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Module 2.Module 2.
Structures of Engineering Materials1.Atomic Structure
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
2.Atomic Bonding
2.1. Atomic Structure
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
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Atomic StructureAll materials consist of elements and all elements consist of atoms. Each atoms has its own characteristics → has different properties → see Periodic Table
proton
Atomic Structure
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
proton
neutron e-
orbital electrons: n = principal quantum number
n=3 2 1
BOHR ATOM
Nucleus: Z = # protons
n=3 Adapted from Fig. 2.1, Callister 6e.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
= 1 for hydrogen to 94 for plutoniumN = # neutrons
Atomic mass A ≈ Z + N
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• have discrete energy states• tend to occupy lowest available energy state.
Electrons...ELECTRON ENERGY STATES
py gye
asi
ng
en
erg
y
2
n=3
n=4
3s2p
3p
4s4p
3d
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
Inc
re
n=1
n=2
1s2s
2p
Adapted from Fig. 2.5, Callister 6e.
• have complete s and p subshells• tend to be unreactive.
Stable electron configurations...STABLE ELECTRON CONFIGURATIONS
Z Element Configuration
2 He 1s2
10 Ne 1s22s22p6
18 Ar 1s22s22p63s23p6
2 2 6 2 6 10 2 6
Adapted from Table 2.2, Callister 6e.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
36 Kr 1s22s22p63s23p63d104s24p6
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• Most elements: Electron configuration not stable.Element Hydrogen Helium Lithi
Atomic # 1 2 3
Electron configuration 1s1 1s2 (stable)
SURVEY OF ELEMENTS
Lithium Beryllium Boron Carbon ... Neon Sodium Magnesium Aluminum
3 4 5 6
10 11 12 13
1s22s1 1s22s2 1s22s22p1 1s22s22p2 ... 1s22s22p6 (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1
Adapted from Table 2.2, Callister 6e.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
• Why? Valence (outer) shell usually not filled completely.
Aluminum ... Argon ... Krypton
13
18 ... 36
... 1s22s22p63s23p6 (stable) ... 1s22s22p63s23p63d104s246 (stable)
ne
rt g
ase
s e
pt
1e
ep
t 2
e
ive
up
1e
e
up
2e
3
e
Metal
• Columns: Similar Valence Structure
THE PERIODIC TABLE
He
Ne
Ar
Kr
Xe
ina
cc
ac
cgg
ive
giv
e u
p 3
F Li Be
Nonmetal
Intermediate
H
Na Cl
Br
I
O
S Mg
Ca
Sr
K
Rb
Sc
Y
Se
Te
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
Rn At Ba
Ra
Cs
Fr
Po
Electropositive elements:Readily give up electronsto become + ions.
Electronegative elements:Readily acquire electronsto become - ions.
Adapted from Fig. 2.6, Callister 6e.
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• Ranges from 0.7 to 4.0,
He H
• Large values: tendency to acquire electrons.
ELECTRONEGATIVITY
-
Ne -
Ar -
Kr -
Xe -
Rn -
F 4.0
Cl 3.0
Br 2.8
I 2.5
At 2.2
Li 1.0
Na 0.9
K 0.8
Rb 0.8
Cs 0.7
2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 7
Smaller electronegativity Larger electronegativity
Fr 0.7
Ra 0.9
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 CornellUniversity.
2.1. Atomic Bonding
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
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Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
P i b d
Atoms may form two types of bond:
Primary bonds: a. Ionic bond b. Covalent bond c. Metallic bond
Secondary bonds: a. Van der Waal bond
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
a. Van der Waal bond b. Hydrogen bond
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Primary BondsElectrons are directly transferred from one atom to another and localised. Atoms are bonded by coulombic force. Bonds are non-
Ionic Bonding
directional.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
• Occurs between + and - ions.• Requires electron transfer.• Large difference in electronegativity required.
IONIC BONDING
Na (metal) unstable
Cl (nonmetal) unstable
electron
• Example: NaCl
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
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However, more energy is released as Na+ and Cl- ions are getting closer.
The attractive energy , EA, is given by:
))((1 eZeZE −=
where ε0 is the permittivity of free space (8.85 x 10-12 F/m), Z1 and Z2 are the valences of the two ions, e is the electronic charge (1.602 x 10-19 C) and r is the separation of the ions.In convention, negative is for the energy released in forming
))((4 21
0
eZeZr
EA πε=
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
g gy gbonds and positive for the energy required to break bonds.
At a separation, r*, the energy released equals the 1.5eV required to transfer the electrons. So at separations smaller than this there will be a net release of energy and so the bond is stable.
