che 42d chapter 1 ed_11!09!07_structure of matls_basics
TRANSCRIPT
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Materials Science andEngineering
CHAPTER 1The Structure of Materials
Basic Concepts
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Materials Science
Involves theinvestigation of the
relation between
structure and
properties of
materials
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Materials Engineering
Involves designing or
engineering the structure of
materials on the basis of
material structure-propertyrelationship to produce a
predetermined set of
properties
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Structure
Relates to the arrangement of its internal
components
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Subatomic
Atomic
Microscopic
Macroscopic
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Property
A material trait in terms of the kind and
magnitude of response to a specificimposed stimulus
Definitions are made independent ofmaterial shape and size
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chemical mechanical
electrical magnetic
dimensional
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Four components of materials
science and engineering
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DesignProduction
Utilization
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Elements as building blocks for all
materialsMatter
Elements
Organic substances Inorganic substances
Liquids Solids Gases Liquids Solids Gases
Fuels
ChemicalsPaints
Oils
Beverages
Living organisms
PolymersNatural resins
Food
Soils
Composites
Fuels
Chemicals
Acids
WaterBases
Chemicals
Metals
CeramicsGlasses
Clays
Cements
Composites
Inert gases
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Bohrs Quantum Model of the
AtomOrbital electron
Nucleus
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Suggested in 1913
Electrons exist in stablecircular orbits of fixed
energy (quantized)
ENERGYSTATES/LEVEL
Electrons can only emit
or absorb energy whenmaking a transition
from one possible orbit
to another QUANTUMJUMP
Electron is a particle moving in a discreteorbital.
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Simple atomic structure model
KL
7e-2e-
K L
8e-2e-
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Wave-mechanical model
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Nucleus
Electron probability density
electrons position is
described by a probabilitydistribution (electron
cloud)
Bohr energy levelsseparate into electron
subshells
Defines the exact location of electrons in orbitals (energy level)
Electrons exhibit both wave-likeand particle-like characteristics
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Bohr and Wave-mechanical
models Comparison of
the (a) Bohr and(b) Wave-
mechanical atom
models in terms
of electronic
distribution
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(a) (b)
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Quantum Numbers
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K
LM
N
O
s s s s s 0p p p p 1
d d d 2
f f 3
g 4
Subshells
s: sphericalp: dumbbell-shaped
d: clover in four cases or dumbbell-shaped in one
Principal quantum
number (n): general energy
levels of the electrons (in
terms of distance from the
nucleus); n is + integer;
max. value: 2n2
Secondary quantum
numbers (l): (angular
momentum); defines theshape of the electron
subshell; values 0 to (n-1)
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Quantum Numbers
Third quantum number (ml):
(magnetic moment); the
orientation of an orbital; number
of energy states in a subshell;
values l to l including 0
Fourth quantum number (l):(spin momentum); motion (-
1/2down), (+1/2 up)
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s p d f1 3 5 7
A moving electron will produce a
magnetic field which is affected byexternal magnetic field
No. of energy states
In the absence of an external magnetic field, the states within each subshell
are identical.
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Number of available electron states
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1
3
57
s
p
df
N4
1
35
s
pd
M3
1
3
s
p
L2
1sK1
Number of
electrons
Number of
states
SubshellsShell
designation
Principal
quantum
number, n
?
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Electron configuration
Pauli Exclusion Principle
No two electrons can have the same fourquantum numbers!!!!
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Each electron state can hold no more than two electrons,
which must have opposite signs.
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Electron configuration
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1 2 3 4 5 6 7
ps
ps
ps
d
psd
psdf
p
s
df
df
s
Principal quantum number, n
Energy
NOTES:
1) The smaller the principal quantum number, the lower the energy level.
2) Within each shell, the energy of a subshell level increases with the value of the (l) quantum number.3) There may be overlap in energy of a state in one shell with states in an adjacent shell.
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Electron configuration
The manner by which states are
occupied by electrons Aluminum (13): 1s22s2p63s2p1
Chromium (24): 1s2
2s2
p6
3s2
p6
3d5
4s1
Krypton (36): 1s22s2p63s2p63d54s2p6
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Electrons fill up the lowest possible energy level first in the electron shells and
subshells, two electrons per state.
