ch-2 compatibility mode
TRANSCRIPT
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ATOMIC BONDING INSOLIDS
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ISSUES TO ADDRESS...ISSUES TO ADDRESS...ISSUES TO ADDRESS...ISSUES TO ADDRESS...
What promotes bonding?
BONDING AND PROPERTIES
What types of bonds are there?
What properties are inferred from bonding?
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orbital electrons:n = principalquantum number
n=3 21
BOHR ATOM
Nucleus: Z = # protons
= 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.
ergy
n=44p
3d
Electrons...ELECTRON ENERGY STATES
Inc
reasinge
n=1
n=2
n=3
1s
2s
3s
2p
3ps
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have complete s and p subshells tend to be unreactive.
Stable electron configurations...
Z Element Configuration
STABLE ELECTRON
CONFIGURATIONS
10 Ne 1s22s22p6
18 Ar 1s22s22p63s23p6
36 Kr 1s22s22p63s23p63d104s24p6
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Most elements: Electron configuration not stable.ElementHydrogen
Helium
LithiumBerylliumBoronCarbon
Atomic #1
2
3456
Electron configuration1s1
1s2 (stable)
1s22s11s22s21s22s22p11s22s22p2
SURVEY OF ELEMENTS
Why? Valence (outer) shell usually not filled completely.
...
NeonSodiumMagnesiumAluminum...
Argon...Krypton
10111213
18...36
...
1s22s22p6 (stable)1s22s22p63s11s22s22p63s21s22s22p63s23p1...
1s22s22p63s23p6 (stable)...
1s22s22p63s23p63d104s246 (stable)
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Columns: Similar Valence Structure
He
iner
tgases
accept1e
accept2e
give
up1e
giveup2e
eup3e
Metal
Nonmetal
Intermediate
H
THE PERIODIC TABLE
Electropositive elements:Readily give up electrons
to become + ions.
Electronegative elements:Readily acquire electrons
to become - ions.
Ar
Kr
Xe
Rn
gi
v
Na Cl
Br
I
At
SMg
Ca
Sr
Ba
Ra
K
Rb
Cs
Fr
Sc
Y
Se
Te
Po
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Ranges from 0.7 to 4.0
He
-
Ne-
Ar-
F4.0
Cl3.0
Li1.0
Na0.9
H
2.1
Be1.5
Mg1.2
Large values: tendency to acquire electrons.
ELECTRONEGATIVITY
Smaller electronegativity Larger electronegativity
-Xe
-
Rn-
2.8I
2.5
At2.2
0.8Rb0.8
Cs0.7
Fr0.7
1.0Sr1.0
Ba0.9
Ra0.9
1.5
1.6
1.8
1.8
1.8
2.0
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Interatomic ForcesInteratomic ForcesInteratomic ForcesInteratomic Forces
Attractive repulsiveAttractive repulsiveAttractive repulsiveAttractive repulsive
Magnitude of eachMagnitude of eachMagnitude of eachMagnitude of eacha fn. ofa fn. ofa fn. ofa fn. of
Net force bet. two atomsNet force bet. two atomsNet force bet. two atomsNet force bet. two atomsFFFFNNNN=F=F=F=FAAAA + F+ F+ F+ FRRRR
When FWhen FWhen FWhen FNNNN and Fand Fand Fand FRRRR balance,balance,balance,balance,FFFFNNNN = 0= 0= 0= 0
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rrrr0000 corresponds tocorresponds tocorresponds tocorresponds toseparation distanceseparation distanceseparation distanceseparation distance
at min. of P.E curve,at min. of P.E curve,at min. of P.E curve,at min. of P.E curve,EEEE0000 and shape of E vs r curveand shape of E vs r curveand shape of E vs r curveand shape of E vs r curve
vary fromvary fromvary fromvary fromMATERIAL TO MATERIALMATERIAL TO MATERIALMATERIAL TO MATERIALMATERIAL TO MATERIAL
and depend on theand depend on theand depend on theand depend on the
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BONDING IN MATERIALS
Primary Bonding Secondary Bonding
(Intra molecular (Intermolecular forces 1 to 40kJ mol-1)150 to 800kJ mol-1 )
(Vanderwaals Bonding)(inert gases)
Ionic Covalent Metallic(NaCl,MgO, (CH4,Ge,C,SiC, (all elemental metals)
a 2 e c. a s, amon
etc.)
