properties structure processing electronic level (subatomic) atomic (molecular level, chemical...
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Properties
Structure Processing
• Electronic level (subatomic)• Atomic (molecular level, chemical composition)• Crystal (arrangement of atoms or ions wrt one another)• Microstructure (can study with microscopes)• Macrostructure (can see with naked eye)
“Materials Science”“Materials Engineering”
Valence electrons determine all of the following properties
1) Chemical2) Electrical 3) Thermal4) Optical
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• What promotes bonding?
• What types of bonds are there?
• Bonding material propertiesWhat properties are inferred from bonding?
(melting point, thermal expansion, elasticity)
• Electronic structure & bonding
Based on Chapter 2 (Callister)
atom – electrons – 9.11 x 10-31 kg protons neutrons
Electric charge of proton = 1.6 x 10-19 C atomic number = # of protons in nucleus of atom
= # of electrons of neutral species
A [=] atomic mass unit = amu = 1/12 mass of 12C
Atomic wt = wt of 6.023 x 1023 molecules or atoms
1 amu/atom = 1g/mol
C 12.011H 1.008 etc.
} 1.67 x 10-27 kg
Nucleus: Z = # protons
= 1 for hydrogen to 94 for plutoniumN = # neutrons
Atomic mass A ≈ Z + N
Adapted from Fig. 2.1, Callister 6e.
electrons: n = principal quantum number
n=3 2 1
n labels shells; shells are composed of sub-shells: s, p, d, f, …
Electrons have wavelike and particulate properties. This means that electrons are in orbitals defined by a
probability. Each orbital at discrete energy level determined by quantum
numbers. (like x, y, z) Quantum # Designation
n = principal (energy level-shell) K, L, M, N, O (1, 2, 3, etc.7)
l = subsidiary (form and number of orbitals) s, p, d, f (0, 1, 2, 3,…, n -1)
ml = magnetic (Orientation of orbitals) 1, 3, 5, 7 (-l to +l)
Pauli exclusion principlems = spin +½, -½
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1s
2s2p
K-shell n = 1
L-shell n = 2
3s3p M-shell n = 3
3d
4s
4p4d
Energy
N-shell n = 4
• have discrete energy states• tend to occupy lowest available energy state.
Electrons...
Adapted from Fig. 2.4, Callister 7e.
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• Why? Valence (outer) shell usually not filled completely.
• Most elements: Electron configuration not stable.Electron configuration
(stable)
...
...
1s22s 22p 63s23p 6 (stable)... 1s22s 22p 63s23p 63d 10 4s 24p 6 (stable)
Atomic #
18...36
Element1s1 1Hydrogen1s22Helium1s22s 1 3Lithium1s22s24Beryllium1s22s 22p 15Boron1s22s 22p 26Carbon
...
1s22s 22p 6 (stable)10Neon1s22s 22p 63s111Sodium1s22s 22p 63s2 12Magnesium1s22s 22p 63s23p 113Aluminum
...
Argon...Krypton
Adapted from Table 2.2, Callister 7e.
Valence electrons – those in unfilled shells
Filled shells more stable Valence electrons are most available for
bonding and tend to control the chemical properties
example: C (atomic number = 6)
1s2 2s2 2p2
12
valence electrons
ex: Fe - atomic # =
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valence electrons
Adapted from Fig. 2.4, Callister 7e.
1s
2s2p
K-shell n = 1
L-shell n = 2
3s3p M-shell n = 3
3d
4s
4p4d
Energy
N-shell n = 4
1s2 2s2 2p6 3s2 3p6 3d 6 4s2
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• Columns: Similar Valence Structure
Adapted from Fig. 2.6, Callister 7e.
Electropositive elements:Readily give up electronsto become + ions.
Electronegative elements:Readily acquire electronsto become - ions.
give
up
1egi
ve u
p 2e
give
up
3e iner
t ga
ses
acce
pt 1
eac
cept
2e
O
Se
Te
Po At
I
Br
He
Ne
Ar
Kr
Xe
Rn
F
ClS
Li Be
H
Na Mg
BaCs
RaFr
CaK Sc
SrRb Y
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• Ranges from 0.7 to 4.0,
Smaller electronegativity Larger electronegativity
• Large values: tendency to acquire electrons.
