atomic structure and bonding in solids

8/4/2019 Atomic Structure and Bonding in Solids

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MECH 221 PM Wood-Adams Fall 2008

POLYMERS CERAMICS METALS

DUCTILITY Varies Poor Good

HighCONDUCTIVITY(ELECTRICAL & THERMAL)

Low Low

HARDNESS/STRENGTHLow

medium Very highMedium

high

CORROSION RESISTANCE Fair good Good Fair poor

STIFFNESS Low High Fair

FRACTURE TOUGHNESSLow

mediumLow High

MACHINABILITY Good Poor Good

Classes of materials


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Electronic structure (distribution of electrons in atomic orbitals) Number of electrons and (tendency for an

atom to attract an electron)

Why study bonding?

Because the properties of materials (strength, hardness,

conductivity, etc..) are determined by the manner in which

atoms are connected.

Also by how the atoms are arranged in space.

What determines the nature of the chemical

bond between atoms?

Crystal Structure

electronegativity


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Nucleus: Z = # protons

orbital electrons:n = principalquantum number

n=3 2 1

= 1 for hydrogen to 94 for plutonium

N = # neutrons

Atomic mass A Z + N

Adapted from Fig. 2.1,

Callister 6e.

BOHR ATOM


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have discrete energy states

tend to occupy lowest available energy state.

I

ncreasingenergy

n=1

n=2

n=3

n=4

1s

2s

3s

2p

3p

4s

4p3d

Electrons...ELECTRON ENERGY STATES

Remember that n is the principal quantum number

s, p,d and f signify thesubshells which the

electrons occupy.

Different types ofsubshells have different

numbers of energy states

Within each energystate there are two

possible spin orientations


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have complete s and p subshells

are unreactive.

Stable electron configurations...

Z Element Configuration

2 He 1s2

10 Ne 1s22s22p6

18 Ar 1s22s22p63s23p6

36 Kr 1s22s22p63s23p63d104s24p6

Adapted from Table 2.2,Callister 6e.

Valence electrons are the electrons that occupy the

outermost filled shell.


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Why? Valence (outer) shell usually not filled completely.

Most elements: Electron configuration not stable.ElementHydrogenHelium

LithiumBerylliumBoronCarbon...

NeonSodiumMagnesiumAluminum...

Argon...Krypton

Atomic #12

3456

10111213

18...36

Electron configuration1s1

1s2 (stable)1s22s11s22s21s22s22p11s22s22p2...

1s22s22p6 (stable)1s22s22p63s11s22s22p63s21s22s22p63s23p1...

1s22s22p63s23p6 (stable)...

1s22s22p63s23p63d104s246 (stable)

Adapted from Table 2.2,

Callister 6e.

SURVEY OF ELEMENTS


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Increasing Electronegativity


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He

Ne

Ar

Kr

Xe

Rn

in

ertgases

accept1e

accept2e

giveup1e

give

up2e

g

iveup3

e

FLi Be

Metal

Nonmetal

Intermediate

H

Na Cl

Br

I

At

O

SMg

Ca

Sr

Ba

Ra

K

Rb

Cs

Fr

Sc

Y

Se

Te

Po

Columns: Similar Valence Structure

Electropositive elements:

Readily give up electronsto become + ions.

Electronegative elements:

Readily acquire electrons

to become - ions.

THE PERIODIC TABLE


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Ranges from 0.7 to 4.0,

Smaller electronegativity Larger electronegativity

Large values: tendency to acquire electrons.

ELECTRONEGATIVITY


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Na (metal)

unstable

Cl (nonmetal)

unstableelectron

+ -CoulombicAttraction

Na (cation)stable Cl (anion)stable

Occurs between + and - ions.

Requires electron transfer.

Large difference in electronegativity required. Example: NaCl

IONIC BONDING

Stable because the s and

p subshells are filled!


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Predominant bonding in Ceramics

Give up electrons Acquire electrons

He

-

Ne

-

Ar

-

Kr-

Xe-

Rn-

F

4.0

Cl3.0

Br2.8

I2.5

At2.2

Li

1.0

Na

0.9

K0.8

Rb0.8

Cs0.7

Fr

0.7

H

2.1

Be1.5

Mg1.2

Ca1.0

Sr1.0

Ba0.9

Ra

0.9

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

CsCl

MgO

CaF2

NaCl

O3.5

Adapted from Fig. 2.7, Callister 6e. (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.

EXAMPLES: IONIC BONDING


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Requires shared electrons

Example: CH4

C: has 4 valence e,needs 4 more

H: has 1 valence e,

needs 1 more

Electronegativitiesare comparable.

shared electrons

from carbon atom

shared electronsfrom hydrogenatoms

H

H

H

H

C

CH4

Adapted from Fig. 2.10, Callister 6e.