Non-directional Directional
(many nonmetallicmolecules)
When a bond is formed the total energy inWhen a bond is formed the total energy inWhen a bond is formed the total energy inWhen a bond is formed the total energy inBONDED STATE is < that in the FREE STATEBONDED STATE is < that in the FREE STATEBONDED STATE is < that in the FREE STATEBONDED STATE is < that in the FREE STATEFor both IONIC and COVALENT BONDINGFor both IONIC and COVALENT BONDINGFor both IONIC and COVALENT BONDINGFor both IONIC and COVALENT BONDING
Association can take place when theAssociation can take place when theAssociation can take place when theAssociation can take place when the
bonded state achieves the stabilitybonded state achieves the stabilitybonded state achieves the stabilitybonded state achieves the stabilityie A STATE OF MINIMUM ENERGYie A STATE OF MINIMUM ENERGYie A STATE OF MINIMUM ENERGYie A STATE OF MINIMUM ENERGY
When two atoms combine,energy must be releasedWhen two atoms combine,energy must be releasedWhen two atoms combine,energy must be releasedWhen two atoms combine,energy must be releasedso that total energy is lowered. This is known asso that total energy is lowered. This is known asso that total energy is lowered. This is known asso that total energy is lowered. This is known as
BONDING ENERGYBONDING ENERGYBONDING ENERGYBONDING ENERGY
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ELECTRONIC CONFIGURATION OF ALLIONS OF A SIMPLE IONIC CRYSTAL
SHOWS CLOSE ELECTRONIC
SHELLSCHARGE DISTRIBUTION IS
SPHERICALLY SYMMETIRC
INERT GASESFOR IONIC CRYSTAL TO BE STABLE,ALL POSITIVE IONS MUST HAVE ASFOR IONIC CRYSTAL TO BE STABLE,ALL POSITIVE IONS MUST HAVE ASFOR IONIC CRYSTAL TO BE STABLE,ALL POSITIVE IONS MUST HAVE ASFOR IONIC CRYSTAL TO BE STABLE,ALL POSITIVE IONS MUST HAVE AS
NEAREST NEIGHBOURS NEGATIVELY CHARGED IONS IN A 3D SCHEME.NEAREST NEIGHBOURS NEGATIVELY CHARGED IONS IN A 3D SCHEME.NEAREST NEIGHBOURS NEGATIVELY CHARGED IONS IN A 3D SCHEME.NEAREST NEIGHBOURS NEGATIVELY CHARGED IONS IN A 3D SCHEME.NON DIRECTIONAL BONDING:NON DIRECTIONAL BONDING:NON DIRECTIONAL BONDING:NON DIRECTIONAL BONDING: MAGNITUDE OF THE BOND IS EQUAL IN ALLMAGNITUDE OF THE BOND IS EQUAL IN ALLMAGNITUDE OF THE BOND IS EQUAL IN ALLMAGNITUDE OF THE BOND IS EQUAL IN ALL
DIRECTIONS AROUND AN ION ( EACH ION HAS A UNIFORMLY DISTRIBUTEDDIRECTIONS AROUND AN ION ( EACH ION HAS A UNIFORMLY DISTRIBUTEDDIRECTIONS AROUND AN ION ( EACH ION HAS A UNIFORMLY DISTRIBUTEDDIRECTIONS AROUND AN ION ( EACH ION HAS A UNIFORMLY DISTRIBUTEDELECTRIC FIELD,ONE CANNOT PREDICT THAT A PARTICULAR ION ISELECTRIC FIELD,ONE CANNOT PREDICT THAT A PARTICULAR ION ISELECTRIC FIELD,ONE CANNOT PREDICT THAT A PARTICULAR ION ISELECTRIC FIELD,ONE CANNOT PREDICT THAT A PARTICULAR ION ISBONDED TO THIS OR THAT ATOM).BONDED TO THIS OR THAT ATOM).BONDED TO THIS OR THAT ATOM).BONDED TO THIS OR THAT ATOM).
