7 structure of materials-bonding (1)
DESCRIPTION
materialsTRANSCRIPT
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Structure of Materials: bonding and properties
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ZnS
NaCl
Cs Cl
1 mm
Crystal units
Microstructure (grains, grain boundaries) of a metal alloy
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Atomic Structure
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atom Ø What are reactive particles?
C 12.011; H 1.008 etc
Atomic number = # of protons in nucleus of atom (Z) = # of electrons of neutral species
Atomic mass unit (amu): 1/12 of the atomic mass of the most common isotop of carbon
Atomic mass A ≈ Z + N (Z = # protons, N = # neutrons)
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Electronic Structure • Electrons have wavelike and particulate properties.
– Orbitals – Quantum numbers.
Nucleus Increasing energy
orbital electrons: n = principal quantum number
n=3 2 1
ω
Fe
Fc
Discrete orbitals
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Quantum # Designation n = principal quantum number (energy level-shells) K, L, M, N, O (1, 2, 3, etc.) l = subshells (orbitals) s, p, d, f (0, 1, 2, 3,…, n -1) ml = magnetic 1, 3, 5, 7 (-l to +l) ms = spin ½, -½
Pauli exclusion principle Each electron state can hold no more than two electrons, which must have opposite spin.
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Electron Energy States
Electrons...
“Absorb or emit energy”
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Stable electron configurations...
Electron Configurations
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• Why?
• Most elements: Electron configuration not stable. Electron configuration 1s1 1s2 (stable) 1s22s1 1s22s2 1s22s22p1 1s22s22p2 ... 1s22s22p6 (stable) 1s22s22p63s1 1s22s22p63s2 1s22s22p63s23p1 ... 1s22s22p63s23p6 (stable) ... 1s22s22p63s23p63d104s246 (stable)
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• What are valence electrons? • Their role in bonding?
– example: C (atomic number = 6)
1s2 2s2 2p2
valence electrons
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• Columns: Similar Valence Structure
Electropositive elements: Readily give up electrons to become + ions.
Electronegative elements: Readily acquire electrons to become - ions.
give
up
1e
give
up
2e
give
up
3e
iner
t gas
es
acce
pt 1
e ac
cept
2e
O
Se Te Po At
I Br
He Ne Ar Kr Xe Rn
F Cl S
Li Be H
Na Mg
Ba Cs Ra Fr
Ca K Sc Sr Rb Y
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• Ranges from 0.7 to 4.0, • tendency to acquire electrons.
Electronegativity
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Bonding
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Bonding force
Bonding energy
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Ø Primary bonds
Ø Secondary bonds
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• Example: NaCl
Ionic Bonding
Na (metal) unstable
Cl (nonmetal) unstable
electron
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
Requirements?
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Give up electrons Acquire electrons
He -
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 0.7
H 2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ra 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
CsCl
MgO
CaF2
NaCl
O 3.5
Ionic bonding What type of materials?
Bonding energy high (150-3700 kcal/mol) Nondirectional
Conductivity? Ductility? Melting temperature?
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Covalent Bonding
• Example: CH4
shared electrons from carbon atom
shared electrons from hydrogen atoms
H
H
H
H
C
CH 4
Requirements?
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He -
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 0.7
H 2.1
Be 1.5
Mg 1.2
Ca 1.0
Sr 1.0
Ba 0.9
Ra 0.9
Ti 1.5
Cr 1.6
Fe 1.8
Ni 1.8
Zn 1.8
As 2.0
SiC
C(diamond)
H2O
C 2.5
H2
Cl2
F2
Si 1.8
Ga 1.6
GaAs
Ge 1.8
O 2.0
co
lum
n IV
A
Sn 1.8Pb 1.8
Covalent bonding
What type of materials?
Bonding energy high (75-300 kcal/mol) Directional
Conductivity? Melting temperature?
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Metallic bonding
What type of materials? Requirements?
Bonding energy high (25-200 kcal/mol) Nondirectional
Conductivity? Ductility?
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Primary Bonding
• Metallic Bond
• Ionic-Covalent Mixed Bonding % ionic character = where XA & XB are Pauling electronegativities
%) 100 ( x
€
1−e− (XA−XB)2
4
#
$
% % %
&
'
( ( (
Ex: MgO XMg = 1.3 XO = 3.5
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van der Waals
Secondary bonding
Hydrogen bonding
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Type
Ionic
Covalent
Metallic
Secondary
Bond Energy
Large!
Variable large-Diamond small-Bismuth
Variable large-Tungsten small-Mercury Smallest but important! Biopolymers….
In What Materials?
Bonding: a summary
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How bonding affects material properties?
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• Melting Temperature, Tm
Properties From Bonding: Tm
r o r
Energy
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• Coefficient of thermal expansion, α
Properties From Bonding : α
= α ( T 2 - T 1 ) Δ L L o
coeff. thermal expansion
Δ L
length, L o unheated, T 1
heated, T 2
r o r
Energy unstretched length
Eo
Eo
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• Elastic modulus, E
ΔL F Ao
= E Lo
Elastic modulus
Properties from bonding: elastic modulus
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
ro unstretched length
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Ceramics (Ionic & covalent bonding):
Metals (Metallic bonding):
Polymers (Covalent & Secondary):
Large bond energy large Tm large E small α
Variable bond energy moderate Tm moderate E moderate α
Secondary bonding dominates small Tm small E large α
Summary
secondary bonding