7 structure of materials-bonding (1)
DESCRIPTION
materialsTRANSCRIPT
Structure of Materials: bonding and properties
ZnS
NaCl
Cs Cl
1 mm
Crystal units
Microstructure (grains, grain boundaries) of a metal alloy
Atomic Structure
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)
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
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.
Electron Energy States
Electrons...
“Absorb or emit energy”
Stable electron configurations...
Electron Configurations
• 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)
• What are valence electrons? • Their role in bonding?
– example: C (atomic number = 6)
1s2 2s2 2p2
valence electrons
• 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
• Ranges from 0.7 to 4.0, • tendency to acquire electrons.
Electronegativity
Bonding
Bonding force
Bonding energy
Ø Primary bonds
Ø Secondary bonds
• Example: NaCl
Ionic Bonding
Na (metal) unstable
Cl (nonmetal) unstable
electron
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
Requirements?
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?
Covalent Bonding
• Example: CH4
shared electrons from carbon atom
shared electrons from hydrogen atoms
H
H
H
H
C
CH 4
Requirements?
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?
Metallic bonding
What type of materials? Requirements?
Bonding energy high (25-200 kcal/mol) Nondirectional
Conductivity? Ductility?
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
van der Waals
Secondary bonding
Hydrogen bonding
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
How bonding affects material properties?
• Melting Temperature, Tm
Properties From Bonding: Tm
r o r
Energy
• 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
• 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
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