Adapted from Fig. 2.7, Callister 7e. (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.
donates accepts electrons electrons
Dissimilar electronegativities
ex: MgO Mg 1s2 2s2 2p6 3s2 O 1s2 2s2 2p4
[Ne] 3s2
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Mg2+ 1s2 2s2 2p6 O2- 1s2 2s2 2p6 [Ne] [Ne]
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• Occurs between + and - ions.
• Requires electron transfer.
• Large difference in electronegativity required.
• Example: NaCl
Na (metal) unstable
Cl (nonmetal) unstable
electron
+ - Coulombic Attraction
Na (cation) stable
Cl (anion) stable
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Energy – minimum energy most stable Energy balance of attractive and repulsive
terms
Attractive energy EA
Net energy EN
Repulsive energy ER
Interatomic separation r
rA
nrBEN = EA + ER =
Adapted from Fig. 2.8(b), Callister 7e.
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• Predominant bonding in Ceramics
Adapted from Fig. 2.7, Callister 7e. (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.
Give up electrons Acquire electrons
NaCl
MgO
CaF2CsCl
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C: has 4 valence e-, needs 4 more
H: has 1 valence e-, needs 1 more
Electronegativities are comparable.
Adapted from Fig. 2.10, Callister 7e.
• similar electronegativity share electrons• bonds determined by valence – s & p orbitals
dominate bonding
• Example: CH4shared electrons from carbon atom
shared electrons from hydrogen atoms
H
H
H
H
C
CH4
• Molecules with nonmetals• Molecules with metals and nonmetals• Elemental solids (RHS of Periodic Table)• Compound solids (about column IVA)
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
colu
mn IVA
Sn 1.8Pb 1.8
Adapted from Fig. 2.7, Callister 6e. (Fig. 2.7 isadapted from Linus Pauling, The Nature of the Chemical Bond, 3rd edition, Copyright 1939 and 1940, 3rd edition. Copyright 1960 by Cornell University.
3.5
• Prevalent in ceramics and polymers
• Arises from a sea of donated valence electrons (1, 2, or 3 from each atom).
• Primary bond for metals and their alloys high electrical conductivity. Why? What about ionic/covalent?
+ + +
+ + +
+ + +Adapted from Fig. 2.11, Callister 6e.
Much easier to deform materials with metallic than with ionic bonding. Why?
• Sliding atom planes over each other (deformation) very unfavorable energetically in ionic solids!
metals are ductile & ceramics (ionic) are brittle
Metallic Bond -- delocalized as electron cloud
Ionic-Covalent Mixed Bonding
% ionic character =
where XA & XB are Pauling electronegativities
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%)100( x
1 e
(XA XB )2
4
ionic 70.2% (100%) x e1 characterionic % 4)3.15.3(
2
Ex: MgO XMg = 1.3XO = 3.5
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Arises from interaction between dipoles
• Permanent dipoles-molecule induced
• Fluctuating dipoles
-general case:
-ex: liquid HCl
-ex: polymer
Adapted from Fig. 2.13, Callister 7e.
Adapted from Fig. 2.14, Callister 7e.
asymmetric electron clouds
+ - + -secondary
bonding
HH HH
H2 H2
secondary bonding
ex: liquid H2
H Cl H Clsecondary bonding
secondary bonding+ - + -
secondary bondingsecondary 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
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• Bond length, r
• Bond energy, Eo
• Melting Temperature, Tm
Tm is larger if Eo is larger.
r o r
Energyr
larger Tm
smaller Tm
Eo =
“bond energy”
Energy
r o r
unstretched length
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• Coefficient of thermal expansion,
• ~ symmetry at ro
is larger if Eo is smaller.
= (T2 -T1)LLo
coeff. thermal expansion
L
length, Lo
unheated, T1
heated, T2
r or
smaller
larger
Energy
unstretched length
Eo
Eo
Why does thermal expansion Why does thermal expansion occur?occur?
As atoms vibrate, easy to move apart, but difficult to move closer
As temperature increases, average bond length increases.
Bond Energy vs. Distance Curve is asymmetric
• Elastic modulus, E
• E ~ curvature at ro
cross sectional area Ao
L
length, Lo
F
undeformed
deformed
L F Ao
= E Lo
Elastic modulus
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
ro unstretched length
E is larger if curvature is larger.
E similar to spring constant