PREDOMINANT BONDING IN CERAMIC MATERIALS IS IONICPREDOMINANT BONDING IN CERAMIC MATERIALS IS IONICPREDOMINANT BONDING IN CERAMIC MATERIALS IS IONICPREDOMINANT BONDING IN CERAMIC MATERIALS IS IONIC
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Attractive bonding forces areAttractive bonding forces areAttractive bonding forces areAttractive bonding forces are COULOMBICCOULOMBICCOULOMBICCOULOMBIC (between positive and negative ions)(between positive and negative ions)(between positive and negative ions)(between positive and negative ions)For two isolated ions ,For two isolated ions ,For two isolated ions ,For two isolated ions ,
he attractive energy Ehe attractive energy Ehe attractive energy Ehe attractive energy Eaaaa is a function of theis a function of theis a function of theis a function of the INTERATOMIC DISTANCEINTERATOMIC DISTANCEINTERATOMIC DISTANCEINTERATOMIC DISTANCE according toaccording toaccording toaccording toEEEEaaaa ==== ----A / r , A=(zA / r , A=(zA / r , A=(zA / r , A=(z1111e )(ze )(ze )(ze )(z2222e)/4e)/4e)/4e)/40000,
(MADELUNG Energy) , Similarly for the repulsive energy, E(MADELUNG Energy) , Similarly for the repulsive energy, E(MADELUNG Energy) , Similarly for the repulsive energy, E(MADELUNG Energy) , Similarly for the repulsive energy, E bbbb = B / r= B / r= B / r= B / rnnnn
A,B and n are constants and their values depend on the particular ionic systemA,B and n are constants and their values depend on the particular ionic systemA,B and n are constants and their values depend on the particular ionic systemA,B and n are constants and their values depend on the particular ionic system
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REAL IONIC CRYSTAL:approximately spherical symmetry ,some distortion near the region of
contact with neighbouring atoms
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Na metal Cl nonmetal
Occurs between + and - ions. Requires electron transfer.
Large difference in electronegativity required.
Example: NaCl
IONIC BONDING
unstable unstableelectron
+ -CoulombicAttraction
Na (cation)stable Cl (anion)stable
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Predominant bonding in Ceramics
He-
Ne-
Ar
F4.0
Cl
Li1.0
Na
H2.1
Be1.5
Mg
CsCl
MgO
CaF2
NaCl
O3.5
EXAMPLES: IONIC BONDING
Give up electrons Acquire electrons
-
Kr-
Xe-
Rn-
.
Br2.8
I2.5
At2.2
.
K0.8
Rb0.8
Cs0.7
Fr0.7
.
Ca1.0
Sr1.0
Ba0.9
Ra0.9
Ti1.5
Cr1.6
Fe1.8
Ni1.8
Zn1.8
As2.0
Good conductors of heat and electricity in molten state or aqueous mediumGood conductors of heat and electricity in molten state or aqueous mediumGood conductors of heat and electricity in molten state or aqueous mediumGood conductors of heat and electricity in molten state or aqueous medium
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LATTICELATTICELATTICELATTICE is a regular geometrical array formed by a large no. of +ve andis a regular geometrical array formed by a large no. of +ve andis a regular geometrical array formed by a large no. of +ve andis a regular geometrical array formed by a large no. of +ve and ve ionsve ionsve ionsve ionsheld togetherby elecrostatic attraction.held togetherby elecrostatic attraction.held togetherby elecrostatic attraction.held togetherby elecrostatic attraction.
Energy released during the formation of a lattice is known asEnergy released during the formation of a lattice is known asEnergy released during the formation of a lattice is known asEnergy released during the formation of a lattice is known as LATTICE ENERGYLATTICE ENERGYLATTICE ENERGYLATTICE ENERGY
LatticeLatticeLatticeLattice is a highly stable arrangement. For high lattice energy, size of cation < size of anion,is a highly stable arrangement. For high lattice energy, size of cation < size of anion,is a highly stable arrangement. For high lattice energy, size of cation < size of anion,is a highly stable arrangement. For high lattice energy, size of cation < size of anion,
more effective is the nucleus of the cation to pull the neighbouring anion towardsmore effective is the nucleus of the cation to pull the neighbouring anion towardsmore effective is the nucleus of the cation to pull the neighbouring anion towardsmore effective is the nucleus of the cation to pull the neighbouring anion towards itititit
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Requires shared electrons
Example: CH4
C: has 4 valence e,needs 4 more
H: has 1 valence e,
shared electrons
from carbon atom
H
H
H C
CH4
COVALENT BONDING
needs 1 moreElectronegativitiesare comparable.