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Electron configuration
GROUND STATE: all electrons occupy the
lowest possible energies in accordanceto Pauli exclusion principle.VALENCE ELECTRONS: electrons in the
outermost filled shell; participate ininteratomic bonding physical/chemicalproperties
STABLE ELECTRON CONFIGURATION:outermost shells are fully filled (s & pstates normally)
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The Periodic Table
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ElectronegativeElectropositive
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Electronegativity Values for Elements
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ELECTRONEGATIVITY the relative tendency of an atom to attract electrons
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Bonding
Secondary bonds
Primary bonds
Ionic Covalent Metallic
Permanent dipole
(also as H bond)London
forces Hydrogen
= attractive bond X O = shared electrons
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Types Bonding
Primary bonds strong atom-to-atom
attractions produced by changes in electronposition of outer (valence) electrons
Secondary bonds weaker than primary
bonds and are formed when atoms ormolecules are attracted by overall electric
fields (dipoles), which often result from the
transfer of electrons in the primary bonding
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Bond Strengths
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0.1-10
10-40
50-1000
200-1000
50-1000
Intermolecular forcesvan der Waals (London,
dipole-dipole) forces
Hydrogen bondingChemical bonding
Ionic
Covalent
Metallic bonding
Energy (kJ/mol)Type of bond
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Ionic bond
M
2e-
L
2e-
LKK
8e- 8e- (ionized)
0e- (ionized)
= attractive bondU
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electrons are transferred from one metal to the nonmetal,
creating ions that attract each other throughout the mass
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Covalent bond
2e-
L
K
8e-
2e-
LK
8e-
FF22U
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electrons are shared between atoms to produce a stable
group of eight
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Metallic bond
++ + +
+ + +
Mg2+ ion cores
Electron cloud from valence electrons
L
8e-
K
2e-
2e-
M
Magnesium atom
Sea of valence electrons
Ion cores
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the electrons are delocalized, or given up to form a
common sea of electrons surrounding the positive ions
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Metallic bond
Electron cloud valence electrons that are not
bound to any particular atom in the solid and aremore or less free to drift throughout the entire
metal; shield the ion cores from mutually
repulsive electrostatic forces and act as a gluethat hold ion cores together
Ion cores remaining non valence electrons
and atomic nuclei form which has a net positivecharge equal to that in magnitude to the total
valence electron charges per atomU
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Dipoles
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+ - + -
Atomic or molecular dipoles
Van der Waals bonding between two dipoles
Atomic nucleus
Electron cloud
Electrically
symmetric
atom
Induceddipole
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Dipole-dipole Forces
the arrangement of electrons and positive
nuclei results in a positively charged field atone end and a negatively charged field at the
other end; e.g., formation of a polar molecule
of HCl Also called polar molecule-induced dipole
bonds
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ClH
+ -
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London forces
attractions are formed when temporary
dipoles are developed due to the motionof electrons; e.g., attraction between
nonpolar molecules and single atoms of
inert gases
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Ne Ne
Attraction between + and -
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Hydrogen bonds (Permanent dipole)
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FH FH
Hydrogen bond
a special case of intermolecular attraction producedbetween certain covalently bonded hydrogen atoms andlone pairs of electrons of another atom; e.g., attractionbetween H atoms in water molecules, H is covalentlybonded to F (HF) & to nitrogen (as in NH3)
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Hydrogen bonding in H2O
molecules
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Seatwork
1. Write the un-ionized and ionized shorthandelectron notation for the following elements:
a. Cu Cu+ and Cu2+ At. No. = 29
b. Fe Fe2+ and Fe3+At. No. = 26
2. Silica (SiO2) can be treated as either acovalently or ionically bonded material.Sketch the valence-electron configuration forsilica both ways. Si = 14, O = 8
3. Despite the strength of the ionic bond, manymaterials with ionic bonds are not considered
good engineering materials. Why not?UniversityofSanCarlos-DepartmentofCh
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