Classes of materials
POLYMERS
CERAMICS
METALS
DUCTILITY
Varies
Poor
Good
CONDUCTIVITY
(ELECTRICAL & THERMAL)
Low
Low
High
HARDNESS/STRENGTH
Low –
medium
Very high
Medium –
high
CORROSION RESISTANCE
Fair – good
Good
Fair – poor
STIFFNESS
Low
High
Fair
FRACTURE TOUGHNESS
Low –
medium
Low
High
MACHINABILITY
Good
Poor
Good
MECH 221
PM Wood-Adams
Fall 2008
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 Æ Crystal
……….Structure
What determines the nature of the chemical
bond between atoms?
• Electronic structure (distribution of electrons in atomic orbitals)
electronegativity (tendency for an
• Number of electrons and ………………
atom to attract an electron)
MECH 221
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BOHR ATOM
orbital electrons:
n = principal
quantum number
1
2
n=3
Adapted from Fig. 2.1,
Callister 6e.
Nucleus: Z = # protons
= 1 for hydrogen to 94 for plutonium
N = # neutrons
Atomic mass A ≈ Z + N
MECH 221
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Fall 2008
ELECTRON ENERGY STATES
Electrons... • have discrete energy states
Increasing energy
• tend to occupy lowest available energy state.
n=4
4s
n=3
3s
n=2
n=1
2s
1s
•s, p,d and f signify the
subshells which the
4p
3d electrons occupy.
3p
2p
• Different types of
subshells have different
numbers of energy states
•Within each energy
state there are two
possible spin orientations
Remember that n is the principal quantum number
MECH 221
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Fall 2008
Stable electron configurations...
• have complete s and p subshells
• are unreactive.
Z
2
Element Configuration
He
1s2
Adapted from Table 2.2,
Callister 6e.
10
Ne
1s22s 22p6
18
Ar
1s2 2s22p63s23p6
36
Kr
1s2 2s22p63s23p63d10 4s24p6
Valence electrons are the electrons that occupy the
outermost filled shell.
MECH 221
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Fall 2008
SURVEY OF ELEMENTS
• Most elements: Electron configuration not stable.
Element
Atomic #
Hydrogen
1
Helium
2
Lithium
3
Beryllium
4
Boron
5
Carbon
6
...
Neon
10
Sodium
11
Magnesium
12
Aluminum
13
...
Argon
18
...
...
Krypton
36
Electron configuration
1s 1
(stable)
1s 2
1s 22s 1
1s 22s 2
Adapted from Table 2.2,
1s 22s 22p 1
Callister 6e.
1s 22s 22p 2
...
1s 22s 22p 6
(stable)
1s 22s 22p 63s 1
1s 22s 22p 63s 2
1s 22s 22p 63s 23p 1
...
1s 22s 22p 63s 23p 6
(stable)
...
1s 22s 22p 63s 23p 63d 10 4s 24 6
(stable)
• Why? Valence (outer) shell usually not filled completely.
MECH 221
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Fall 2008
Increasing Electronegativity
MECH 221
PM Wood-Adams
Fall 2008
accept 2e
accept 1e
inert gases
give up 1e
give up 2e
give up 3e
THE PERIODIC TABLE
• Columns: Similar Valence Structure
H
Li Be
Metal
Nonmetal
Intermediate
He
Ne
O
F
Na Mg
S
Cl Ar
K Ca Sc
Se Br Kr
Rb Sr
Te
Y
Cs Ba
I
Xe
Po At Rn
Fr Ra
Electropositive elements:
Readily give up electrons
to become + ions.
MECH 221
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Electronegative elements:
Readily acquire electrons
to become - ions.
Fall 2008
ELECTRONEGATIVITY
• Ranges from 0.7 to 4.0,
• Large values: tendency to acquire electrons.
Smaller electronegativity
Larger electronegativity
IONIC BONDING
•
•
•
•
Occurs between + and - ions.
Requires electron transfer.
Large difference in electronegativity required.
Example: NaCl
Na (metal)
unstable
Cl (nonmetal)
unstable
electron
Na (cation)
stable
+
Stable because the s and
p subshells are filled!
MECH 221
Coulombic
Attraction
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Cl (anion)
stable
Fall 2008
EXAMPLES: IONIC BONDING
• Predominant bonding in Ceramics
H
2.1
Li
1.0
Na
0.9
K
0.8
Rb
0.8
Cs
0.7
Fr
0.7
NaCl
MgO
CaF2
CsCl
Be
1.5
O
F
3.5 4.0
Cl
3.0
Mg
1.2
Ca
1.0
Sr
1.0
Ti
1.5
Cr
1.6
Ba
0.9
Fe
1.8
Ni
1.8
Zn
1.8
As
2.0
Br
2.8
I
2.5
At
2.2
He
Ne
Ar
Kr
Xe
Rn
-
Ra
0.9
Give up electrons
Acquire electrons
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.
COVALENT BONDING
• Requires shared electrons
• Example: CH4
C: has 4 valence e,
needs 4 more
CH4
H: has 1 valence e,
needs 1 more
H
H
C
H
Electronegativities
are comparable.
shared electrons
from carbon atom
H
shared electrons
from hydrogen
atoms
Adapted from Fig. 2.10, Callister 6e.