shared electronsfrom hydrogenatoms
H
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Covalent Bond ELECTRONS IN COVALENT BOND AREELECTRONS IN COVALENT BOND AREELECTRONS IN COVALENT BOND AREELECTRONS IN COVALENT BOND ARE
LOCALIZED IN THE REGIONS BETWEENLOCALIZED IN THE REGIONS BETWEENLOCALIZED IN THE REGIONS BETWEENLOCALIZED IN THE REGIONS BETWEENTHE TWO ATOMS JOINED BY THE BONDTHE TWO ATOMS JOINED BY THE BONDTHE TWO ATOMS JOINED BY THE BONDTHE TWO ATOMS JOINED BY THE BOND
POSSIBLE NUMBER OF COVALENT BONDSPOSSIBLE NUMBER OF COVALENT BONDSPOSSIBLE NUMBER OF COVALENT BONDSPOSSIBLE NUMBER OF COVALENT BONDS
NO. OF VALENCE ELECTRONS = NNO. OF VALENCE ELECTRONS = NNO. OF VALENCE ELECTRONS = NNO. OF VALENCE ELECTRONS = NNO. OF COVALENT BONDS = 8NO. OF COVALENT BONDS = 8NO. OF COVALENT BONDS = 8NO. OF COVALENT BONDS = 8----N (by octet rule forN (by octet rule forN (by octet rule forN (by octet rule forstability of atoms)stability of atoms)stability of atoms)stability of atoms)
INTER ATOMIC BONDING : PARTIALLY IONIC andINTER ATOMIC BONDING : PARTIALLY IONIC andINTER ATOMIC BONDING : PARTIALLY IONIC andINTER ATOMIC BONDING : PARTIALLY IONIC andPARTIALLY COVALENTPARTIALLY COVALENTPARTIALLY COVALENTPARTIALLY COVALENT
GREATER THE DIFF. IN ELECTRONEGATIVITY MOREGREATER THE DIFF. IN ELECTRONEGATIVITY MOREGREATER THE DIFF. IN ELECTRONEGATIVITY MOREGREATER THE DIFF. IN ELECTRONEGATIVITY MOREIONIC NATURE OF THE BOND (IONIC NATURE OF THE BOND (IONIC NATURE OF THE BOND (IONIC NATURE OF THE BOND (larger the separation bothlarger the separation bothlarger the separation bothlarger the separation both
column wise or row wise in the Periodic Table, more ionic nature ofcolumn wise or row wise in the Periodic Table, more ionic nature ofcolumn wise or row wise in the Periodic Table, more ionic nature ofcolumn wise or row wise in the Periodic Table, more ionic nature ofthe bond)the bond)the bond)the bond)
Percentage ionic character of a bond betweenPercentage ionic character of a bond betweenPercentage ionic character of a bond betweenPercentage ionic character of a bond betweenelements A and B,elements A and B,elements A and B,elements A and B,
(A being more electronegative) = {1(A being more electronegative) = {1(A being more electronegative) = {1(A being more electronegative) = {1---- exp[exp[exp[exp[----(0.25)(X(0.25)(X(0.25)(X(0.25)(XAAAA----
XXXXBBBB))))2222]} X 100]} X 100]} X 100]} X 100
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Covalent bonds are less dense than the Ionic/Metallic bonds,Covalent bonds are less dense than the Ionic/Metallic bonds,Covalent bonds are less dense than the Ionic/Metallic bonds,Covalent bonds are less dense than the Ionic/Metallic bonds,because Covalent bonds are directionalbecause Covalent bonds are directionalbecause Covalent bonds are directionalbecause Covalent bonds are directional
and they cannot pack together in as dense a manner, yielding a lower mass densityand they cannot pack together in as dense a manner, yielding a lower mass densityand they cannot pack together in as dense a manner, yielding a lower mass densityand they cannot pack together in as dense a manner, yielding a lower mass density
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He-
Ne-
Ar-
Kr
F4.0
Cl3.0
Br
Li1.0
Na0.9
K
H2.1
Be1.5
Mg1.2
Ca Ti Cr Fe Ni Zn As
SiC
C(diamond)
H2O
C2.5
H2
Cl2
F2
Si1.8
Ga Ge
O2.0
columnIVA
EXAMPLES: COVALENT BONDING
Molecules with nonmetals Molecules with metals and nonmetals Elemental solids (RHS of Periodic Table)
Compound solids (aboutcolumn IVA)
-
Xe-
Rn-
.