EEA
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The variation of the attractive coulomb interaction,EA, with ionic separation, r.
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The approaching ions also suffer a repulsive force due to the interaction of the inner electron shells of the two ions.A repulsive energy, ER, is given by:
nR rBE =r
where n is an exponent ~8 and B is an empirical constant.
ER
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The variation of the repulsive interaction, Ur, withionic separation, r.
ER
EA
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
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The overall energy of interaction, Etot, is the combination of the attractive energy, EA and the repulsive energy, ER.
ntot rBeZeZ
rE +−= ))((
41
210πε
ETOT
EW
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The equilibrium spacing between the two ions, given as r0, is where the energy of interaction is a minimum.r0 = bond length
Structure of Salt (NaCl)
Schematic model Lattice
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
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• Predominant bonding in Ceramics
MgONaCl
MORE EXAMPLES: IONIC BONDING
He -
Ne -
Ar -
Kr -
Xe -
Rn
F 4.0
Cl 3.0
Br 2.8
I 2.5
At
Li 1.0
Na 0.9
K 0.8
Rb 0.8
Cs
H 2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
CsCl
g
CaF2O
3.5
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 9
Give up electrons Acquire electrons
-At
2.2Cs 0.7
Fr 0.7
a 0.9
Ra 0.9
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 CornellUniversity.
• Requires shared electrons• Example: CH4
C: has 4 valence eshared electrons from carbon atomH
CH
COVALENT BONDING
C: has 4 valence e,needs 4 more
H: has 1 valence e,needs 1 more
Electronegativitiesare comparable.
from carbon atom
shared electrons from hydrogen atoms
HH
H
C
CH4
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 10
Adapted from Fig. 2.10, Callister 6e.
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He -
H 2.1 SiC
C(diamond)
H2OH2
Cl2
F2
co
lum
n IV
A
EXAMPLES: COVALENT BONDING
Ne -
Ar -
Kr -
Xe -
Rn -
F 4.0
Cl 3.0
Br 2.8
I 2.5
At 2.2
Li 1.0
Na 0.9
K 0.8
Rb 0.8
Cs 0.7
Fr
2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ra
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
SiCC
2.5
Cl2
Si 1.8
Ga 1.6
G A
Ge 1.8
O 2.0
c
Sn 1.8Pb 1.8
Ad t d f Fi 2 7 C lli t 6 (Fi 2 7 i
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 11
• Molecules with nonmetals• Molecules with metals and nonmetals• Elemental solids (RHS of Periodic Table)• Compound solids (about column IVA)
Fr 0.7
Ra 0.9 GaAsAdapted 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.
Sharing electrons between adjacent atoms. Bonds are directional.
The energy released, Ua, associated with electron sharing:
a
Covalent Bonding
If the two atoms become too close together, the repulsive energy Ur is given by: b
nr raU −=
where r is the distance between the two atoms, n is ~6, and a is a constant.
g y
mr rbU =
where m is an exponent (~12) and b is a constant.
The overall covalent interaction energy Utot is given by:
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
mntot rb
raU +−=
energy Utot is given by:
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Covalent bonding in amolecule of methane (CH4)(CH4)
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
In a metal, valence electrons leave their parent atoms and combine to form an electron "gas" which freely wander around metal ions.
Metallic Bonding
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The electrons are completely delocalized and provide a bonding force between the metal ions. The bonding between the atoms is non-directional.
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+ + + 1. The complete delocalization of the electrons means metal "ions" are readily i t h bl diff t
Metallic Bonding
+ + +
+ + +
interchangeable → different alloys.
2. The electrons are easy to move → metals are good electrical conductors.
3. The metal ions pack
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 12
• Primary bond for metals and their alloys
Adapted from Fig. 2.11, Callister 6e.
ptogether very well → metals have high densities and assume simple crystallographic structures.
Arises from interaction between dipoles• Fluctuating dipoles van der waals
H2 H2ex: liquid H2asymmetric electron
clouds
SECONDARY BONDING
• Permanent dipoles-molecule induced hydrogen bond
+ - secondary bonding + -
HH HH
H2 H2
secondary bonding
clouds
+ - + -secondary
bonding
-general case:
Adapted from Fig. 2.13, Callister 6e.
Adapted from Fig. 2.14,Callister 6e.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 13
bonding
H Cl H Clsecondary bonding
secondary bonding
g
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.14,Callister 6e.
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Secondary Bonds
A dipole may be formed when the electrical symmetry for some atoms or molecules is instantaneously distorted. Two opposite dipoles may attract each and form a eak bond
Van der Waal Bonding
attract each and form a weak bond.