COVALENT BONDING


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He-

Ne-

Ar-

Kr-

Xe-

Rn-

F4.0

Cl3.0

Br2.8

I

2.5

At2.2

Li1.0

Na0.9

K0.8

Rb

0.8

Cs0.7

Fr

0.7

H2.1

Be1.5

Mg1.2

Ca1.0

Sr1.0

Ba0.9

Ra0.9

Ti1.5

Cr1.6

Fe1.8

Ni1.8

Zn1.8

As2.0

SiC

C(diamond)

H2O

C2.5

H2

Cl2

F2

Si1.8

Ga1.6

GaAs

Ge1.8

O2.0

columnIVA

Sn1.8

Pb1.8

Molecules ofnonmetals

Molecules ofmetals and nonmetals Elemental solids (RHS of Periodic Table) Compound solids (aboutcolumn IVA)

EXAMPLES: COVALENT BONDING

It is possible for bonds to be partially covalent and partially ionic in

nature. Look in Chapter 2 to see how to evaluate this aspect of bonds


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MECH 221 PM Wood-Adams Fall 2008 12

Arises from a sea ofdonated valence electrons(1, 2, or 3 from each atom).

Primary bond for metals and their alloys

+ + +

+ + +

+ + + Adapted from Fig. 2.11, Callister 6e.

METALLIC BONDING


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Arises from interaction between dipoles

Permanentdipoles-molecule induced

Fluctuating dipoles

+ - secondarybonding

+ -

H Cl H Clsecondarybonding

secondarybonding

HH HH

H2 H2

secondarybonding

ex: liquid H2asymmetric electron

clouds

+ - + -secondary

bonding

-general case:

-ex: liquid HCl

-ex: polymer

Adapted from Fig. 2.13, Callister 6e.

SECONDARY 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

SUMMARY: BONDING


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Net force is given by the sum of an

attractive force and a repulsive force

Potential is given by the integral of the net

force curve with respect to distance:

Note: equilibrium separation occurs

where the net force = 0 and the energy is

at a minimum.

=

dFE

Bonding Forces and Energies

repulsive, attractive, and net

forces

repulsive, attractive, and netenergies

Bond length, r

FF

r


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Indicates how much energy must be supplied to completely

disassociate the two atoms

Depth of the potential well indicates bonding strength

Deep well

Shallow well

Bonding energy: Minimum of the potential vs. distance curve.

strongly bonded

weakly bonded

Bonding Forces and Energies

Eo=

bond energy

Energy (r)

ro

r

unstretched length


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Bond length, r

Bond energy, Eo

FF

r

Melting Temperature, Tm

Eo=

bond energy

Energy (r)

ror

unstretched length

r

larger Tm

smaller Tm

Energy (r)

ro

Tm is larger if Eo is larger.

PROPERTIES FROM BONDING: TM


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Example: Bonding energy and TM

Use the data below to estimate the bonding energy of

copper which has a melting temperature of 1084C.

34108.8W

15384.2Fe

6603.4Al

-390.7Hg

E0, eV/atom TM, C

tungsten


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Solution: Plot the data

-500

0

500

1000

1500

20002500

3000

3500

4000

0 2 4 6 8 10

Bonding Energy (eV/atom)

Meltin

gTemperat

ure(C)

E0=3.6 eV/atom

With this analysis we estimate E0 of copper = 3.6 eV/atom.

The measured value is 3.5 eV/atom.


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16

Elastic modulus, E

E ~ curvature at ro

crosssectionalarea Ao

L

length, Lo

F

undeformed

deformed

LFAo

= E Lo

Elastic modulus

r

larger Elastic Modulus

smaller Elastic Modulus

Energy

rounstretched length

E is larger if Eo is larger.

PROPERTIES FROM BONDING: E


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Coefficient of thermal expansion,

~ symmetry at ro

is larger if Eo is smaller.

L

length, Lo

unheated, T1

heated, T2

= (T2-T1)LLo

coeff. thermal expansion

r

smaller

larger

Energy

ro

PROPERTIES FROM BONDING:


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A comparison of the type of bonding found in different materials:

For brass, the bonding is .. since it is a metal alloy.

For rubber, the bonding is with some .Rubber is composed primarily of carbon and hydrogen atoms

For BaS, the bonding is predominantly .. (but with some covalentcharacter)

on the basis of the relative positions of Ba and S in the periodic table.

For solid xenon, the bonding is . since xenon is an inert gas.

For nylon, the bonding is .. with perhaps some ..

Nylon is composed primarily of carbon and hydrogen

For AlP the bonding is predominantly (but with some ionic character)

on the basis of the relative positions of Al and P in the periodic table.

metallic

covalent Van der Waals

ionic

Van der Waals

covalent Van der Waals

covalent

secondarybonding

Bonding Types: Summary


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Ceramics

(Ionic & covalent bonding):

Metals

(Metallic bonding):

Polymers(Covalent & Secondary):

secondarybonding

Large bond energylarge Tm

large E

small

Variable bond energymoderate Tm

moderate E

moderate

Directional PropertiesSecondary bonding dominates

small Tsmall E

large

SUMMARY: PRIMARY BONDS

atomic structure and bonding in solids

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