I2.5
At2.2
.
Rb0.8
Cs0.7
Fr0.7
.
Sr1.0
Ba0.9
Ra0.9
. 1.6 . 1.8 1.8 ..
GaAs
.
Sn1.8
Pb1.8
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F0RMATION OF COVALENT BOND:1.SHARING ELECTRONS BETWEEN ATOMS OF SAME TYPE, eg: H2,Cl2,O2 etc
2.SHARING ELECTRONS BETWEEN ATOMS OF DIFFERENT KINDS,
eg:H20,CH4,HCL etc
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Arises from a sea ofdonated valence electrons(1, 2, or 3 from each atom).
+ + +
METALLIC BONDING
Primary bond for metals and their alloys
+ + +
+ + +
Metal is a network of ions in a sea of electrons,Metal is a network of ions in a sea of electrons,Metal is a network of ions in a sea of electrons,Metal is a network of ions in a sea of electrons,with the electron sea providing the binding force which holds the metal ions togetherwith the electron sea providing the binding force which holds the metal ions togetherwith the electron sea providing the binding force which holds the metal ions togetherwith the electron sea providing the binding force which holds the metal ions together
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Electrons are shared by all atoms,that is they are delocalized throughout the crystalElectrons are shared by all atoms,that is they are delocalized throughout the crystalElectrons are shared by all atoms,that is they are delocalized throughout the crystalElectrons are shared by all atoms,that is they are delocalized throughout the crystal
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Arises from interaction between dipoles
Fluctuating dipoles
HH HH
H2
H2
secondary
ex: liquid H2asymmetric electron
clouds
+ - + -secondary
bonding
SECONDARY BONDING
Permanentdipoles-molecule induced
+ - secondarybonding
+ -
H Cl H Clsecondarybonding
secondarybonding
-general case:
-ex: liquid HCl
-ex: polymerCoulombic interactionCoulombic interactionCoulombic interactionCoulombic interaction
between +ve end of one dipolebetween +ve end of one dipolebetween +ve end of one dipolebetween +ve end of one dipoleandandandand ve region of the adjacent dipoleve region of the adjacent dipoleve region of the adjacent dipoleve region of the adjacent dipole
Strength of these types of forces decreasesStrength of these types of forces decreasesStrength of these types of forces decreasesStrength of these types of forces decreaseswith increase in intermolecular separationwith increase in intermolecular separationwith increase in intermolecular separationwith increase in intermolecular separation
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Polar moleculePolar moleculePolar moleculePolar molecule----Induced dipole bondsInduced dipole bondsInduced dipole bondsInduced dipole bonds
Polar HCl MoleculePolar HCl MoleculePolar HCl MoleculePolar HCl Molecule
ATTRACTIVE FORCES between TEMPORARY DIPOLE and INDUCED DIPOLE are known asATTRACTIVE FORCES between TEMPORARY DIPOLE and INDUCED DIPOLE are known asATTRACTIVE FORCES between TEMPORARY DIPOLE and INDUCED DIPOLE are known asATTRACTIVE FORCES between TEMPORARY DIPOLE and INDUCED DIPOLE are known as
Van der waals forcesVan der waals forcesVan der waals forcesVan der waals forcesFLUCTUATING DIPOLE BONDS ARE BETWEEN NON POLAR ATOMS/MOLECULES
Molecule develops aMolecule develops aMolecule develops aMolecule develops aTEMPORARY DIPOLETEMPORARY DIPOLETEMPORARY DIPOLETEMPORARY DIPOLE
(INSTANTANEOUS DIPOLE(INSTANTANEOUS DIPOLE(INSTANTANEOUS DIPOLE(INSTANTANEOUS DIPOLE)
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HYDROGEN BONDING
EXISTS IN THE MOLECULES THAT HAVE HYDROGEN BONDEDTO NITROGEN, OXYGEN or FLUORINE
ELECTROSTATIC FORCE OF ATTRACTION EXISTS BETWEEN THECOVALENTLY BONDED HYDROGEN ATOM OF ONE MOLECULE ANDTHE ELECTRONEGATIVE ATOM OF THE OHER MOLECULE
+ - + ---------H F ------------H F-----------
Hydrogen Bond Covalent Bond* H atom must be bonded to a highly electronegative atom
* Size of electronegative atom should be small(HCl does notcontain a Hydrogen bond,because size of Cl is large)It coexists with the covalent bond ,weaker than primary bonds
but stronger than secondary bondsM.P and B.P is high compared to other intermolecular bondings
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Type
Ionic
Covalent
Bond Energy
Large!
