominates
small Tsmall E
large
SUMMARY: PRIMARY BONDS