MECH 221
PM Wood-Adams
Fall 2008
EXAMPLES: COVALENT BONDING
H2
H
2.1
Li
1.0
Na
0.9
K
0.8
Be
1.5
Mg
1.2
Ca
1.0
Rb
0.8
Cs
0.7
Sr
1.0
Ba
0.9
Fr
0.7
Ra
0.9
column IVA
H2O
C(diamond)
SiC
Ti
1.5
Cr
1.6
Fe
1.8
Ni
1.8
Zn
1.8
Ga
1.6
C
2.5
Si
1.8
Ge
1.8
F2
He
O
2.0
As
2.0
Sn
1.8
Pb
1.8
GaAs
F
4.0
Cl
3.0
Br
2.8
I
2.5
At
2.2
Ne
-
Cl2
Ar
Kr
Xe
Rn
-
• Molecules of nonmetals
• Molecules of metals and nonmetals
• Elemental solids (RHS of Periodic Table)
• Compound solids (about column IVA)
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
METALLIC BONDING
• Arises from a sea of donated valence electrons
(1, 2, or 3 from each atom).
+
+
+
+
+
+
+
+
+
Adapted from Fig. 2.11, Callister 6e.
• Primary bond for metals and their alloys
MECH 221
PM Wood-Adams
Fall 2008
12
SECONDARY BONDING
Arises from interaction between dipoles
• Fluctuating dipoles
ex: liquid H2
asymmetric electron
H2
H2
clouds
+
- secondary +
bonding
-
H H
Adapted from Fig. 2.13, Callister 6e.
• Permanent dipoles-molecule induced
-general case:
+
-ex: liquid HCl
H Cl
-ex: polymer
secon
d
-
a ry b
secondary
bonding
+
secondary
bonding
H Cl
ondin
g
-
H H
secondary
bonding
SUMMARY: BONDING
Type
Bond Energy
Comments
Ionic
Large!
Nondirectional (ceramics)
Covalent
Directional
Variable
large-Diamond semiconductors, ceramics
small-Bismuth
polymer chains)
Metallic
Variable
large-Tungsten
small-Mercury
Nondirectional (metals)
smallest
Directional
inter-chain (polymer)
inter-molecular
Secondary
MECH 221
PM Wood-Adams
Fall 2008
14
Bonding Forces and Energies
Bond length, r
F
F
r
Net force is given by the sum of an
attractive force and a repulsive force
repulsive, attractive, and net
forces
Potential is given by the integral of the net
force curve with respect to distance:
E = ∫ F • dr
Note: equilibrium separation occurs
where the net force = 0 and the energy is
at a minimum.
MECH 221
repulsive, attractive, and net
PM Wood-Adams
energies
Fall 2008
Bonding Forces and Energies
Bonding energy: Minimum of the potential vs. distance curve.
¾ Indicates how much energy must be supplied to completely
disassociate the two atoms
¾ Depth of the potential well indicates bonding strength
• Deep well Æ strongly bonded
• Shallow well Æ weakly bonded
Energy (r)
unstretched length
ro
r
Eo=
“bond energy”
MECH 221
PM Wood-Adams
Fall 2008
PROPERTIES FROM BONDING: TM
• Bond length, r
F
• Melting Temperature, Tm
Energy (r)
F
r
ro
• Bond energy, Eo
r
Energy (r)
unstretched length
ro
smaller Tm
r
Eo=
“bond energy”
MECH 221
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larger Tm
Tm is larger if Eo is larger.
Fall 2008
15
Example: Bonding energy and TM
Use the data below to estimate the bonding energy of
copper which has a melting temperature of 1084°C.
E0, eV/atom
TM, °C
Hg
0.7
-39
Al
3.4
660
Fe
4.2
1538
W
8.8
3410
tungsten
MECH 221
PM Wood-Adams
Fall 2008
Solution: Plot the data
Melting Temperature (C)
4000
3500
3000
2500
2000
1500
1000
500
E0=3.6 eV/atom
0
-500
0
2
4
6
8
10
Bonding Energy (eV/atom)
With this analysis we estimate E0 of copper = 3.6 eV/atom.
The measured value is 3.5 eV/atom.
PROPERTIES FROM BONDING: E
• Elastic modulus, E
length, Lo
undeformed
cross
sectional
area Ao
ΔL
deformed
F
Elastic modulus
F
ΔL
=E
Ao
Lo
• E ~ curvature at ro
Energy
unstretched length
ro
r
E is larger if Eo is larger.
smaller Elastic Modulus
larger Elastic Modulus
16
PROPERTIES FROM BONDING: α
• Coefficient of thermal expansion, α
coeff. thermal expansion
length, Lo
unheated, T1
ΔL
Lo
ΔL
heated, T2
= α (T2-T1)
• α ~ symmetry at ro
Energy
ro
r
α is larger if Eo is smaller.
larger α
smaller α
MECH 221
PM Wood-Adams
Fall 2008
17
Bonding Types: Summary
A comparison of the type of bonding found in different materials:
metallicsince it is a metal alloy.
• For brass, the bonding is ……..
Van der Waals
covalent with some ……….
• For rubber, the bonding is ………
Rubber is composed primarily of carbon and hydrogen atoms
secon
d
a ry b
ondin
g
• For BaS, the bonding is predominantly ionic
….. (but with some covalent character)
on the basis of the relative positions of Ba and S in the periodic table.
der Waals since xenon is an inert gas.
• For solid xenon, the bonding is Van
…………….
Van der Waals
• For nylon, the bonding is covalent
…….. with perhaps some ………..
Nylon is composed primarily of carbon and hydrogen
• For AlP the bonding is predominantly covalent
……… (but with some ionic character)
on the basis of the relative positions of Al and P in the periodic table.
MECH 221
PM Wood-Adams
Fall 2008
SUMMARY: PRIMARY BONDS
Ceramics
Large bond energy
large Tm
large E
small α
(Ionic & covalent bonding):
Metals
Variable bond energy
moderate Tm
moderate E
moderate α
(Metallic bonding):
Polymers
Directional Properties
(Covalent & Secondary):
secon
d
MECH 221
a ry b
ondin
g
Secondary bonding dominates
small T
small E
large α
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Fall 2008
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