Variablelarge-Diamond
-
Comments
Nondirectional (ceramics)
Directionalsemiconductors, ceramics
SUMMARY: BONDING
Metallic
Secondary
Variablelarge-Tungstensmall-Mercury
smallest
Nondirectional (metals)
Directionalinter-chain (polymer)
inter-molecular
PROPERTIES FROM
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Bond length, r
Bond energy, Eo
FF
r
Melting Temperature, Tm
Energy (r)
ro
PROPERTIES FROM
BONDING: TM
Eo=
bond energy
Energy (r)
ror
unstretched length
r
larger Tm
smaller Tm
Tm is larger if Eo is larger.
PROPERTIES FROM BONDING E
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Elastic modulus, E
crosssectionalarea Ao
L
length, Lo
F
undeformed
deformed
LFAo= E Lo
Elastic modulus
PROPERTIES FROM BONDING: E
~ curva ure a ro
r
larger Elastic Modulus
smaller Elastic Modulus
Energy
rounstretched length
E is larger if Eo is larger.
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Coefficient of thermal expansion,
L
length, Lo
unheated, T1
heated, T2
= (T2-T1)
LLo
coeff. thermal expansion
PROPERTIES FROM BONDING:
~ symmetry at ro
is larger if Eo is smaller.r
smaller
larger
Energy
ro
Small dimensional alterationsfor changes in temperature
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Ceramics(Ionic & covalent bonding):
Metals
(Metallic bonding):
Large bond energylarge Tmlarge Esmall
Variable bond energymoderate Tmmoderate E
SUMMARY: PRIMARY BONDS
Polymers(Covalent & Secondary):
secondarybonding
moderate
Directional PropertiesSecondary bonding dominates
small Tsmall Elarge
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Problems:2.13 to 2.18
2.13 The net Potential Energy bet. two adjacent ions,E N = EA + ER= - A/r + B/r
n ------(1)By differentiating EN w.r.t r,
dEN
/dr = - A/r2 + nB/rn+1 = 0(r is minimum at E0 = r0r0= equilibrium inter-ionic spacing),or, A/ r2 = nB/rn+1, A/nB = r2/rn+1, A/nB = r1-n,
Therefore, r0 = (A/nB)1/1-n-----(2) ,(A=1.436,B=7.32 x 10-6,n=8)
Putting these values n the above expression,r0= 0.236nm.
Substituting the expression for r0from (2) in (1) and solving forE (= E0 )and solving it, we get, E0 = -5.32ev
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2.15 r0 = (A/nB)1/1-n ,
E 0 = - A/(A/nB)1/1-n + B/(A/nB)n/1-n
Thus we have two simultaneous equations with twounknowns (A and B),substituting the values of r0 and E0
in terms of n, we get,0.35nm = (A/10B)1/1-10
-6.13ev = - A/(A/10B)1/1-10 + B/(A/nB)10/1-10
simultaneous solutions to these two equations give,A=2.38 and B=1.88X10-5,
EA =- A/r = -2.38/r
ER = B/rn = 1.88X10-5 / r10