Bonds in the condensed halogen molecules, such as liquid and solid forms of Cl2, Br2 and I2.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The energy of the Van der Waals bond
mntot rb
raU +−=
rr
where r is the distance between the two atoms, n is an exponent ~6, m ~12 and a and b are constants.
Van der Waals bond:
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
• the weakest bond• forces fluctuate with time • non-directional
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When a single hydrogen electron is shared with dissimilar atoms in covalent bonding, a net positive charge is displaced towards the hydrogen atom. The positively
Hydrogen Bonds
charged hydrogen can form a hydrogen bond with the negative end of a neighboring molecule.
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
• Approximately 30 times weaker than a normal covalent bond, because only one of the contributing atoms is supplying
Hydrogen Bonds
contributing atoms is supplying electrons to it.
• Relatively easily broken.
• Directional.
• Such bonds may also exist in many polymers linking discrete
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
a y po y e s g d sc etechain molecules together.
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Interatomic ForcesThe bonding force, F, varies as a function of the separation, r, between atoms.
drdUF =
F(r)
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
The variation of force between two atoms, F, as a function of atomic separation
Some engineering materials have mixed bonds.• In ceramic: ionic/covalent mixed bonds. e.g. SiO2.• In polymer: covalent/secondary mixed bond.
Mixed Bonds
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
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• Two atoms of similar electronegativity form either a metallic bond or covalent bond.
• Two atoms of different electronegativity form partial ionic bond. The ionic character increases with the difference in electronegativity.
For a AB compound:
e.g. NaCl:|ENa-ECl|= |0.9-3.0|=2.1
hi hl i i
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
→ highly ionic
e.g SiC: |ESi-EC|= |1.8-2.5|=0.7→ highly covalent
• Bond length, r
F F
• Melting Temperature, Tm
Energy (r)
PROPERTIES FROM BONDING: TM
• Bond energy, Eo
r
Energy (r)
unstretched length
r
larger T
smaller Tm
ro
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
Eo=
“bond energy”
ro r
unstretched length larger Tm
Tm is larger if Eo is larger.
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Bond Strength and Melting PointBonding Type Substance Bonding Energy
(kJ/mol)Melting
Temperature (oC)Ionic NaCl 640 801
MgO 1000 2800C l t Si 450 1410Covalent Si 450 1410
C (diamond) 713 >3550Metallic Hg 68 -39
Al 324 660Fe 406 1538W 849 3410
Van der Waals Ar 7 7 189
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
Van der Waals Ar 7.7 -189Cl2 31 -101
Hydrogen NH3 35 -78H2O 51 0
• Elastic modulus, E cross sectional area Ao
ΔL
length, Lo
undeformed ΔL F E
Elastic modulus
PROPERTIES FROM BONDING: E
• E ~ curvature at ro
ΔL
F deformed
ΔL F Ao
= E Lo
Energy
unstretched length
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 16
r
larger Elastic Modulus
smaller Elastic Modulus
ro E is larger if Eo is larger.
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• Coefficient of thermal expansion, α
ΔL
length, Lo
unheated, T1 ΔL
coeff. thermal expansion
PROPERTIES FROM BONDING: α
• α ~ symmetry at ro
ΔL
heated, T2 = α (T2-T1) ΔL
Lo
Energy
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 17
α is larger if Eo is smaller.r
smaller α
larger α
ro
TypeIonic
C l t
Bond Energy
Large!
Variablel Di d
Comments
Nondirectional (ceramics)
Directionali d t i
SUMMARY: BONDING
Covalent
Metallic
Secondary
large-Diamondsmall-Bismuth
Variablelarge-Tungstensmall-Mercury
smallest
semiconductors, ceramicspolymer chains)
Nondirectional (metals)
Directionalinter-chain (polymer)
Prof. Dr. Ir. Bondan T. Sofyan, M.Si.
Secondary smallest inter hain (polymer)
inter-molecular
• There is an equilibrium spacing between two atoms determined by an attractive force associated with electrostatic attraction and a repulsive force associated with the interaction of inner shell electrons.
• The nature of bonding has a strong effect on the properties of materials.
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Ceramics(Ionic & covalent bonding):
Large bond energylarge Tm
large Esmall α
SUMMARY: BONDING
Metals(Metallic bonding):
Polymers
Variable bond energymoderate Tm
moderate Emoderate α
Directional Properties
Prof. Dr. Ir. Bondan T. Sofyan, M.Si. 18
(Covalent & Secondary):
secondary bonding
Secondary bonding dominatessmall Tsmall Elarge α